byrdman454

What are the differences in auto transmissions from a '88 and one from a '91? The '91 uses PSD and the '88 does not. The gearing is also different 2.54 versus 2.20. Is one better than the other? Can the '91 be used in my '88 if the PSD is bypassed? How is this done?

IcemanG17

The 89+ tranny has the 2.54 rear end + slightly different gear ratios...90+ has PSD....PSD can be bypassed and retrofitted to an early car....but its quite a bit of work

Vilhuer

All but 4th gear have different ratios to counter different final drive ratios. I think its possible to mix late style boxes and differentials without problems. Maybe not much gain to be had but possible anyway.

Putting PSD box in '87-89 car shouldn't be a problem as it should be direct swap. Having working PSD is other matter entirely. Bypassing means just replacing PSD slave cylinder at differential cover with fixed bolt.

Giovanni

Anyone knows the dimensions and approximate weight of an auto transmission?

Vilhuer

Total lenght including differential is little under 90cm and weight close to 100kg IIRR.

Giovanni

Hartelijk bedankt.

byrdman454

drnick and Mark Anderson have both suggested not to bypass the PSD and use the mechanical LSD. I have a LSD in my '88 autobox and I will have a '91 autobox delivered next week. I can probably use either tranny and put the old style LSD with the 2.54 gears in it. I do have a question about the internal ratios from the '88 and '91. Which one matches up best with the 2.54 R&P? I know the differences will be small, but would like to have the lowest 1st gear for better off the line acceleration. I am not interested in road racing this car and am also not interested in going 170mph.
Thanks

byrdman454

I am planning on using a '91 auto in my '88 for the better gear ratio, 2.54 vs. 2.20. I also plan to swap my LSD from my '88 into the '91. Do I need to swap the entire diff case and internals, or can I just swap the chunk and make a cover plate for the slave cylinder hole in the '91 case? I have abandoned trying to trick the PSD into working after hearing comments from drnick and Mark Anderson.

John Veninger

Think their suggestions were based on track use. Might still be an option for the street.

byrdman454

John,
If I understand the PSD correctly, the slave applies a certain amount of lock when 1 wheel spins. If I replace it with a bolt, this basically makes this unit a locker arrangement to where I have X% applied to both wheels at all times, even around turns. I was under the impression that going around turns, even on the street would pre-maturely wear out the unit and in slick conditions might be dangerous.

Do you have a different understanding?

Vilhuer

Bolt basically turns PSD into LSD but question is will it work as constant LSD or is it weaker than real LSD. Its also possible PSD has different shape plates which will give different result in deceleration compared to LSD.

Since you have a choice to use LSD just use it as there isn't anything preventing its usage. It shouldn't matter which casing you use. You just need to use ring & pinion from newer diff and LSD from older diff. All other parts can be from either one.

PorKen

Install a clutch pedal, and plumb the PSD so you can operate it with your left foot.

Jim bailey - 928 International

I tried to talk Mark into adding another pedal but in the race car he said his feet were already pretty busy ! But on an automatic it could work and give full lockup. The late style mechanical slip is only a 40 % lockup.

Jim bailey - 928 International

If you really want to rock and roll have the Mercedes torque convertor reworked for a somewhat higher stall speed ....

byrdman454

Jim,
I already have the stall planned. Took the TC out of the tranny last night. According to some old info from Steve Pappin (remember him?), the stock converter is set to 1875 rpm. The S4 cams optimum stall for its cams is 2200 rpm and the GT is 2400 rpm.

Does anyone disagree?