DEZERTE

I believe the automatic has the 3.9 rearend and the 5speed has a 4.1 or 4.5 (please correct me if im wrong)
But how noticeable will the change be in swapping out the auto for a 5speed one?

1987RX7guy

iTrader: (3)

Auto rear is 3.9 N/a and TIIs have 4.1 and GTU's have 4.3 so from auto to N/a 4.1 is the same thing as 4.1 N/a to 4.3 N/a so I would think it will be noticable.

DEZERTE

come again?

cbrinega

What he means is that most 5 speed non-turbo diff. gears are in a 4.1 to 1 ratio.

The GTUs model (in the US) came with a 4.3 to 1 ratio. This ratio of ring and pinion can also be purchased from many vendors (rx7 store, rx7.com, racing beat, ...) for around $450.
http://www.rx7.com/store/rx7/fcdrivetrain_rearend.html

I changed my GXL from a 4.1 to 4.3 ratio (from a GTUs), and I liked the change (5% lower gearing). I noticed that taking off in normal city traffic was easier and that it was easier to keep up with other traffic in the 0 to 20 mph range without getting into the throttle too much. (Of course the entire RPM range receives a small but noticeable boost in acceleration.)

I would go for the change from a 3.9 to a 4.1 at least. If you can afford it or find a good deal, then the 4.3 would be fun too. Your engine RPMs will go up when crusing, and you may want to change out your speedometer gear so it reads correctly.

DEZERTE

ahh, thanks for the great information! does the "rear end" ratio have anything to do with the differential? because i was going to get a limited slip. So should i worry about the gear ratio when purchasing my LSD?

theonlygreat

other than being bolted to the differential, one does not change the other in anyway. if you are making a lot of power, then i'd say leave it the way it is. you'll get mildly better gas milage with the 3.9 and you'll be able to stay in gear a little longer (if you have a manual you could take 2nd to almost 70mph give or take, depending on tire size). the higher ratios like 4.3 are to get more acceleration out of the low end because the rotary does't make a whole lot of down low torque. if you are NA, i'd say the 4.3 is the way to go though.

DEZERTE

Thanks thegreat... ill be going with 4.3:1 most likely

Kenteth

EDIT: (*for reference...)

---

ummm... 3.9 --> 4.1
is *not* the same as 4.1 --> 4.3

Look...

4.1/3.9 - (1.051282 ~~ 105.13% ~~ 5.13% accel increase)
4.3/4.1 - (1.048780 ~~ 104.88% ~~ 4.88% accel increase)

The changes would not be the same. But in terms of notability... You're looking at .25% of a difference... on a 160ft/lb torque car Thats roughly .4ft/lbs torque difference... hardly noticeable. Difference would be more noticable in speed differences.