Chris2k

My friend has a '91 convertable w/ a decent amount of work done. Unfortunately his car feels much slower than my old '87 Rx-7 with the same mods. I told him I believe the problem lies in the trans / diff gearing. He's looking to do what it takes to get better gearing for acceleration.

I'm curious to what his gearing is now and what he needs to change it to. I did a search and some people are saying that the 3.90 diff is really sluggish, is that which one he has? If so should he change that out to a 4.1 or a 4.3 (or is there something even better than that).

Also are the trans gears for all the N/A 7's the same, or would he be able to optimize his setup with better trans gearing also?

Rotorific

iTrader: (1)

3.9 stock diff vert??, 4.1 is the standard diff in all cars 4.3 is the diff you want for acceleration but from what i hear top speed is severely reduced... i.e. 55mph 5th gear at 4500 rpms....defenitely not good on gas mileage. So theres your options

Spraintz

iTrader: (1)

sorry, verts have 3.909 final drive.
http://beyondtheredline.fd3s.net/Convertible.html

i would prolly just swap in a good 4.1 NA LSD rear. That would prolly be easiest.

Chris2k

Ok I'm going to have him look at the 4.1 and the 4.3 diffs.

Does anyone know what RPMs you run at for 60mph and 80mph with either diff.

He wanted the setup I had, but I don't remember which gearing I had in the rear (had the car 7 years ago). I know my car turned just under 4,000RPM at 80, and I believe 3,000RPM at 60.

Spraintz

iTrader: (1)

a GTUs 4.3 rear end will prove to be a tad hard to come by. should be able to aquire a 4.1 NA LSD relatively easily.

Can't give any info on the RPM' as its been a little too long since i've driven the 7 but yer #'s sound like a 4.1 rear.

mattwangler

your problem is its a convertable intended to be driving my the prom queen its a mild heavy rx-7 my suggestion is to detail it and drive around looking pretty oh yeah did i meantion its heavey

Rotorific

iTrader: (1)

hmm i beg to differ <- NA LSD 80mph is 3000 and 90 is 3500 and 100 is around 4100 (just did this and then some this weekend)

REFLUX

since we're on this topic....are there any options for a final drive swaps??

GTUs' have 4.3 for a final drive.....but like someone said..hard to come by

any aftermarket FD's that are more than 4.3???
like maybe 4.8? 4.9?

Spraintz

iTrader: (1)

i don't know of any other swaps that can be made but you can buy aftermarket ring and pinion sets to get the 4.3 ratio on whatever model you have( pretty pricey and can be a pain to install) but i have yet to find anything othe than 4.1.

Icemark

actually only the 88 and 91 verts had the 3.9, the 89 and 90 got the 4.1 (regardless of tranny intresting enough).

Icemark

RP has a 4.777 for the FD, but its like $700 for just the gear and pinion.

Spraintz

iTrader: (1)

Thats awfully goofy of em

guess i was tryin to at least make the point that no, 4.1 was not the standard in all cars.

Icemark

Well in 88 the 3.9 and only available as a manual tranny was used for the CAFE regulations to keep the FC3C out of gas guzzler taxes.

But when the 'vert got heavy in 89 and 90, and the CAFE regulations got relaxed (as well as Mazda started getting fuel credits for the protege and 626) they put the 4.1 on even it the car came with a automatic.

But 90% of the '91 and '92 productions run on the FC3C were automatic trannies, they switched back to the 3.9 just like the auto coupes to helpbring up mileage as well as the vert was already slow, and how much slower can it get with a auto tranny... Plus they wanted the Miata to be just as fast, since they were stopping convertible FC production and were not going to do a FD3C.

Chris2k

RP? Sorry I'm not familiar with all these abbreviations.

Can anyone post links of sites that sell diffs and/or the ring and pinion gears?

Rather than swapping the diff, would it be easier and/or cheaper to just get a new lower gear tranny (like from an '83 GSL) and then resell the vert's tranny?

Icemark

FD rear gear sets

http://www.rx7.com/cgi-local/3catalog.cgi?cat=2&part=6

Spraintz

iTrader: (1)

RP=Rotary Performance

Here is there FC final drive gear page
http://www.rx7.com/cgi-local/2catalog.cgi?cat=2&part=6

Mazdatrix
http://www.mazdatrix.com/g7.htm

Icemark, will the FD pinion fit an FC? I didn't think it would. I know you can put an FD diff in a FC but i thought the pinion lengths were different.

Icemark

and don't forget the Good Ol RX-7 Store.. They have the Kaaz 4.33 rear gear sets:

http://66.216.67.51/product.asp?0=207&1=294&3=242

Icemark

I don't think it will work on a FC, but the guy keeps asking for FD

mattwangler

just drive it its a convertable its never going to be that fast unless your cut it into peaces and remove the reinforcement beams. hey way don't you try that and tell me how it works out