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Engine Modifications

  Experience in a Book
Engine Modifications


The following is a collection of comments regarding modifications for the XJ-S. Most are performance-related, but a few are not. The owner who merely wishes to drive at the speed limit nevertheless may still benefit from this section; many of the suggested modifications (such as the replacement of the steering rack mounts) are suggested for everyone, and many suggestions are worth considering for their ease of implementation.


AIR FILTER REPLACEMENT: K&N Engineering, Inc., makes permanent air filters consisting of special fabric sandwiched between aluminum mesh and treated with oil. They have much less flow restriction than stock paper filters, while still providing excellent filtration. Tests on race engines show only a slight drop in power compared to no filters at all! Since these filters are permanent and cleanable, they can even save money in the long run.

The part number is 33-2011 for any pre-1991 XJ-S. Note that stock air filters changed when the ABS brakes were added; since the housing shifted forward, the blank-off area over the throat had to move rearward. The K&N filters have no blank-off area, so the same part number fits either application.

Roger Bywater of AJ6 Engineering says, "...we gave up using K&N filters some time ago because they fall to bits around the edges where the air box clamps up. We raised the matter with K&N Europe, for whom we do consultancy work on occasions, but they claim not to be aware of the problem..." AJ6 Engineering now offers their own design permanent washable foam air filters.

Glen E. MacDonald notes, "Roger Bywater gave up using K&N filters because "they fall to bits around the edges where the airbox clamps up". True enough. However, I was determined not to trash an otherwise perfectly acceptable (and expensive to replace) pair of filters. The fix I found was to take four suitable lengths of windshield washer hose, split lengthwise. These are then slid over the edges of the filter, after cleaning off whats left of the 'bits' Bywater refers to. I made this alteration over 3 years ago and haven't had any problem since."

Of course, K&N filters come with a "million mile warranty", so another option would be to gripe to K&N about the problem.

For some of us, performance is not the issue with air filters; the wear rate of an engine can be closely related to the filtration of the air intake, and any risk of inferior filtration would not be worthwhile. K&N claims that a NASCAR stock car fitted with its filters will outperform a car with no filters over a 500-mile event, since the deterioration of the engine due to contaminated air over 500 miles will exceed the power loss due to the intake restriction of a K&N filter. K&N claims excellent filtration -- although there have been some contradictory reports from other sources. K&N filters contain a layer of cotton soaked with sticky oil; as the air passes through the cotton, particles should stick to the oil. Such a system normally is not described as a particular size micron filter, since it really should stop all particles regardless of size. The stock paper filter, on the other hand, is a porous media filter, and particles below a certain size are free to pass right through.

Either K&N or the stock paper filter is probably acceptable. However, speaking as an engineer, I must express doubts about the filtration efficiency of most of the foam filters I have seen (I haven't seen the AJ6 Engineering foam filters). A foam filter works on a similar theory as the K&N, the foam providing a media for the air to pass through that is coated with sticky oil. However, the passages between the cotton fibers on a K&N filter are very tiny indeed, it's hard to imagine how a speck of dust could get through without sticking, but some foam filters you can see through. And even those you can't see through appear to have passages that are quite large. And I've also seen foam deteriorate, with crumbs of foam breaking off and going into the inlet.


AIR INTAKE MODS - VERSION 1: Each of the air cleaner housings has a long tapered tube with a relatively small opening for the air intake. The purpose of the small intake opening is to accelerate the air to near Mach 1 at wide open throttle. This prevents intake sound from coming forward through the intake. In other words, it makes the car quiet.

Unfortunately, the air is now moving fast, and there's an air filter up ahead. If the air is allowed to simply run into the filter, the energy associated with the speed will be wasted, and the result is a loss of pressure. Therefore, Jaguar provides the tapered tube to gradually and efficiently slow the air down, recovering most of the energy and pressure. Jaguar designed this tapered tube as long as they could fit under the hood.

Also unfortunately, once the air reaches Mach 1, the passage is "choked" and all the sucking the engine can manage will not increase the amount of air flowing through it. The opening therefore forms an absolute control on the maximum amount of air going into the engine -- and therefore the HP generated.

If you cut off this intake tube and form a large opening for the air intake into the air cleaner housing, you will eliminate this restriction. If you interfere with the mounting of the temperature sensor in the left side intake tube, relocate it into the housing itself (drill a hole and use a nut on the inside); it can sense the air temperature anywhere in there. Because the EFI system detects manifold vacuum, the system will automatically compensate for the increased airflow; no tuning modifications are required. There is no effect on emissions, so there should be no complaints from inspectors.

At part throttle (most of the time with an engine this powerful), the butterfly acts as an air-accelerating restriction, and no sound gets out anyway. The car will sound like it did before. However, when you open it up, you will be greeted with a sonorous growl from under the hood. This is a very sexy sound, but some Jag owners may not like it.

The performance improvement goes along with the noise. In any situation where there is still no sound, there is no change in performance either. When the engine growls, there is more airflow than there was before.

This mod will have no effect on fuel economy, except when you hear the growl; at that point, increased fuel use accompanies the increased airflow and increased power.

You can give this mod a trial run without much effort. Unscrew the air temperature sensor, then reconnect the wire to it and tape it down anywhere convenient. Then, remove both air filter covers and tie the air filters in place with some wire. This setup will provide the same performance (and a little more noise) than the intake tube removal described above.


AIR INTAKE MODS - VERSION 2: Any engineer will tell you that an internal combustion engine will run more efficiently (more power and better fuel economy) on cooler intake air. Most automobiles nowadays (including some Jaguars) have hoses directing cool outside air into the air cleaners rather than the hot air of the engine compartment. The XJ-S is a notable exception; perhaps they felt that the hoses would make their engine compartment less attractive.

While opening up the intakes on the air filter housings as described above, a flange can be provided for an intake hose. One such installation used 3" exhaust pipe, but 2" pipe would work nearly as well (still four times the area of the stock inlet) and would probably be easier to install. Intake hoses in many sizes can be found at most parts stores, and some have built-in clamps. Remember to provide flexibility in the system, since the engine moves around on its mounts.

All of that was easy. The hard part is routing the intake hoses somewhere. The best place on the XJ-S appears to be the back end of the headlight compartments. By making a hole and providing a flange for the hose (2" or 3" pipe again), cool air can be routed from existing openings just inside the grille through the space behind the headlights and into the intakes. An opening must be made in an unseen panel within the headlight compartment to permit air flow. The modification will also require relocating the headlight relays. Be sure to clean up the intake path as well as possible when you're finished, and check your air filters for debris after driving a while.

On cars with ABS brakes, the air cleaner assemblies are essentially unchanged except that they are moved forward to avoid the brake system. It is uncertain what impact that would have on the above modifications, if any.

The good news here is that this mod will reduce the noise from the Version 1 mod. The intake hoses, as well as the sheer length of the intake path, will help dampen the growl.

Since the EFI temperature sensor will detect the cooler intake air temperature, once again no tuning mods are needed. The cooler air provides improvements at all operating conditions, and will improve fuel economy. There is even less tendency to overheat. The only effect on emissions is a reduction in nitrogen oxide emissions.

Peyton Gill reports, "I used a Dremel to cut the horn out of the air filter cover and then rounded the "not so perfect hole" with a grinding stone mounted in a drill press. I did not use the exhaust pipe Kirby mentioned, I went to Home Depot and got 2 inch gray PVC couplings. The ones I got came from the electrical dept. The couplings I used have threads on one side and open for a PVC pipe on the other. The conduit works good because you can get a "nut" that fits the threaded side. This nut I'm talking about is more like a threaded ring, it is very common in the electrical conduit world. The rounded hole you leave when you cut the horn out is close to the same size as the threaded side of the coupling and the nut inside the air filter holds the coupling tightly in place. The hole I cut was not perfect so I sealed any openings with black RTV (silicon). I did have to relocate the ballast resistor pack on the right hand side and managed not to have to relocate the relays on the left hand side.

"I used a hole saw ( maybe 2 1/4 inch) or whatever size slightly larger than the conduit coupling threads to cut holes in the panels behind the headlights. Since the hole was only slightly larger than the threads so they screw fairly tightly in plus a little more RTV to be sure they hold. I got a flexible hose from Pep Boys (they have hoses made for air filter intakes). I took my conduit coupling with me and found a hose that fit snugly over the pipe side. The hoses have clamps that snap into place and one of the hoses fit my conduit perfectly.

"All that was fairly easy; the good part comes when you attack the "hidden panel" described in Kirby's book. This panel is between the headlight opening in the front and where the hole was cut in the engine compartment. You can see it easily enough when the headlight assembly is removed. Seeing the panel is one thing, cutting a hole in it's another. There is not much room to work or cut. I got a 12 inch extension for my drill and put the hole saw on this and cut a couple 2 inch holes in the panel.

"The air temp sensor also has to be relocated. I drilled a hole in the air filter cover and placed the sensor in the direct air path. I did not have a nut with the same size threads as the sensor so I cut the fitting off the old horn and used it as a nut. I don't think the threads are anything special, I just did not want to make a trip to the hardware store to get one. I had to extend the wires a little."


AIR INTAKE MODS - VERSION 2A: Scott Horner, apparently before he became aware of this book, designed his own cold air intake system: "I based the cold air intake on my car on a friend's XJ-S race car's set-up. He has approx. 4" tubing going into the front of the inner guards, with the air filters mounted within that panel (in front of the wheels). But this is a bit rough for a road car as all you can hear is induction roar...

"For my car, I visited a panel beater friend to modify the guards, etc., as I wanted the mod to look as if it had come out of the factory...

"I'll try and describe this without pictures...We used 2 1/2" tubing which is bent to come out of the inner guard and face up to the original air intakes, which have been cut down (with new tubing) to just a stub and attached via flexible tubing.

"Within the guard, the tubing is bent down (to vertical) which then meets up with another piece of tubing (via flexible hose) that is welded into the valance on the outer corners. These tube holes are cut just under the bumper, so are fairly unobtrusive and with the angle of them look fairly sexy anyway...

"The great thing about them (apart from the cold air), is if you get someone to give the engine a rev while you've got your hand over one of the intakes, you can really feel the engine sucking! There'll be hell to pay the day I hit a sparrow!"


AIR INTAKE MODS -- BOLT-ON VERSION: John Goodman reports: "My (unmodified) '89 XJR-S 6.0 has different air cleaner boxes (part numbers SPE 1008 and SPE 1009 ) with large bore intakes. These take I believe the standard air filters and fit the standard intake manifolds. In other words it's all simple bolt-on improvements for any V12.

"Even more interesting is a special very neatly designed radiator top crossbeam (part number SPD1164) which has quite large smooth oval air intakes incorporated in it. The air intakes in this crossbeam line up perfectly with the air boxes.

"It all looks very neat, but they are JaguarSport parts and won't come cheap!!!"

Regarding that top crossbeam SPD1164, Goodman adds: "I think the part no. has been superseded with SPD 1428. This is the part no. listed for '93 XJR-S (American spec). The only difference AFAIK is that this part has mounting holes for the location of the Ign power amp, (necessitated because the manifolds were different) and rubber connecting hose. There is a good picture of this component in Jaguar World vol.2 no.2; anybody handy with a welder could modify a standard one."


AIRFLOW IMPROVEMENT -- HOME MACHINIST VERSION: If you have access to a machine shop, you can make a minor improvement in intake airflow with little fuss. Remove the air filter housings, and remove the butterfly housings. You will need a #30 Torx screwdriver to remove the butterfly housings. NOTE: The ports the hoses connect to may look similar, but they may be different! Be sure to carefully record which hose connects to which port prior to disconnecting the hoses.

Referring to Figure 25, machine a rounded edge on the inlet into the butterfly housing. This modification should not be attempted by hand, as smoothness and consistency is important. It is important that the radius blend smoothly into the inner surface of the passage, but it will form an edge with the air filter mounting surface.

Before starting, ensure that the machining will not interfere with the butterfly seat nor with any of the vacuum ports near the butterfly seat; by avoiding machining more than 3/8" into the throat, problems should be averted. Also, avoid cutting into the openings for the two bolts that hold the butterfly housing onto the manifold; while doing so wouldn't cause any leakage, it may cause a whistle as the air flows past the opening. Keeping the outer diameter at 2-13/16" should be acceptable.

Enlarge the opening in the air filter housing to 2-13/16". The gaskets typically are already this size, but final trimming can be done after assembly by using a razor knife before installing the filter.

This modification has not yet been tried, so performance improvements are unquantified. In theory, this mod can provide an airflow improvement (and hence a horsepower increase) of several percent. Considering the proximity of the blanked-off section of the air filter, the improvement may be even more significant. There should be no change in performance other than at wide-open throttle. Since the ECU senses and corrects for changes in manifold vacuum, no tuning changes are required.


AIRFLOW IMPROVEMENT -- MAIL ORDER VERSION: AJ6 Engineering offers revised butterfly housings on an exchange basis in which the entire bore is enlarged from 2-1/2" to 2-7/8" (32% airflow area increase) and larger butterflies are fitted. Note that their kit includes low-loss foam filters and air filter housings with enlarged openings, fulfilling some of the other suggestions above. The kit also includes an electronic gadget to revise the ECU response, since the larger butterflies will behave differently than the stock ones at a given throttle position. AJ6 Engineering claims a 20-25 horsepower increase with this kit.

Roger Bywater points out that "...our large bore versions have radiused entries as well so they will certainly flow a lot more than just radiused stock items."


INTAKE MANIFOLD MODIFICATION: AJ6 Engineering offers a service called the Plus Torque Conversion where they cut the intake manifolds open and modify the openings into each runner, then weld the manifolds back together.


CARBURETORS: Replacing the EFI with carburetors is not recommended. Carburetors have several disadvantages: First, a standard carburetor does not correct for changes in altitude, temperature, fuel density, or other variables that make an engine run at less than optimum performance. Corrections usually require an excessive amount of complication in the carburetor design.

Second, since the carburetor relies on a pressure reduction due to drawing air through a venturi, there is always a flow restriction due to the venturi. Using a carburetor with a larger venturi reduces the loss, but the airflow at idle is so small that they have difficulty drawing fuel consistently, and a rough idle and poor low speed performance is the result. The American solution was the progressive 4-barrel, which uses one set of venturis at low speed and an additional set at high speed.

Third, carburetors tend to have problems ensuring proper fuel/air mixtures at all cylinders. With most arrangements, the corner cylinders on a V-8 tend to run leaner, because the path for the fuel/air mixture to get to them is more convoluted, and the fuel gets left behind and drawn into a nearer cylinder. Such problems would be even more serious on a V12.

Problems occur when a cylinder runs lean; burned pistons usually result. In the old days, the engines would simply be adjusted rich enough to avoid any problems. When environmental regulations and fuel efficiency demands rendered this solution unacceptable, the incidence of burned pistons increased.

The time-honored solution to this problem was multiple carburetor systems. The intake path for each cylinder must be similar, and sometimes even a dedicated barrel for each cylinder was used (Webers, etc.).

Obviously, fixing all these problems simultaneously would involve a truly elaborate carburetor system, possibly requiring multiple, progressive barrels for each cylinder.

EFI systems generally come in two types: throttle-body and multi-port. Each type will correct the first and second problem of carburetors, because all EFI systems automatically correct for variables and no EFI requires a venturi. The throttle-body system, which consists of a single injector in the same housing as the butterfly valve, shares the same distribution problems as carburetors, but is much simpler and cheaper than multi-port.

Multi-port EFI, what the Jaguar comes with, is the ideal fuel supply system. Since the injectors serve each cylinder individually, there is little chance of a cylinder not getting its share.

The only disadvantage of EFI is the difficulty in modifying it -- which, of course, the EPA considers an advantage. However, contacting AJ6 Engineering will likely alleviate all such concerns; they will modify ECU's for very reasonable fees.


EFI SYSTEM REPLACEMENT: Electromotive Inc. makes a system called the TEC-I (TEC stands for Total Engine Control) that replaces the EFI computer and the entire ignition system as well. It's got more modern and comprehensive electronics than the Jaguar original, including an optional knock sensor. Also, it is programmable using a PC-compatible computer, allowing the owner to customize the operation to suit his needs. It's not cheap; but if your ECU has already died and you're looking at the cost of the Jag original to get running again, it starts to look cheaper. And if you're having ignition problems as well (this system renders the entire distributor superfluous, you might as well put in a blank-off plate), it starts to look downright reasonable.

If you are performing serious engine modifications, this system is just the ticket. Since it can be calibrated, you are not restricted to the original response curves on your non-stock engine.

AJ6 Engineering also makes replacement EFI systems and components, and will modify the stock ECU on an exchange basis. According to Jeffrey Gram, "if experimenting was wished they could do a single EPROM version , which has up to 8 MAPs. It would be possible to switch between the MAPs if the ECU is depowered between switches by means of a "dial". AJ6 do such programmes to experimenters which then have 8 maps to choose from trying to find the best setting."

Scott Horner found an aftermarket system in New Zealand:

Link Electro
243D Annex Rd
Christchurch, ph +64 3 348-8854

"From them you can buy the ECU, Ignitors, Injector Ballast and Tuning Module (a handheld LCD interface) & the Crank Angle Sensor... You still need to supply a lambda sensor (aftermarket Bosch will do), and the coils...On my car, we used VN Holden Commodore (Australian GM car) coils, 2 coil packs. The Holden uses the Buick 3.8 liter engine, so it's presumable the coils would be available in the US in some form. I have heard these coils only last about 20,000km, but we'll see.

"Any 3 or 6 pack coil combination would do, I have heard Audi have a very tidy solution..."

Robert Joseph Dingli reports: "Here in Australia there are many, many aftermarket injection ECU's available. I've tested many of them in cars and on the bench and have found most of them to be garbage. Common faults are production quality, lack of temperature compensation and other instabilities, and general difficulties with setting up a system from scratch. Basically you get what you pay for. A $2000 Motec or Autronics unit is highly recommended over sub-$1000 Injec, Linx EMX, Microtec or Haltec. The expensive units also control ignition. The Motec, Autronics and Haltec require a portable PC (to be added to the cost if not available) to make proper adjustments.

"As far as "high performance" chips are concerned, I'm rather skeptical as to their worth. The problem (??) with modern day EFI systems is that they are very close to optimum, being a fine balance of performance, economy and exhaust emissions. Some of the chips available advertise that the systems will still comply with pollution levels. The specs reveal that power increases are only in the order of a few percent for most cars."

"Systems where chip changes may actually make a noticeable difference are:

  • Systems with electronically-governed rev limits.
  • Racing use where pollution isn't a concern.
  • Turbo cars where the electronically-governed boost limits are raised.
  • Electronic ignition systems without knock sensors where the timing maps can be advanced to take advantage of better quality fuels.
  • Electronically-controlled auto gearboxes where upshift points are raised.
  • Older poorly-calibrated systems, e.g. SIII XJ6 Jags set up for cold climates but being run in warmer climates."

According to Ken Wallace, "If you live in a regulated area the smog police will not approve of any of this even if it improves your emissions. You can have your installation certified, but this is very expensive like more than $10,000 from what I hear."

Robert Gee adds a warning about aftermarket EFI systems: "I worked for a car manufacturer who made their own injection systems (Rover MEMS). One or two people had fitted MEMS to their own cars, mostly for competition use. One guy had a modified peugeot 205 GTi with one in. He had spent a fortune on the thing.

"Now, given that this guy had access to rolling roads, vehicle calibration experts and the like, the driveability was never perfect (an important Jaguar feature - low throttle response crucial on an auto). Also he went through about 3-4 tankfuls of petrol doing the calibration (not too bad at US petrol prices I suppose - more than the cost of an ECU at UK prices). And this was still with the very fine tuning carried out automatically (This was done with some special adaptive software we were working on which used the lambda sensor feedback signals to adjust the fuelling maps).

"To cap it all, when he came to sell the car it was quite hard since it was non-standard and hence not supported by any garages.

"Put it this way, I could have converted my XJ-S 3.6 to full sequential injection with fully programmed ignition for next to no cost - but preferred to keep it standard and I probably would not have got it any better than the Lucas P digital.

"If there one thing I learn't at Rover it's that the low throttle drivability is what separates a good car from a bad one - and it also takes about 95% of the engineering effort. Anybody can make a car which goes well at full throttle - and I suspect it's the latter market at which the after market fuel injection systems are aimed."


IGNITION SYSTEM REPLACEMENT: Welsh Enterprises offers at least two different models of Lumenition electronic ignition system for the XJ-S, each featuring an optical pickup and compatible with their engine rev limiters. Note that these systems do not offer electronically controlled advance; they merely replace the existing electronic ignition with another, continuing to use the seizure-prone centrifugal advance and the leak-prone vacuum advance that come with the car. These kits are terrific upgrades for points and condensers, but benefits over the stock XJ-S electronic ignition system are probably minimal.


NITROUS: Martin R. Fooks has a nitrous system in his XJ-S: "The Nitrous system was supplied by Trevor Langfield and is a customized "High Power Nitrous" system. Because of the size of the hit to the engine (150BHP) they installed a progressive controller, which fits neatly in my car where the trip computer used to be. This enables me to control the way the extra power is delivered to the engine, such as starting power, ending power, time delay and time from starting level to ending level.

"I am very happy with the Nitrous installation, which really seems to be very smooth on the V12. Trevor's people have the computer equipment to work out power and 0-60MPH times and that is where the figures came from. As a side note, it ran 5.3 seconds 0-60 with a standard TH400 and only 75BHP jets in the NOS system instead of the 150 jets.

"Nitrous got a very bad press in the past, caused mostly by people adding too much power to their engines, or by not richening the fuel mixture when the NOS was injected. The casualties in my case have been the torque converter and the rear IRS mounts, which were all bar 1 ripped off." (See the notes regarding rear suspension mount weakness.)

"The engine is stock except for the exhaust and intake, and I have had no problems at all with it (unlike the torque converter).

"The builders of my system told me that they do not recommend putting any more than 50-75BHP into your engine without using a progressive system as avoidable damage to the engine could result.

"Their main concerns with adding 150BHP were not with the engine as they believe it to be a very strong and reliable item. The standard transmission and torque converter were their main objection to the increase which as it turns out was well founded."

"The amount of power provided in total by the NOS system can be changed by installing new jets at an English price of 7 pounds per pair, so if 150BHP proves too much to handle, you can always fit a smaller pair taking very little time and expense. 200BHP is the maximum obtainable by my kit. My suppliers stated to me that as a rule they do not fit systems with more than 50% of the original engine power (which is why I'm only using 150 Jets instead of 200). Obviously this is not the case if you bullet-proof your engine."


ADDING EMISSIONS CONTROLS: Huh? Well, Germany now has a taxation system that penalizes cars without emissions controls so severely that many German XJ-S owners are seeking to retrofit. For many years the German version had no catalytic convertors or oxygen sensors even though they were provided in the US.

According to Jeffrey Gram, the following outfits (all in Germany) will install emission controls in cars that were not originally equipped with them:

  • Ernst Apparatebau, Hagen, Tel +49 (0)2331 3600-0
  • GAT-Abgastechnik, Gladbeck, Tel +49 (0)2043 24021
  • Gutmann, Breisach, Tel +49 (0)7667 1091
  • G+M Kat, Gladbeck, +49 (0)2043 42410
  • HJS Abfgastechnik, Menden, +49 (0)2373 9870
  • Oberland, Garmisch-Partenkirchen +49 (0)8821 1036
  • Oettinger, Friedrichsdorf +49 (0)6172 7053-55
  • Walker, Viernheim, +49 (0)6204 738-0
  • Waschkuttis, Wiesenthau, +49 (0)9191 96495
  • Wurm, Stuttgart, +49 (0)711 420071.


CAMSHAFT REPLACEMENT: Most performance enthusiasts will agree that replacing the camshaft (or camshafts; the Jag V12 has two, some cars have four) is the most effective way to change the performance of an engine. The entire personality of a car can be radically altered by merely changing the camshafts.

Chad Bolles reports that Isky makes high performance camshafts for the Jaguar V12.

Rod Beere Racing Services also offers hot cams, as well as tappet shims in extreme thicknesses that may be necessary for such installations.

Note that any camshaft alterations should be accompanied by an EFI system modification; there will be increased airflow at wide open throttle, but since the feedback circuit is disabled under those conditions the fuel supply will remain at the original fixed map with no trim. The engine will therefore run lean at full throttle, a situation that begs for burned pistons and valves.

There has been some confusion regarding the difference between pre-H.E. and H.E. cams, since published valve timing data on the two engines seemed to differ. However, Bywater points out that they actually use the same part number camshafts. "When the V12 was launched the valve timing was quoted in Walter Hassan's SAE paper 720163 as being 17,59/59,17. When the H.E. version was launched in 1981 the supplement to the manual quoted cam timing as 13,55/55,13. In fact, the same cam profile had been in use since the early 1970s and continued through into the 1990s under part numbers C42176/7, therefore all EFI V12s were produced with these cams. Now it is not widely known that for some time the V12 was mildly plagued with excessive tappet noise and in the course of dealing with the problem the quietening ramps on the cams were altered at least once around 1972-3. I was personally involved in an investigation into the causes of a spate of tappet noise around 1978 and am not aware of any cam change ever being made to the V12 in production for any other reason than to reduce valve gear noise.

"Measurement of true cam timing is not a straightforward matter and it has been accepted practice to measure from the point where the quietening ramp ceases and the lift curve proper commences. For those who are not familiar with the term, a quietening ramp is an area at the flank of the cam where the rate of lift is small, at around 0.0005" per cam degree, and to be fully effective must extend rather higher than the widest clearance likely to be encountered. The idea is that any reasonable running clearance found in use it will always be taken up at a predetermined velocity which should not give rise to noise. If the ramp geometry is changed, as we know happened on the V12, then the timing as measured at the top of the ramp could also vary and this accounts for the small difference of timing quoted at different times in its life."

Mike Cogswell elaborates: "Duration itself can be misleading, since almost nobody measures duration from the instant the valve leaves the seat until the instant it returns. Instead, duration is commonly measured at some point where there is noticable flow, albeit typically a very low fit. This is important, since different cam grinders measure duration at different lift, hence one man's long duration might be less than another's short duration. Caveat emptor, as always."

Bywater again: "It is perhaps of interest to note that over the years Jaguar used virtually identical valve timing on all their mainstream engines. Consider the following: 

SS 2.5 & 3.5 litre 16,56/56,16
XK 3.4,3.8,4.2 15,57/57,15
V12 17,59/59,17 (13,55/55,13)

Clearly having found something that worked there was great reluctance to change from it, although the smaller displacement engines often had softer timing to beef-up the low speed torque as the following examples show:

 SS 1.5 litre 10,50/50,10
XK 240 saloon 10,50/50,10 (5/16" lift)

Note also that hot cams might not have the expected effect. AJ6 Engineering (page *) once offered a milder cam than stock, and the result was higher mid-range torque and a nearly one full second quicker 0-to-60 time. Bywater explains: "...we introduced some short duration cams for the V12 back in about 1984. They certainly boosted mid-range performance, especially appreciated with the 3 speed BW and GM transmissions, but suffered a marketing problem because they gave slightly less peak power so we eventually discontinued them. We sense that attitudes are changing and many drivers are realising that a performance gain around 40-70 m.p.h. is far more valuable than adding to a rarely seen top speed so we may well offer a modernised version of such a camshaft again in the near future."


VALVE ADJUSTMENT (!): Roger Bywater of AJ6 Engineering (and formerly with Jaguar) says, "we knew back in the 1970s that running with the exhaust valve clearances set at 0.016" gives a slight but measurable gain in mid-range torque and reduced fuel consumption. Noise is not excessive at this setting because the actual running clearance closes up with the higher temperature of the exhaust valves compared to the inlets which must be set as normal.

"Anyone wishing to measure a V12 cam, as I have done in the past, will find that the timing quoted for the H.E. occurs at 0.010" lift whilst the timing at the point at which 0.012" clearance is taken up is an almost unbelievable 36,78/78,36. The difference in overlap between 0.013" and 0.016" clearances is about 12 degrees so the need to avoid tight clearances will be obvious and although the extra lift may be insignificant it can profoundly effect the HC emissions generated, quite apart from the modest effect on torque. In my view the best compromise regarding noise and performance is to aim for 0.013" for inlets and 0.015-0.016" for exhaust which, because of greater expansion of the exhaust valve stem, results in a similar true running clearance for both.

"...If mid-range torque could be improved by just opening up the clearances, why did the factory not do it in production? Well maybe they did (I am not prepared to be more positive than that) -- but you will not find any manual telling you so because the reason for doing it would have been to reduce HC emissions at a critical time, at the risk of introducing more tappet noise problems."


CAM COVERS: Strictly an appearance mod here. Some of us think those Jaguar decals on the cam covers are not in keeping with the general class of this automobile. One suggestion is to remove the decals and attach some brass insignias with screws -- making sure not to cause a leak. Might even find just the right key fob to use.

A suggestion from Steve Averill: "If you want to find something a little better looking that the Jaguar decal on the valve cover, why not either get it photo etched or alodyned? That'd be more apropos than sticking on something that'd probably wind up looking "tacked on" & you could pick any pattern that appeals to you."

If you have them off anyway and have a milling machine at your disposal, you might consider skimming the top of the ribs, leaving the black paint in the grooves between the ribs. It'll really make it look snazzy.


SO YOU'RE DOING A VALVE JOB: To many of us, doing a valve job means pulling the heads off the car, taking them to a machine shop, picking them up later, and bolting them back onto the engine. However, the valves are a prime place for easy improvements when the heads are off. I will mention a few ideas.

The valve guides are sleeves that are press-fit into the heads, and protrude a little bit into the inlet and exhaust ports. Jim Isbell suggests that, before you install the valves, you take a hand grinder and grind off the protruding parts of the guides flush with the surface of the port.

The valves and seats on the Jaguar V12 are supposed to be machined with 44 angles. Most machine shops will suggest that the seats be "triple cut", in which additional cuts are made at angles greater and less than 44 in order to control the width and location of the contact area. Typical angles for these cuts would be 32 and 60 and the contact area width should be about 1 mm wide. Chad Bolles suggests that a similar triple cut on the valves themselves would be beneficial.

John Milne suggests that, after the valve contact surfaces have been machined, some machinist's bluing be applied and the valve trial fit to determine the location of the actual contact on the valve surface. Then, carefully avoiding the contact area, the inner edge of the machined surface should be blended to form a smooth continuous surface with the "tulip" shape of the valve. This helps the flow through the valve, since it makes a smooth passage instead of that corner. It also slightly enlarges the opening, since the smallest flow area when the valve is open is between this inner corner and the seat. "It's kinda like getting a little extra valve lift for free." Blending the inner edges of the machined surfaces of the seat may have similar benefits.

Of course, every high-performance enthusiast will suggest that you do a little "porting" while you're in there. This means carefully enlarging the intake and exhaust passages. In the case of the intake passages on the Jaguar V12, however, this may produce undesirable results. These intake passages are designed to provide a certain amount of resonance-induced flow enhancement, and this requires that the flow rates be fairly high. Enlarging the ports makes for slower flow, which means better flow at high RPM but less boost at low RPM. In other words, enlarging the intake ports may increase high-speed horsepower at the expense of low-speed torque.

The next automatic suggestion is "polishing". Quite literally, the passages may be polished using successively finer abrasive compounds until a mirror finish is achieved. This supposedly will reduce surface friction of the flow.

Before you reinstall the head, Jim Isbell suggests you "cc" it. This means that you measure the volume of each combustion chamber, and grind a little metal away here and there to make sure they are all the same. This makes for a smooth running engine. To measure the volume, you can set the heads upside down on a level surface and fill each chamber with a carefully measured amount of light oil.

Cc'ing must be done after the valves are installed for the final assembly. Clearly, if valves are ground or relocated after cc'ing, they will sit at a different level than they did before and this would significantly change the volume of the chamber. And this is the reason that cc'ing is recommended whenever the valves have been redone.


TEFLON VALVE STEM SEALS: Michel Carpentier says, "Teflon valve seals are the way to go. They last forever, plus you can fit them on the exhaust guides as well. Less blow-by, reduced crankcase pressure, no more oil leaks, and cleaner oil to boot.

"Our engines came from the factory with 24 valves but only 12 valve stem seals. Jaguar probably realized that the British rubber intake rendition would soon fry on the exhaust side.

"Why would any (effective) exhaust valve seal affect crankcase pressure? When the exhaust valves open, gases gush out of the combustion chamber with tremendous pressure. Even though the valve itself acts as some sort of an umbrella, there is still a significant pressure differential between both sides of the exhaust guide. Any clearance between valve and guide will let exhaust gases into the crankcase area. Jaguar old trade secret to keep engines leaking oil!"

Of course, the decision to switch to Teflon seals, and to install 24 instead of 12, may not be quite that simple. This author had some discussion on the phone with the rep at Silver Seals about the differences between Teflon valve stem seals and normal nitrile seals. There is apparently a functional difference: the nitrile seals always leak, and the leakage allows a small amount of oil down the valve guides to lubricate them. The Teflon items will not leak, so the valve guides run dry. According to the rep, this is a problem on some cars and not a problem on others. I'm betting it's not a problem on the Jag, since it uses good materials on both the valves and guides. Also, the action of the cam on the tappet applies no sideways load to the valve stem, as there is in any valve train with rockers. Still, recognize that you may be trading oil burning for faster valve guide wear.

Does anyone make Teflon valve stem seals for the XJ-S? Carpentier: "Perfect Circle does, although unwittingly since I doubt they ever considered Jaguar engines as a possible application for their product. Our valve guide diameter is 0.502 to 0.501 in, that's essentially 1/2" or sixteen (16) thirty secondths of an inch. Valve stem diameter is 0.3092 to 0.3093 in, and that's within 3 thou of 5/16" or ten (10) thirty secondths of an inch. These dimensions are a bit unusual but luckily match those of the first generation Chrysler Hemi engines. Call Perfect Circle (they are a DANA subsidiary) and they will tell you: "Yes, we manufacture such Teflon seals, part number D1610, available at any NAPA auto store".

"There is a catch: if you go to NAPA and ask for this reference, they look it up in their computer and tell you it does not exist (never mind telling them what kind of car it is for...). The last time I was in the US, I finally found a place in Los Angeles that would order them for me. The seals actually came from a company called Silver Seals Products/Whip-L Products, Trenton, Michigan, 1-800-521-2936; their reference is DT1610 but the seals carry the Perfect Circle logo." This author called that 800 number, and had a set of 24 Teflon valve seals on order in a matter of minutes and in hand in a week at a cost of less than a dollar each. For those outside the US where an 800 number won't work, their regular phone number is +1 (734) 479-2255.

The OEM nitrile seals on the inlet valves are pressed over the valve guide until they snap into a groove around the guide just above the surface of the head. The exhaust valve guides have no such groove. This turns out to be a non-issue with the Teflon seals; these seals press fit onto the OD of the valve guide, and therefore do not use the snap groove. While the OEM seals are a simplistic piece of rubber, the Teflon seals include a metal ring on each diameter to provide a secure fit on the guide and on the valve stem itself.

Besides the diameters of the guide and stem, there are a few other things to consider whenever fitting a non-OEM valve stem seal to an engine. In this case, the OD of the base fits within the inner spring just fine. The Teflon seal is a little taller than the original seal, which makes for a really close fit; the valve lift is 3/8", and that's pretty much right where the valve keepers sit down on the top of the seal's lip. If you intend to run an aftermarket cam with more lift, you'll need to either use the shorter OEM seals or make some other changes, like shortening the keepers or the guides.

Installation is not as simple as with the nitrile seals. Carpentier: "You cannot push them in as you would plain vanilla seals. With your package you should get a clear and rather flimsy plastic tube closed at one end. Slipped over the valve stem, it protects the seal as you first push it over the stem end (push with your thumb with a slight rocking motion to get started) then slide it over the cotter groove. Trim this sleeve so it still covers the groove but does not stay trapped under the seal when fully home." Note: on the valve stems, just below the groove, the part number is engraved; might as well leave the sleeve long enough to cover the number too, just to make sure you don't damage the seals going over it.

"When you start, remember (24 times!) to put the spring seat first as it does not fit over the seal. This is especially important as a teflon seal cannot be removed without being destroyed: 1) it is hard to pull it from the guide without distorting it and 2) as it goes back over the cotter groove, the sealing surface between stem and seal will be damaged." Clearly, you need to have the valve in place before installing the seal and you can't remove it afterward, so make sure you are actually at the final assembly stage (all lapping completed, etc.) before installing seals.

"Now you need to push the seal over the guide. Use a tube (e.g. a deep socket) with ID greater than the narrow upper section and a rubber mallet. A first blow will get you over the guide chamfer, then tap it all the way in. Don't hammer too hard though or the upper portion will shear off as you hit bottom." Note: this author has done this job, and it's a lot easier than it sounds.


EXHAUST FLOW LIMITATIONS: Roger Bywater explains an inherent shortcoming of the H.E. engine: "The small and pocketed exhaust valves are the real problem and is why all the high performance racing V12s have been based on the old "flat head" design. Putting big exhaust valves in doesn't help because the chamber walls are so close and leave no room for flow around the edge of the bigger valve head. In fact if the HE were not of abnormally high compression ratio, always a useful trick to get a bit more top end power out of an engine that doesn't breathe, it would struggle to produce the barely adequate power that it gives as standard. You can build an HE to 6 or more litres (we used to do a 6.3 using 98 mm bore and standard crank) but all you really get is more torque low down but not much more power."


TOTAL SEAL PISTON RINGS: Total Seal, Inc., makes a type of piston ring set in which the second ring is a two-piece ring. Effectively, it puts two rings in a single groove with the gaps staggered -- so gases cannot pass either gap. As a normal ring wears, the gap gets larger and the leakage increases accordingly. With Total Seal rings, the gap is covered by the second ring, so it won't leak no matter how much it wears.

Apparently, these rings are highly recommended by just about everyone who has ever used them -- including high percentages of competitors in several types of racing. Testing shows considerably less leakage even compared to new conventional rings. Not only will performance improve, but the reduced blowby should result in less contamination of the oil.


MAJOR ENGINE MODS: The Jaguar V12 H.E. engine is a technically advanced powerplant. It is recommended that major engine modifications such as enlarged bore or stroke be attempted only by knowledgeable and experienced performance enthusiasts.

Changes to the engine displacement would require modifying or replacing the EFI. The stock EFI is hard to modify; it has a fixed (trimmable, but fixed) program for intake manifold vacuum and RPM versus fuel. If the displacement is altered, this relationship changes. One solution is an aftermarket EFI system that is completely programmable and re-programmable (see above). Most choose to ditch the EFI at this point for carburetors, beginning the problems outlined above. And, the emissions inspector will not smile at them.

With an advanced combustion chamber design, the H.E. engine has 11.5:1 compression and runs on 89 octane unleaded (Unleaded Plus). Almost any mods to the engine internals would endanger the integrity of this design, and the owner would risk having to lower the compression radically and/or face buying octane boosters ($$$). His fuel economy would get worse, and if he didn't do his job right, his performance would get worse, too.

One modification worth considering is to install the European pistons, raising the compression to 12.5:1. This will obviously require higher octane fuel. The European XJ-S H.E. reportedly produces about 30 more HP than the US version. According to Chad Bolles, the only difference in the engine is the piston, which has a slightly different pin bore location.


ENGINE ENLARGEMENT: There are two ways to get more power out of an engine: tune it to obtain more horsepower per liter, and enlargement to provide more liters. Of the two, enlargement has some definite advantages: if the horsepower per liter is not changed significantly, the durability may not suffer; the "manners" of the engine, important in street applications, may remain as stock or even improve; fuel economy may remain nearly unchanged; and the use of higher octane fuel or octane boosters may not be necessary.

Indications are that the Jaguar V12 has a lot of room for expansion. A 5.3 liter (90mm bore x 70mm stroke) for two decades, it was enlarged by Jaguar in the early 90's to 6.0 liter (90mm bore x 78mm stroke). AJ6 Engineering once offered engines bored and stroked in sizes up to 7.1 liter and 405 BHP.

The room for expanding the bore seems to be limited to around 98mm. The stroke, however, can go a long way, and since it is so over-square to begin with, getting too under-square is not a problem. Bill Whitehas prepared a Jaguar V12 for use in a 3/4-scale replica of a Spitfire fighter plane. He expanded the bore and stroke to 96mm x 95mm -- almost an inch of additional stroke, for a displacement of 8.4 liters! This was done without significant modification to the block, and the stock H.E. heads were used. At 3000 rpm, this engine produces 500 ft-lb of torque in naturally-aspirated form, and 820 ft-lb when supercharged as it is in the aircraft.

According to White, that isn't the limit by any means. He reports that an outfit in the UK called Forward Engineering prepares Jaguar V12's for use in offshore racing boats. They install a spacer 3/4" thick between the block and the head, and use liners the same amount longer than stock to provide longer cylinders. Longer studs hold the head on, and since 3/4" is the length of a link of the timing chain the addition of four links allows an otherwise completely stock timing chain scheme to be used. White says the engines so assembled are 9.3 liter, and are very successful at offshore boat racing.


ALUMINUM CYLINDER LINERS: Apparently aluminum cylinder liners are available for the Jaguar V12 from GKN Squeezeform in the UK. Such use would require the replacement of the pistons as well, since the stock pistons are designed (via the use of a special alloy, as well as steel expansion-control inserts) for the expansion rate of the stock iron liners.

Jeffrey Gram contacted Rob Beere Engineering, which reported: "In the 1980's the Jaguar Group C endurance racers used aluminum liners. It is actually not pure aluminum but a compound called nickasil or similar. This material is very light and is treated (don't know with what). In 1986-1988 the alu-liners were not used anymore for endurance races since the wear was too big and inferior to cast iron. Apparently this nickasil material has a tendency to pick up material by which the pick-up process is accelerated and the material wears out quickly. The nickasil was only used on race engines with frequent liner renewal."

The V8 engine used in the XK8 and XJ8 comes with nickasil cylinders.


SEEEEERIOUS ENGINE MODS: There have been experiments to adapt the 4-valve DOHC head from the Jaguar AJ6 2.9 engine to the V12. The cylinder spacing and bolt patterns are the same -- according to Roger Bywater, not because the AJ6 was derived from the V12, but rather because there was a plan to put the V12's H.E. head on some versions of the AJ6. If the DOHC heads are just bolted onto the V12, on one side the intake ports will be on the outside and the exhaust ports will be toward the center -- not good. So, one of the AJ6 heads must be turned around backwards -- and some complicated fabrication work is required to get such things as the cam sprockets rearranged. Bill White appears to be reviving this idea, and since he generally does good work he's likely to produce an attractive product.

Steve Averill reports that the Autumn 1988 issue of Jaguar Quarterly has an article on "a 60 valve DOHC V12 that was under development by Warrior Automotive Research. They expected to achieve 100 bhp/litre in low tune with a 5.8l engine. The head had 3 inlet & 2 exhaust valves per cylinder." No word on what's happened since, but Warrior's phone number was given as 061-928 3284 in Cheshire if anyone wants to try a call.

There has been at least one experiment in Australia in turbocharging the XJ-S, but the results were apparently not good. Officially the problems were blamed on the inability to assemble a drivetrain that would handle the 1000+ hp for more than a few seconds.

Chris Sleeman (also in Australia) reports on a 1998 endeavor: "When I picked up my Daimler Double Six yesterday from my local Jaguar specialist, he showed me an XJ-S he is working on. It is a '76 model, with a 6.8 Litre Twin Turbo V12. The motor was built by them, and the customer is apparently going to enter it into the Targa Tasmania in April. The motor is being dynoed next week, but they say it puts out around 700hp. It is running 0.8 bar of boost at the moment, and will be using Motec injection."


AIR INJECTION SYSTEM: Removing the system that injects air into the exhaust system is not recommended, as this would obviously be a modification of an emission control system and would be illegal in many areas. Too bad, too; this system accounts for a great deal of plumbing under the hood as well as a belt-driven load on the engine. And it only functions when the engine is cold -- the pumped air is wastegated after the engine warms up to prevent interference with the operation of the oxygen sensors. And removal would be so easy: making a bracket and mounting a standard Ford adjustable idler pulley in place of the pump, being sure to mount the idler low enough that the belt clears the fan belt idler (a different A/C belt would be needed); and plugging the holes in the intake manifold.

Of course, some people insist on genuine Jaguar parts. Peter Smith: "I removed my air injection system and used a jockey pulley to carry the V-belt. These pulleys were standard equipment on English cars which did not have to meet emission requirements in the early days and some of these jockey pulleys may still be available in wrecking yards in England." Smith went on to provide enough data from the parts books to create a parts list:

C36014 Pulley
C37886 Hub
C37889 Bearing
C23128 Setscrew
C37879 Bearing Holder
C37875 Bracket an assortment of generic bolts, nuts, etc., that you can get locally. Here's a guess: judging from the drawings, it's entirely probable that the pulley used as an idler is the very same part that's used on the air pump -- so you may be able to skip buying C36014.

If you happen to be using your car for competition or other applications where the air injection system is unnecessary, John G. Napoli devised a method for replacing the air pump with a GM alternator, allowing the elimination of the Lucas alternator and eliminating one of the four belts on the XJ-S. Since a GM alternator hardly costs any more than an idler pulley (!), it's a low-cost mod. And if your Lucas alternator is acting up, it will assuredly save you $$$. His description follows:

"First, remove the original alternator. The original Lucas unit then gets tossed." Note: you might wanna just store it -- the guy you eventually sell the car to might wanna return the car to original condition for some reason.

"I elected to use what automotive electrical rebuilders refer to as a ëlarge case GM 70 amp alternator with 12 and 6 mounting'. Obviously, this should not have a serpentine belt pulley; a standard GM V-belt pulley is needed. This unit is available up to 100 amps. (You could probably use some other alternator.) The cost of this rebuilt alternator at my rebuilder was $30.00 (US) with no core charge. And a replacement, should one ever be needed, is as close as any auto parts store." Note: be sure the GM alternator selected has an internal regulator.

"The air pump and its vacuum valve get tossed, too. Now, take the steel bracket that supported the base of the air pump. Take a piece of flat steel or aluminum sheet (I used aluminum) about 5/32" thick (you want stiffness but too thick and you'll need longer bolts, and who needs the extra weight) to use as an adapter plate. The piece should be as wide as the stock air pump bracket and twice as long (high).

"The idea is to use the stock air pump bracket to secure the alternator right side up in its new position, shifted ëdown' one set of bolt holes to mount the alternator a little lower in the engine bay for thermostat housing and radiator hose clearance.

"The stock air pump bracket has four bolts that secure it to the side of the block (via an aluminum block bracket casting itself attached to the block with two large bolts). The plate you'll make will be drilled for six bolt holes, and is mounted sandwiched between the alternator (via the steel air pump bracket) and the block bracket. The upper two holes are used to bolt the adapter plate to the upper two holes of the block bracket. The middle two holes are used to bolt the air pump bracket and adapter plate to the lower two holes of the block bracket. The lower two bolt holes of the adapter plate bolt the air pump bracket to the adapter plate. Got it? Use the air pump bracket as a pattern for the bolt holes on the adapter plate.

"Bore a large hole in the top center of the adapter plate to clear the forward bolt that attaches the block bracket to the block -- once again, use the air pump bracket as a guide. If you made the adapter plate too wide, notch it to clear the alternator (nee air pump) adjusting bolt. Polish the adapter plate to a high gloss to add several miles per hour. Make a spacer that fits between the lower alternator mounting lug and the back of the air pump bracket, so that the lower pivot bolt can snug everything up.

"Mount the adapter plate and air pump bracket on the engine. Take the brass terminal lugs off the Lucas alternator and transfer them to the GM unit. Loosely bolt in the GM alternator, using the stock air pump adjuster. You will see that the alternator will line up perfectly with the plane of the A/C-air pump belt.

"Now the aggravating part: remove all the belts! Toss the original alternator and air pump belts. Buy a belt 2 inches shorter than the stock A/C-air pump belt. Install this new belt and reinstall the power steering and fan belts. Tighten the alternator.

"If you have a GM alternator plug, use it for this next step. Otherwise, standard solderless connectors (which I always solder and insulate with heat-shrink tubing) can be used providing they are well insulated to prevent accidental grounding to the case of the alternator. The original Jag idiot light sensor wire goes to terminal 2 of the GM alternator. Hot wire terminal 1 of the alternator to the ëbattery' terminal of the alternator. (This does not seem to be a parasitic current draw problem -- if you prefer, wire a relay thru the ignition circuit so this connection is only ëhot' when the ignition is on.) Plug in the two main Jag alternator leads to the ëbattery' terminal via the brass lugs you transferred from the Lucas unit.

"Plug the air pump hoses in the air cleaner and the exhaust manifolds as needed. Replace the battery cable, radiator hose and air cleaner. Start it up, watch that baby charge, stand up straight, get the kinks out of your back, and feel proud of yourself.

"Notice that you have picked up 300 or 400 rpm of idle speed. Readjust the idle. Note the number (length) of the new alternator belt and scratch it into the fender of the car. Alternately, write it down and keep it in the glove box or trunk."

Michael Aiken points out that if you started with the Bosch 115-amp alternator instead of the Lucas, you will be reducing your capacity with this mod -- and the Bosch seems to be far more reliable than the Lucas, so you may not be improving reliability significantly either. The loss of generating capacity is not likely to cause problems unless you have something added to your car that uses a lot of juice. The removal of the air pump and the reduction in the number of drive belts may still justify the mod.

Napoli offers a suggestion for plugging the air injection holes in the intake manifold: "Buy 1/4" nail anchors from Home Depot. They are aluminum and look like a large pop rivet. Get them 1" long. Cut them back to 5/8" long with a band saw or hack saw (only cut the body, don't worry about the ënail'). Put the two o-rings from each air tube on the shank of the anchor. Smear on some engine-grade silicone. Insert into the manifold and hammer down the nail all the way. Omit the ëplates' and retighten the intake manifold to the engine. Looks great and works great."


EXHAUST SYSTEM: The stock XJ-S exhaust system is quite restrictive, especially the area right around the oxygen sensors. However, there appears to be little available in the way of aftermarket exhaust systems, other than stainless steel systems similar to the stock system. There is probably not enough room under the hood for a header. To improve exhaust flow, there appear to be few options other than custom-making your own system -- a real challenge, considering the spaces to work with.

One option is to add a crossover pipe: a connection between the dual exhaust systems to permit each exhaust pulse to escape through both systems. This method has been used with dramatic results on V8's, and some cars come with them now. To avoid screwing up the EFI, the crossover should be after the oxygen sensors; but the closer it is to the engine, the more effective it is.

It should be noted that the improvements would not be expected to be as great as on a V8; six cylinders per bank results in smoother exhaust flow than four, so the benefits of a crossover are smaller. However, the Series III E-type WAS fitted with a crossover pipe, so it must have had some benefit.

A crossover has two disadvantages: First, it can make the exhaust system, already a pain to work on, even more difficult to assemble and disassemble. Second, being able to check the exhaust from each bank helps with engine fault diagnosis. Both of these problems can be minimized by including a flat face flange in the crossover, and making a flat blank-off plate that can be bolted into the connection when doing engine analysis.

Another option is to replace some of the mufflers with straight pieces of exhaust pipe. Obviously the car would leave the realm of the totally silent, but perhaps a little exhaust sound is acceptable.

It is not recommended that the catalytic converters be removed to improve performance. There would obviously result a discussion with the emissions inspector. Furthermore, tests have indicated that the convertors don't offer much restriction, so benefits would be small. The only good reason that ever really existed for removing convertors was to allow the use of cheaper leaded gasoline, but that benefit has all but disappeared.


OIL COOLING: The standard oil cooler on an XJ-S is a bypass unit, meaning it only cools the oil that doesn't go through the engine. However, as pointed out by Bob Tilley, the XJ-S sold in Germany is fitted with a full flow oil cooler system, and the parts are available through Jaguar. We can make assumptions about why the German cars would be different than other cars, possibly involving those Autobahns.

John Goodman adds, "it's worth pointing out to others who own later model V12's who may not be aware that all V12 HE engines after engine no. 8S44317 had full flow oil cooling, can't remember the year this was introduced."

Both oil systems, as well as almost any other in automotive use, work like this: Oil is drawn from a pickup in the sump into the oil pump. From there it goes to a pressure relief valve, which relieves enough of the oil flow to prevent excessive pressure in the system. The relieved oil goes back to the sump, while the remaining pressurized oil goes through the filter (which has its own bypass in the event of clogging) and into the galleys that feed the bearings, cam followers, and other parts of the engine requiring lubrication.

In the basic bypass oil cooling system in the Jaguar V12, only the oil that is relieved by the pressure relief valve is piped to the oil cooler in front of the radiator, and from there back to the intake of the oil pump. In the full flow system used in German cars, the oil destined to go to the galleys is piped to the oil cooler at the front of the radiator and then back to the relief assembly to continue through the filter and into the galleys.

Physically, the distinction is like this: in the bypass system, oil feeds out of the outlet elbow at the bottom front of the relief assembly to the right side of the cooler. From the left side of the cooler, it goes to a fitting on the bottom front of the sandwich plate on the crankcase. It doesn't simply return into the sump here, but instead goes directly into the inlet elbow on the bottom of the oil pump. Hence, the oil pump actually draws suction from two places: the sump pickup and the return from the cooler.

In the full flow system, oil to the cooler starts at the relief assembly as well, but at a different place on it -- higher up on the front, from one of a pair of fittings that don't exist on the bypass relief assembly. This feed is piped to the right side of the cooler. From the left side of the cooler, it goes back to the other fitting on the relief assembly. The oil pump therefore has only one intake, from the pickup in the sump, and there is no opening in the bottom of the sandwich plate.

The bypass system has an inherent shortcoming in that it tends to vary the amount of cooling incorrectly. When the oil is cold, it is also thick, and the pressure relief valve has to bypass a great deal of it in order to limit the pressure. As a result, flow through the cooler is high -- precisely when not needed. On the other hand, when the oil is hot and thin, very little or none at all is bypassed, and hence flow through the cooler is minimal -- precisely when it is most needed.

The full flow system always flows the oil through the cooler before it goes through the engine, so there is always cooling. And, the coolest oil in the system is the oil fed to the galleys, so it can be expected to reduce the incidence of burned bearings.

If you wish to incorporate the full flow system to a car that didn't come with it, you will need the following parts. The corresponding part numbers for the bypass system are also shown:



Bypass p/n

FF p/n

Oil Suction Pipe



Oil Suction Elbow



Oil Suction Elbow Gasket



Relief Assembly



Relief Outlet Elbow



Oil Cooler



Oil Feed Pipe



Oil Feed Pipe



Oil Return Pipe



Oil Return Pipe










Unk (1)

EAC6800 (2)


Unk (1)

EAC6790 (2)




Plus a few bolts, nuts, O-rings, etc., all of which can be purchased locally. If you're the industrious type, you can probably improvise all those clamps and brackets too.

A different sandwich plate, without the opening for the oil return, is also required, but I don't know the part number. It is probable that most would prefer to simply make a blank-off cover for their existing sandwich plate rather than pay for a new one anyway.

Now for some hints that may make the job easier. To install all these parts would require pulling the sump off the engine to replace the oil suction pipe, oil suction elbow, and the sandwich plate itself. However, in theory anyway, these replacements may not be necessary, and the retrofit might be accomplished without pulling the sump. The difference in the oil suction elbow is that the one for the bypass system has the second inlet on it, and this inlet is readily accessible via the opening in the bottom of the sandwich plate. If this second inlet is securely plugged, it will serve the purpose of the EAC6422 part. This can be done by making a suitable part that plugs both the second inlet on the oil suction elbow and the opening in the sandwich plate, or by using a separate plug -- perhaps like the rubber expansion-type freeze plugs -- on the suction elbow, along with a simple blank-off on the sandwich plate. This would not only reduce disassembly requirements, but it also eliminates the need for purchasing a new oil suction pipe, oil suction elbow, gasket, and sandwich plate.

Note that the difference in the oil suction pipe -- which is, in fact, the pickup -- is unknown. But there is no reason to believe the one designed for the bypass system won't work properly for the full flow system. Perhaps the design was changed slightly to fit the revised oil suction elbow.


FLYWHEEL INTERCHANGABILITY: According to Mike O'Neill, the bolt pattern for the V12 flywheel is the same as the bolt pattern for the 6-cylinder Jaguar XK flywheel. I dunno what use this info may be, since the starter ring gear and other features are not the same; it is included here for general info.


TORQUE LINK: When the engine/transmission turns the driveshaft, the reaction is a twisting force trying to tilt the engine/transmission assembly on its mounts. There are only three mounts, two soft rubber mounts under the engine and the complicated spring assembly under the tranny. The tranny mount really does little to counter this force; the torque is entirely taken by the two motor mounts.

Since the mounts are soft, the torque can move the engine around quite a bit. If the car is not stock and producing more torque than originally intended, the left-side mount may actually be damaged since it is put in tension under extreme conditions.

A racing trick is to add a fourth connection between the engine and the chassis. By adding a link, torque can be taken up before the engine moves very far or stresses the mounts too much. Newer FWD cars are usually designed with such a link, but front-engine/rear-wheel-drive cars typically rely on rubber in tension.

Under torque, the engine twists, which means the top moves to the right, the left side moves up, etc. A link can be added anywhere that restricts this movement, but it is usually preferable to put it either on the left side of the engine connecting downward to the chassis, or from somewhere near the top of the engine connecting to the left side of the compartment. Either of these locations puts the new link in tension (preferable for such parts) and helps keep the motor mounts in compression (protecting the rubber).

Backyard mechanics have been known to accomplish this fix by bolting a length of chain between the left-side exhaust manifold and the chassis. Under normal conditions, the chain is slack and does nothing but rattle. When the engine tries to lift, the chain pulls tight and stops the motion. This method does work, but it is hardly a suitable fix for an XJ-S; the chain makes too much noise, and the sudden jolt when the chain gets tight is not conducive to an impressive ride.

To do a professional job, a better idea is to install a link made from threaded rod with some rubber bushings (available at any auto parts store), washers, nuts, and some fabricated brackets to provide holes for the bushings to fit into. Rubber bushings are essential, since a rigid connection would transmit vibration directly to the chassis. If necessary, shield the rubber parts from radiant heat from the exhaust system.

Alternatively, the ingenious mechanic may find a way to make a link from one of those FWD cars fit.

It should also be noted that the motor mounts on the XJ-S are not actually between the engine and the chassis, but between the engine and the front suspension subframe. This provides two layers of isolation between engine vibrations and the chassis: the motor mounts and the subframe mounts. Hence, adding a torque link from the engine directly to the chassis would defeat some of this isolation and perhaps expose the occupants to increased noise and vibration. If possible, it would be preferable to connect the torque link to the subframe as well. Or, use really soft bushings on it.



On to Cooling System Upgrades


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