GM 400
Transmission Modifications
Helpful sources include B&M
Racing, TCI, and
Turbo Action.
GM 400 Transmission Fluid
The owner's manual specifies Dexron II automatic transmission
fluid for the GM 400 transmission. Draining this fluid and refilling
with Type F fluid will result in firmer shifts. The Type F fluid is
less slippery.
Alternatively, there are aftermarket fluids available that are
less slippery than either Dexron or Type F, resulting in firmer
shifts yet.
If you have the early XJ-S with the Borg Warner automatic, the
owner's manual specifies Type F fluid. However, there are reports
that some Jaguar dealers refill these transmissions with Dexron II
anyway, because they feel their customers appreciate the smoother
shifts resulting from the slipperier fluid. This might not be a good
idea.
Raising The Line Pressure
Tim Blystone sends this idea, in response to the above suggestion
of using Type F fluid:
The F-Type fluid is somewhat thinner and is supposed to allow
the tranny to shift faster and more positively. It does no harm and
neither have I ever been able to see any good it does either. I would
rather use Dexron 3, remove the pump and replace the pump pressure
spring with a new red spring. Used in Corvettes. This will raise line
pressure and make the whole thing shift more positive.
Vacuum Modulator Modification
The 5.3 liter Jaguar V-12 was designed prior to the fuel crunch of
1973 and has "over-square" engine dimensions the bore (90 mm) is
considerably larger than the stroke (70 mm). Such a design is
conducive to high performance at high rpm and is therefore ideal for
racing applications, but results in poor low end torque. However, to
optimize fuel economy, it is best if an engine is turning rather
slowly, so the stock GM 400 automatic transmission is set up to keep
the V-12 in the rpm range where its performance is poorest.
The point at which the GM 400 shifts is determined primarily by a
governor that responds to engine rpm, and is varied by two things: 1)
a vacuum modulator that makes the car upshift promptly when vacuum is
high (light throttle); 2) an electrical switch on the throttle
linkage that forces the tranny to downshift, and remain in lower
gears until 5000 rpm, when the pedal is pressed all the way to the
floor ("kickdown").
The vacuum modulator is a bolt-on device on the right side of the
transmission. Some of these modulators are adjustable
by removing the vacuum hose and inserting a straight blade
screwdriver into the vacuum fitting. Turning the adjustment changes
the preload on the spring, therefore altering the shift points.
The vacuum modulators are available in several sizes, and since
they are a standard GM part, are found in any auto parts store for
reasonable prices. In general, the smaller they are, the more vacuum
required to make the tranny upshift, so the longer the car will
remain in the lower gears. Of course, it is recommended that you make
sure to buy one that is adjustable.
Rather than buying a smaller modulator, Greg Meboe modified his
existing one:
I needed to increase the spring pressure against the diaphragm.
I did this by collapsing the modulator body around part of the spring
in a vise, effectively shortening the working length of the spring. I
imagine I took about 30% of the travel out of the spring. This had
the effect of INCREASING the effective spring
rate. The vacuum modulator modifications produce an upshift between
500 and 1500 rpm's higher than before, depending on throttle
position.
Shift Point Governor
For absolute maximum performance, shift points should be selected
that surround the peak on the horsepower curve -- without exceeding
the redline, of course. The engine should be allowed to run past the
power peak, so that when shifting the engine doesn't drop back too
far below the power peak.
The published stats rate the Jaguar V-12 at 5000 rpm. With the
large steps between gears on a three-speed transmission, the engine
should be taken to the redline of 6500 before shifting to optimize
performance.
The GM 400 slushomatic transmission, of course, was designed with
the 1950's-design pushrod V8 in mind. Not only does it normally shift
at relatively low rpm, it absolutely refuses to allow the
engine to rev beyond 5000 rpm; it will shift at this point even if
you have selected low gear with the lever. The above modification to
the vacuum modulator will not affect the limit where the GM 400 will
force a shift.
The governor assembly is contained under a sheet metal cover on
the right rear side of the transmission, and there is an access cover
under the carpet of the XJ-S to get to it. If you remove the governor
and grind a little weight off of each of the counterweights, the
engine will shift at higher rpm under all conditions. This
modification will allow the engine to run faster than 5000 rpm, where
the stock GM 400 will force a shift no matter what.
It may make sense to modify the governor as described, and install
a large vacuum modulator to keep the light-throttle shifts at
a reasonable rpm.
According to Chad
Bolles, the GM 400 in the 1991-92 XJ-S was programmed differently
than earlier cars, providing higher rpm shifts under power, better
downshifting, and holding low gear forever if the shifter is held in
first gear. It may be possible to retrofit the changes into earlier
cars (replace the governor, etc.) or to buy the later transmission
and retrofit it.
GM 400 Shift Kits
If you want serious performance from your XJ-S, install one of the
many available "shift kits" for the GM 400. Keep in mind when you
select your kit that your objective is to allow higher rpm shift
points at full throttle (when the kickdown switch on the throttle
cable is activated) or when manually shifting; High rpm shifts when
taking it easy are of little benefit and will hurt fuel economy and
passenger comfort. Many shift kits offer more firm shifting, some
shift at redline at all times (!) and some even render your car a
3-speed manual.
Most shift kits can be installed by simply lowering the pan from
the transmission and removing the valve body within. Replace a few
springs and the like, and reassemble. See the note in the Maintenance
section regarding the differences between pre- and post-1988 GM 400
transmissions.
Per Chad Bolles: B&M makes a
product called a Transpak, "with this you can set up the trans to
shift like you want, from very hard, to normal street." B&M also
makes a Racing Kit, "this is a race very hard shift kit (neck
snapping)." Finally, in what should probably be listed as GM 400
Transmission Mod #3A, there is a replacement valve body. "The B&M
valve body changes the shift pattern to 123NRP instead of PRN321."
Jan Wikström had his local tranny shop "lock up the freewheel
on 1st gear, which gives me respectable engine braking and a better
"feel" in the really low-speed twisty bits. Caution: it's now
possible to engage 1st at any speed..."
Torque Convertor Replacement
There are torque convertors available that have a higher "stall
speed", which sorta means they engage at a higher rpm. This is
wonderful for the Jaguar V-12, but does require removing the tranny
to change.
Michael Kenrick reports that the torque convertor from a 3.8 Buick
Regal Turbo can be used to provide a more suitable stall speed.
According to Chad
Bolles, the later model XJ-S uses the HI-Stall convertor. "It is
the same converter used in the 427 L-88 and LS6 and LS7 Chev, flash
stalls around 2600-2800rpm, the difference being the Jaguar converter
has 6 lugs on it where the Chev has 3." He adds that a Chevy
convertor may be used in the Jag, leaving the other three bolt holes
unused.
Bolles also explains the difference between stall speed and "flash
stall." Stall speed is the rpm the engine runs at continuously if the
brake is held and the accelerator is floored in gear. Note: this is
very hard on the equipment, and causes a lot of heat in the
transmission; such tests should only be done for a couple seconds,
and the car should be driven around at normal speeds for a while
afterward to allow the transmission fluid cooler in the radiator to
cool the transmission back down. "Flash Stall, hold the brake, trans
in gear, quickly mash the accelerator pedal, watch the Tachometer, it
will flash to about 2500-2800 rpm, but if you just hold the brake and
ease the acc pedal down the converter will not reach the same rpm
before it tries to spin the rear tyres. Don't ask me why, I just know
that thats how it works."
First Gear Ratio Change
Parts are available from TCI to
replace the 2.48:1 low gear in the GM 400 with a 2.75:1. Although not
a major change, this is just the thing to make the car feel less
sluggish when starting from a standstill.
GM 400 Replacement
The shift points, cruise speed rpm, fuel economy and top end can
all be optimized by replacing the GM 400 with a 5- or 6-speed manual
transmission. A major task, requiring the installation of flywheel,
bell housing, clutch, clutch pedal, console modifications, etc.
Changing the final drive ratio (differential gears) should be
considered, since 5- and 6-speeds generally have an overdrive top
gear and the final drive ratio is already tall enough.
The author of this book HAS performed this modification on his
car, using a 5-speed Borg-Warner NWC transmission from a Camaro. If
you desire to know more about such a project, you are welcome to
call. However, Chad Bolles
of Jaguar South has also done such projects, and has prepared a
5-speed kit. Also, Gran Turismo
Jaguar now offers kits for 5- and 6-speed conversions.
General info for those considering such mods follows. There are
four transmissions commonly considered: The Borg Warner NWC 5-speed,
which comes in Camaros, Mustangs, and several other vehicles; it has
an integral shifter (no external linkage), and there are a couple
different locations for this shifter, reportedly the Mustang
configuration is better for the XJ-S than the Camaro configuration.
This tranny is adequate for a stock XJ-S but may be marginal if
extensive engine mods are done. There are lots of ratios available.
The second possibility is the Borg Warner 6-speed, used in the
Dodge Viper and the latest Firebird models. It has some advanced
features such as carbon composite synchros. This is a massive tranny,
and will probably handle whatever a Jag V-12 can be made to dish out.
There is a wide variety of ratios available; GT Jaguar reportedly
offers a kit with suitable ratios to allow use of the stock 2.88:1
final drive ratio.
Third possibility is a Getrag 5-speed. It is supposedly also
adequate for all conceivable uses.
Fourth, and possibly the most popular, is the Toyota Supra/Celica
5-speed. It might be from a Japanese four- or six-cylinder car, but
it has nevertheless been used with considerable success in Jaguars
with either 6- or 12-cylinder engines. First note: These
transmissions came with three type of cases, a smooth aluminum, a
cast iron, and a ribbed aluminum. The ribbed aluminum is supposedly
the best, the cast iron is acceptable, the smooth aluminum is not.
Second note: Before installing, it is necessary to remove the front
cover of the transmission which holds the input shaft bearing and the
input end of the layshaft, and replace the light-duty layshaft
bearing with a generic industrial-duty bearing of the same size -- a
five-minute job.
More info on the Toyota tranny from
Dellow, according to Mike Frank:
There are 4 gearing variations available:
|
|
W55
|
W57
|
W58
|
W59
|
|
1st
|
3.57
|
3.28
|
3.28
|
3.95
|
|
2nd
|
2.06
|
1.89
|
1.89
|
2.14
|
|
3rd
|
1.38
|
1.27
|
1.27
|
1.27
|
|
4th
|
1.0
|
1.0
|
1.0
|
1.0
|
|
5th
|
0.85
|
0.86
|
0.78
|
0.85
|
There are four possible stick positions: 18", 19", 20 1/2" and
21", as measured from the front of the box.
Reportedly, the XJ-S was available with a five speed stick shift
for 1992 and 1993. Apparently there was insufficient demand to
continue offering it.
Final Drive Ratio Change
There are many ratios available to fit the Jag final drive.
However, be aware that if the ratio is changed significantly, the
differential carrier will also have to be replaced. Since the lower
ratios (higher numbers) require a smaller pinion, the ring gear needs
to move closer to center to mesh properly. This could be accomplished
by making the ring gear thicker or adding a spacer, but instead a
replacement carrier is used with the mounting flange in a slightly
different position. Unfortunately, the carrier costs more than the
gears.
Please see the secion on the Final
Drive for other considerations on the final drive unit.
On to
Suspension
& Steering Modifications
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