Acesanugal

iTrader: (5)

I've got an '88 GXL autocraptic that as some of you know is on the waiting list for a transmission conversion. I will most likely be using a S5 'vert transmission. My original plan was to swap the 4.10 LSD out of my other GXL into this car for the swap, but I wanted to do a little research first. I know that the auto diff is a 3.9.

So the question is (Because I utterly fail at Math), what kind of difference am I looking at running the 3.9 with an S5 gearbox verus a 4.10? Does anyone run this? I've read it'll decrease my revs slightly but by how much is the question.

Discuss.

RotaryRocket88

iTrader: (7)

Use the 4.1 if anything just for the fact that it's a LSD. The S5 vert used a 4.1 rear end, so the speedometer gear in the 5spd transmission will match the 4.1. If you use a 3.9 with that trans, you will see a little more error in your actual speed. Unless you swap the speedometer gear with the one from your auto trans.

The difference between engine speed & mph with the two diffs will vary between gears.

You can calculate mph at redline in each gear with the gear ratios from here:
https://www.rx7club.com/2nd-generation-specific-1986-1992-17/specifications-what-did-2nd-generation-rx-7-come-w-options-standard-features-642372/
Speed calculator:
http://www.catherineandken.co.uk/sti/tyres.html

3.9:
1st: 37mph @ 7000rpm
2nd: 65mph
3rd: 96mph
4th: 131mph
5th: 188mph

4.1:
1st: 35mph @ 7000rpm
2nd: 62mph
3rd: 91mph
4th: 125mph
5th 179mph

Acesanugal

iTrader: (5)

If I were going to use the 3.9 LSD out of the automatic car, I would swap the speedo gears. I suppose in the long run it would be better to use the 4.1 LSD out of my car, but the 3.9 has less than half the mileage on it. The car has 76,000 dead. But, as I autocross the car...2nd with the 4.1 diff is tall enough and I don't wanna add to that problem now do I?

Thanks for your help.