Manifold/Downpipe
Nuts
The nuts that hold the downpipes to the manifolds look like an
extra long nut. They actually contain a self-locking helicoil, a
special type of helicoil in which a couple of the coils near the
center are deformed to provide some friction when turning. These
helicoils are held within the nut by a staking on each end that
deforms the thread just beyond the end of the helicoil. Whenever
these nuts are removed and reused, it is recommended this staking be
checked, and re-staked if necessary.
The helicoil design means the parent metal of the nut is thinner
than normal nuts. This is countered by the nut being so long. If
overtorqued, these nuts will split lengthwise, and internally grip
the socket used to torque them. If this occurs, of course, the nut
must be replaced. The stud is a normal fine thread, so a normal nut
will fit. It is recommended that a stainless steel or brass nut be
used due to corrosion problems at red-hot temperatures, and a locknut
or lockwasher be used since a normal nut lacks the self-locking
feature of the original. Obviously, a locknut that uses a nylon
insert for self-locking is unacceptable.
Be sure to use anti-seize compound when assembling, whether using
original nuts or substitutes.
Interestingly, Jaguar provides four threaded holes in each header
when only two studs are used. If the threads in one pair of holes get
boogered up, it should be possible to install studs in the alternate
two holes, and rotate the collar on the downpipe 90° and
reinstall.
Transmission/Exhaust
Pipe Brackets
When considering the mounting of exhaust pipes, it must always be
kept in mind that the engine moves around on its mounts. When power
is applied in low gear, the entire engine/transmission assembly can
tilt significantly to the right. Typically, exhaust systems are
rigidly mounted to the exhaust manifold and unsupported from there to
the rear of the car, where they are supported by soft rubber mounts.
The intention is that the pipes will flex enough over this length
that the engine can move without damage or fatigue.
The XJ-S, of course, has a problematic
transmission mount.
Problems with the transmission mount can cause the engine to move
more than intended, increasing the stress on the exhaust system.
The XJ-S also has several joints in the exhaust system around the
catalytic converters, which happen to be in the middle of the
unsupported span. These joints can vibrate loose under the stress.
Add to this the fact that the catalytic converters themselves get
very hot when running, and are made of a high-temperature, brittle
material. There are reports of the catalytic converters actually
cracking within about 50,000 miles.
Jaguar's solution is to add a pair of small brackets from the
torque converter cover to the pipes. The intention is to render the
portion of the exhaust system between the manifold and the catalytic
converter rigidly attached to the engine/transmission assembly,
thereby seeing no stress. The flexing of the system would have to
occur from this point rearward.
This solution is mediocre at best. On earlier models, the torque
converter cover is made of sheet metal; it is not strong enough for
this load and generally ends up pretty mangled. The fix also causes
noise; some of the vibration is transmitted to the sheet metal cover,
adding a tinny sound to the car.
Later, a beefier cast cover was used. This may represent a
solution to the problems, or at least part of one.
Exhaust Pipe Unions
The three-bolt flanged joint in the exhaust pipe just forward of
the rear axle is a very interesting device. The seal within it serves
as a sort of ball joint. The angle of the pipes at the connection can
be changed by tightening some of the flange bolts while loosening
others. This enables corrections to alignment so the exhaust system
doesn't bang against anything.
Unfortunately, few muffler shop weenies have ever seen such a
feature. If they just zip the nuts on those bolts with the air wrench
and proudly announce they finished the job in under 30 minutes, it's
not likely things will line up.
If you have disassembled your exhaust system at this point and
need to reassemble it properly, you will find it most helpful to have
an assistant -- or a jackstand, if you are short of assistants --
hold the forward end of the rear muffler up in its proper position
while you tighten the three bolts on this fitting. When you think you
are done, grab the pipe and give it a good shaking. If it bangs
anything, you're not done.
Exhaust System Assembly
Jan Wikström sends this procedure for assembling each side:
- Hang the bent pipe in place. Make sure the insulating
compressed-fibre bead is in place inside the rubber. Smear Loctite
exhaust joint compound in the joint and offer it up to the flange.
Do the screws up looser than finger-tight.
- Insert the second muffler and engage its hanger. Smear the
Loctite stuff on the sliding joint and join it up.
- Push in a finger-thick stick (conveniently brought by crazy
dog) on each side of the tail muffler to jam it in the centre of
its asbestos-lined pocket.
- Push the bent pipe clear of the brake calipers and jam in
another stick between the pipe and the bleeding nipple.
- Shove the front muffler over and up until it sits right
with another finger-thick stick between it and the heat shield.
(memo: get a bone for the helpful dog)
- Look at the tail muffler to check that it isn't sitting too
high or low through the see-saw action of the pipe. Waggle the
pipe accordingly.
- While holding everything in place, tighten up the flange
bolts about half tight.
- Fit clamp and slot cover piece, liberally smeared with
you-know-what. And do up the sliding joint about half tight. Check
that both hangers have ample space for the pipe to move.
- Remove all sticks, hold your breath and check clearances.
If the exhaust can shake normally on the hangers without touching
anything, tighten flange and clamp fully. If not, first apply
brute force as required...
The Loctite stuff is great; when the heat comes on, it foams to
maybe three times its volume and makes a tight seal. A tap with a
hammer will break it when you want to open the joint.
Heat Shields
The Jag seems to have them everywhere: those hokey little pieces
of sheet metal that seem to do nothing but get in the mechanic's way.
Not so! While some performance cars are designed to go fast for the
duration of the magazine's road test and to never see 100 mph
otherwise, the Jaguar is designed for the Autobahn -- not just 140
mph, but 140 mph all day. Under these conditions, the exhaust
manifolds and downpipes can get red hot. Items that hot radiate heat
(emit heat as infrared light -- you can feel it from a distance). The
belts, electrical insulators, boots, hoses, O-rings, etc. (all of
which happen to be black) absorb this radiant heat and cook. The heat
shields are not there for passenger protection or to aggravate the
mechanic; they are necessary to prevent the rubber and plastic parts
from destruction.
Particular attention should be paid to the heat shields around the
catalytic converters, because they get hot even when you're driving
slow. If these heat shields are not in place, the boots on the
steering rack ($$$!!!) won't last long.
Heat shields are no more complicated than they look. You can
easily make them out of scrap sheet metal. Hoses and the like can
actually be provided considerable protection by merely wrapping them
with ordinary aluminum foil.
Catalytic Convertors
According to Randy K. Wilson, the XJ-S has two different types of
catalysts in series. The first cat after the engine is a threeway.
The second is a single function reduction cat. Since it is a dual
exhaust system, there are two of each for a total of four catalytic
converters.
There is a honeycomb insert in the downpipe, immediately adjacent
to the exhaust manifold. This is part of the catalytic converter
system.
According to John G. Napoli:
I happened to be at my local Jag dealer as someone was taking
delivery of a new pair of V-12 head pipes. I noticed that these pipes
did not have the corrugated metal inserts in each of the four down
tubes. These inserts are ostensibly part of the catalyst system, and
are present on many V-12s (including my 1982 XJ-S H.E.). Anybody who
has seen head pipes with these inserts in place would reasonably
conclude that they add a lot of restriction to the exhaust. The
replacement pipes I saw today have a different shape to the cats more
streamlined and seemingly smaller than the cylindrical cats on the
head pipes with the inserts. The parts man said that both types of
pipes are available (with the inserts and without), both are fully
certified smogwise, and that the design without the inserts is
considered a better performer because of the reduced back pressure.
The downstream cats are required with both designs. I did not inquire
as to price, but it seems that anyone replacing their cats should
consider these replacements, as the Jag V-12 rewards you for anything
that helps it breathe better.
Exhaust Pipe Tips
The XJ series Jaguars have a unique style of tip on the exhaust
system, a sort of S-shaped extension that places the outlets right
out on the corners of the car. This is not a mere styling feature;
these tips were supposedly designed to correct a problem with exhaust
fumes recirculating back into the car. The shape puts the outlets out
into the airstream coming around the car rather than into the dead
air space behind the car. If you wish to replace these extensions
with some generic straight replacement tips, be prepared for fume
problems when driving.
Of course, the aerodynamics are significantly different between
the various XJ models and all of them have the S-shaped tips. While
the fume problem may be the case with some of them, it's not likely
to be the case with all of them. Some people claim to have no
problems, while others complain bitterly of the odors. Pay your own
money, take your own chances.
You might not really want to replace them anyway. The generic pipe
tips are usually cheap chrome-plated steel, and the chrome comes off
quickly and the steel underneath rusts away to nothing. The stock
Jaguar tips are made of stainless steel, so there is no plating to
flake off. No matter how dirty they get, they can always be made to
look new again with a little work with a scouring pad. However, if
you wish, J. C. Whitney offers
stainless steel straight tips.
On to the
Drivetrain
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