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Drivetrain Modifications

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Drivetrain Modifications


AUTOMATIC TRANSMISSION FLUID: The owner's manual specifies Dexron II automatic transmission fluid for the GM400 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, since -- in any automatic transmission -- firmer shifts cause less wear.


GM400 TRANSMISSION MODS: Helpful sources include B&M Racing, TCI, and Turbo Action.


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."


GM400 TRANSMISSION SHIFT POINT MOD #1: The 5.3 liter Jaguar V12 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 GM400 automatic transmission is set up to keep the V12 in the RPM range where its performance is poorest.

The point at which the GM400 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.

Alan Jenks "fitted a B&M adjustable vacuum modulator (B&M #20234). This fits all TH400's."

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."


GM400 TRANSMISSION SHIFT POINT MOD #2: 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 V12 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 GM400 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 GM400 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 GM400 will force a shift no matter what.


GM400 TRANSMISSION SHIFT POINT MOD #3: If you want serious performance from your XJ-S, install one of the many available "shift kits" for the GM400. 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 GM400 transmissions.

Alan Jenks reports on a "Shift Improver Kit' from B&M Racing to the Turbo Hydromatic 400 automatic transmission. There are two kits, one for pre-1988 TH400 (B&M #20260) and one for post-1988 (B&M #20261). The kit can be fitted in two forms, either ëheavy duty' (stage 1) or ëstreet/strip' (stage 2). Stage 1 provided firmer shifting without compromising the quality of the ride, which is what I wanted."

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 GM400 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: First off, you might wish to note Roger Bywater's comments on the stock GM400 torque convertor.

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 V12, but does require removing the tranny to change. According to Thomas E. Alberts, "B&M tells you to try for 500-750 RPM less than the RPM at peak torque."

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. Reportedly, some GM torque convertors use six bolts as well.

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 acc pedal, watch the Tach, 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 that's how it works."


GM400 RATIO CHANGE: Thomas E. Alberts says, "...I agree that the standard gearset is poorly matched to the XJ-S... Note that the modified gearsets, like TCI's, lower the second gear ratio as well as first. Observe:

TH 400

TCI gearset










I think it would make a very nice improvement and without any sacrifice of top end speed."


ADDING OVERDRIVE TO THE GM400: John Goodman has heard about "overdrive units manufactured by American Overdrive Inc. designed for autoboxes, GM400 included? This effectively changes your GM400 into a six speed.

"Two possibilities here would give much better acceleration. The unit can be supplied with the internals reversed so you have underdrive first, underdrive second and underdrive top. Or install a lower axle ratio and leave the overdrive in and switch it out for "Sport Mode"."


GM400 REPLACEMENT: The shift points, cruise speed RPM, fuel economy and top end can all be optimized by replacing the GM400 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. AJ6 Engineering reports that the ECU needs the overrun cutoff disabled "to avoid driveline shunt at low speed."

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. 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, much of it provided by Mike Frank. There are four transmissions commonly considered. The first is 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 V12 but may be marginal if extensive engine mods are done. There are lots of ratios available.

There are 5-speed conversion kits using the "JT5" transmission, which is supposedly the same thing as a BW NWC only different. Only conversions for E-types and other older Jaguars are mentioned, but since the E-type SIII V12 is included, it may be possible to figure something out for the XJ-S. JT5 kits are offered by Terry's Jaguar and Vicarage.

The second possibility is the 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 V12 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. The Getrag is a novelty in the US, but is reportedly fairly common in Europe. SNG Barratt offers kits using this transmission; again, only E-types are mentioned, but the SIII V12 is included.

Fourth is the Toyota Supra "oval case" 5-speed. It might be from a Japanese six-cylinder car, but it has nevertheless been used with considerable success in Jaguars with either 6- or 12-cylinder engines. This transmission is described as "an all-alloy case 5-speed and can be distinguished by 9 bolts and 2 dowels holding the box to its bellhousing." If you get your own tranny from a junkyard, note that before installing it is recommended 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 Supra tranny from Dellow, a major supplier of these kits: "There are 4 gearing variations available:






























There are four possible stick positions: 18", 19", 20 1/2" and 21", as measured from the front of the box. Note that the W57, W58, and W59 may be difficult to locate, with the W58 described as "very rare".

Dellow also offers some Supra 4-speed boxes; ratios are 3.57/2.06/1.38/1.00.

There is also some history of the Toyota "steel case" 5-speed from Celicas, Crowns, Coronas, Cressidas, etc., being used in Jaguars with success, even though it is described as not quite as strong as the Supra. This tranny has 7 bolts holding it to the bellhousing.

Also note that, despite the fact that Dellow is located in Australia, getting all the hardware needed from them might still be cheaper than buying such things as the transmission itself locally, once the exchange rates, shipping, etc., are all figured in.

John Napoli tells us about Tilton: "They cater to the racers. They sell a full range of bellhousings for the V12 that accept popular manual transmissions. You can even get a small-diameter, multi-plate clutch with a small, reverse-rotation, gear reduction starter that mounts backwards on the bellhousing, alongside the transmission. Imagine -- starter changes in the XJ-S without removing exhaust pipes, and from inside the car!! Be still my heart."

Jaguar made a manual transmission XJ-S -- with the AJ6 engine. Still, maybe some parts would be helpful for the V12 conversion. Chad Bolles reports, "The pedal box from the 5sp XJ-S will fit any XJ-S, but the problem is it will only work on an XJ-S with ABS, and I have found no way to make it work on a non-ABS car." Aaargh! Note that that AJ6-powered XJ-S existed in Europe before the advent of ABS brakes, so perhaps there is still a possibility there.


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 page the secion on the Drivetrain for other considerations on the final drive unit.



On to Suspension and Steering Modifications


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