Rear end is shot, stock FD RX-7 3.90. Making about 530 at the wheels, was wondering what people would think would be too tall, and wondering what rear ends would be reccomended. Debating a 3.08 rear gear or a 3.23. I hear a lot of people are using Ford 8.8 rears. Looking to maintain drivability, and get her on the track/auto-x occasionally. Any help would be greatly appreciated.

 

I run a 3.73 rear gear in a Cobra IRS 8.8" housing and DSS axles/inners. No issues with around 550-575rwhp. Street, road race, drag car.

 

A 3.08 is way too much gear for that. I'd be looking at a 3.27-3.55 range.

 

Presumably F-body T56 trans?
I'm running a 3.55 8.8" behind my 465rwhp LS2 w/ F-body T56, and honestly I'd prefer shorter (numerically higher) gearing, like 3.73. 1st gear is pretty tall, and a cammed engine doesn't do anything good for low-rpm torque... That said, 3.55 worked very well for Texas Mile run

 

3.73 is optimal is you will be road racing. Which T56 are you running?

 

First of all, wow, that time/mph is nuts. How many RPMs did you hit at the end of the mile??

Sorry for the late response, was busy with work and schedule changes. It currently has a 98 F-Body T-56.
I was really considering the 2006.5-2010 Ford Explorer rear end with the 3.55 gears, and going with the Ronin kit to minimize the amount of fab work that would need to be done, and keep from messing with the exhaust.
Has anyone else used that kit specifically, or have any other good recommendations for a kit to use? Not trying to break the bank, but also don't want to skimp either. Thanks for the responses btw!

 

6600rpm, on 25.6" diameter 275/35-18 tires. With 3.73 gears, would have been 6934rpm, approaching 7000rpm rev-limit, but hey, woulda worked!
Finding 3.55 a bit tall around town on the street...

Mine has the Samberg kit. I had some issues with cracks in the old style diff mount "batwings", since replaced them with his newer/much-beefier design. All is well so far...

 

Very cool. Will look into the Samberg kit since you have been having as much luck as you have with the newer design, and I know they have been around a while. Thanks!

 

Also look into the Ronin Speedworks setup. It requires welding but is cheaper and has a LOT more exhaust clearance. It uses the Explorer diff which can be had really cheap(aluminum with two front mounting tabs like the cobra). The Ronin is a better way to go in every respect EXCEPT you have to weld it in to the rear sub frame.

The Samberg requires welding too if you have a 93 car. 94 and 95 have the mount tabs already.

I second the 3.55 gear. 3.23 or lower is just too tall. You cannot do wrong with the 3.55 or 3.73.