I have an 88 vert that has the stock tall diff gear ratio. I'm wondering what's involved to get the the ratio all other fc's are using. Is it as easy as pulling a diff out of a non vert fc of the same year? Or does the pinion need to to go as well?

 

Just swap the whole differential from a manual coupe. All NA differentials will bolt right in.

 

While looking around you might want to try to source a LSD unit if possible. Would be a nice upgrade but also more cost. Otherwise AGreen has the right of it.

 

Whole diff as in housing too? I've been hunting for a gxl but no luck at the local pick n pulls.

 

Yes the whole diff/housing. Keep looking for a gxl rear but make sure its an S4 since those are clutch type lsd's. But honestly most clutch type lsd are worn out after all these years and could use a rebuilding.

 

I'm not too picky. I won't be drifting. Just want a funner ratio. The clutch type works better but with the age would need a rebuild. The viscous I'm assuming just will need new fluid.

 

Miatas (94-97 I think) had Torsen limited slip differentials. You can't use their ring/pinion, but the LSD unit bolts right up to the FC's ring gear.

 

I read that they were compatible but I would be stuck with my vert ratios

 

Not if you found an open 4.10 diff, then slapped the torsen unit in. I picked up a 4.10 differential for $50 locally. Just put up a wanted ad in the West Rx-7 classifieds forum.

 

The Miata Torsen unit is pretty challenging to find. It was only available on one specific trim package and obviously is in high demand in Miata circles. Another Torsen option is out of the S2000. It's also a Torsen but is preloaded and is considered to be a better Torsen design then the Miata. It should be cheaper and easier to find as it was stock for most (if not all S2000s) and does not have a good reputation in the S2000 community as it's seen as weak when they start putting a lot of power down. Don't let that scare you as their idea of a lot of power would break just about any of the OE solutions for the 7" rear but will be plenty strong for what an NA motor can put down even with moderate porting.

The one thing I've read about the viscous LSDs is that they may too wear out as the viscous fluid inside of the unit breaks down. Mazda used a sealed unit and it is not rebuildable. I can't validate that they actually do break down but it makes sense.

 

And the S2000 torsen unit bolts right in?

 

any other older car with a compatible diff. S2ks are too new for the pick n pulls in my area.

 

Yes.

 

Nope that's it.

 

The 88 vert was only available in manual so it would have come with the 4.1 diff. 95% of rx7s use the 4.1 diff unless they were automatic coupes with the 3.9 or the GTUs with the 4.3. So you DO have the same ratio as "all other FC's".

 

88 verts were the only fc manual with the 3.9 rear end for sure. Think I am going to pass on the torsen diff. I get stuck when I enter exit drive ways with coilovers. The Miata torsen units don't have any preload as far as I know. Maybe the s2k have a type2 torsen.

 

The S2K is a R type torsen. But as far as I know it still suffers the same problem as any torsen as you mentioned. If you lift a wheel all power goes to the lifted wheel. It's a superior street diff to all the other options in my mind but as a track or very stiff chassis where wheel lift occurs the mechanical diff is the way to go.

AGreen - An update on the S2k torsen according to the solomiata page you've got to use the s2k stub shafts and half shafts as they are a different size than the miata/rx7 bits.

Solomiata : Drivetrain : Interchange

 

So then drop in a standard 4.1 if that is what you want? I will give you one from the next NA I part out if you want to pay for shipping.

 

I will take you up on that. I just found a GTU at the local pick n pull and tried to get the diff out. Looks like the whole sub frame needs to be dropped to get it?

 

^ for most the part yes the entire subframe needs to be dropped. I have pulled and swapped rear ends with out dropping the sub frame but its a pain in the ***.

 

A lot of work for a diff that may be bad.