I'm about to install a B&M "Shift Improver Kit" on my 1987 XJ-S V12 with the TH400 transmission.

This kit can be configured two ways, as a "heavy duty" or "street/strip" modification.

I understand that the "street/strip" mode provides MUCH firmer shifting than the "heavy duty" mode, but I don't know how much "MUCH" I want.

If you've done this installation, please comment on your impressions. I don't want to radically alter the personality of the car, but I do want it to shift much closer to redline than it does now.

Thanks!

John
1987 XJ-S V12, 63,000 miles

 

I installed a TRANS-GO kit years ago and I used the lowest modification in the kit.
You can always go with the more aggressive hole drilling later if that is how the kit is setup.

I think some kits just have bigger holes or holes in different places on the VB plate.

I don't know about the B&M kit, but if it involves drilling holes, you can always make them bigger but making them smaller is more difficult.

Springs are easy to move around.

bob gauff

 

Just adjust your modulator all the way. You don't even need the shift kit to do a later/redline up shift. Mine has the Trans Go kit and the B&M modulator (your stock GM unit is also adjustable) turned all the way up....mine shifts at about 5500 under WOT from a start and pleasingly '"Launches" you into 3rd gear at that RPM.

Here's a section from "The Book"

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..."

 

I agree with the governor mod. You can buy lighter governor weights (or grind weight off them, but it needs quite a bit off) and this keeps the box with a standard feel and shifts, but it revs much higher at full throttle or on the stick.

Greg