I was wondering if there was something that could be done with the transmission or differential on my XJS to improve the low end gearing? Is it a characteristic of this car to have a sluggish take off? The car does very well once you get it moving but it does not take off with anything nearing what I would think it could or should. I am looking fora better launch. Any ideas would be appreciatted.

 

"The Driven Man" makes a 5-speed conversion kit that, according to amny, tranforms the car entirely.

Others use a TH700 trans which has a deeper first gear (plus an overdrive 4th) and are very happy. "Johns Cars" in Texas makes a conversion kit.

"TCI Automotive" makes a low ratio first geat kit (2.75 instead of 2.48) which would help a bit. They can also fix up up with a nice 2500 rpm stall speed torque converter.

Before changing anything make sure the engine is running 100% up to snuff. Many are not. Clogged injectors, throttles not synchronized, etc.

A curb weight of 4000+ pounds and a 2.88 axle ratio don't work well with the powerband of the V12 as far as off-the-line punch goes.

Cheers
DD

 

Aren't you the guy who had the coil problem? If so, you'd best pull off your exhaust and have a look at your catalytic converters. Coil failure (orcap/ rotor failure)= melted honeycomb = restricted exhaust = sluggish acceleration.

If the right side converter overheats, it can also melt the vacuum hose to the transmissionmodulator, or ruin the diaphragm in the modulator itself. It also doesn't get a proper vacuum signal if the RH cat is blocked, even if the hose and modulator are undamaged. The good news is that your car didn't burn to the ground, as that is where the Marelli fires sometimes start.

On a properly tuned XJS, you can install a trick valve body and an adjustable modulator for more positive shifts at the rev range you want them. That's good bang for the buck, without removing the trans. I don't have specific parts information, this was a job I did quite a while ago for a customer who supplied the bits. I think it was Jegs street racer stuff. I installed the Lister air cleaner inlet trumpets at the same time, and my seat of the pants dyno showed happy numbers (not number two).

My opinion was that theFactorydiff gear was selected as a compromise for the lousy fuel mileage. I'd be interested to know the mpg results with an alternate ratio.

 

No that wasn't me but I will check the cats. Thanks

 

Im not sure if the later pumpkin will exchange but the 94 has a 3:54 posi. A 2:88 gear is too high for anything short of a top fuel dragster. I had an 83 XJS with a 402 big block chev and it was better but still a turd out of the hole.