Mazda RX-7 GSL-SE Rear End Swap -- Spriso Motorsports

[st3ph3nm]

Our solution was to create something that could be bolted into ANY roadster with no fabrication to the existing chassis (as there are many people who prefer it that way!) Yes, a four-link like Alvin's would be great, but they are outside the scope of many roadster owner's abilities.

Michael

Thanks for all of this! I'd been thinking that the rear axle of an Alfa Romeo spider or 105 could have been used in much the same way you've done the Mazda axle. All the Alfa's, I've been told, apparently have LSD's. And I agree about the suspension setup. Whilst the multi link setups are great, there's a major problem to my mind, which is eligibility for historic competition. I know I've gone on about this in other threads, but I believe that a single leaf setup will give about 70% of the handling improvements of the multi link setup, at about 30% of the cost and time - and making no changes to the geometry of the car means that it's still eligible for the most stringent historic race classes. (For example, in the classic tarmac rallies we have in Australia, to remain in "unmodified" class, you can't even change the exhaust manifold!).

Cheers,
Steve

[Daryl Smith]

Any information on this "single leaf" rear spring setup? Pictures?

[st3ph3nm]

I'm about to post an article in the tech section right now, explaining what we did. Photos will follow

Cheers,
Steve

[mward]

I tried to locate one of these rearends & came up empty.
Can't seem to find one in northwest through the salvage yard data base.

Mike

[Daryl Smith]

Any updates?

Also, I have one of these stock rear diffs for sale if anyone is still looking. $250 Canadian. I can deliver to Ross's shop on Highway 15 (176th St.) or there abouts.
No driveshaft tho that shouldn't be hard to find.

[SLOroadster]

Our solution was to create something that could be bolted into ANY roadster with no fabrication to the existing chassis (as there are many people who prefer it that way!) Yes, a four-link like Alvin's would be great, but they are outside the scope of many roadster owner's abilities.

Michael

Thanks for all of this! I'd been thinking that the rear axle of an Alfa Romeo spider or 105 could have been used in much the same way you've done the Mazda axle. All the Alfa's, I've been told, apparently have LSD's. And I agree about the suspension setup. Whilst the multi link setups are great, there's a major problem to my mind, which is eligibility for historic competition. I know I've gone on about this in other threads, but I believe that a single leaf setup will give about 70% of the handling improvements of the multi link setup, at about 30% of the cost and time - and making no changes to the geometry of the car means that it's still eligible for the most stringent historic race classes. (For example, in the classic tarmac rallies we have in Australia, to remain in "unmodified" class, you can't even change the exhaust manifold!).

Cheers,
Steve

Not all Alfa's have LSD, only the 71 and later, and the gear ratios would be really short. I'm not sure if there is anything taller than a 4.1 rear end, and the stock setups from the '70s mostly used a 4.56, that combined with the gearing from the roadster box would be stupid short especially on the freeway. It should be noted that they weigh a ton. The roadster one is much much lighter. If I had to guess, the Alfa rear end is about 33% heavier. Since I have both an Alfa that uses trailing arms, and a roadster with the mono-leafs, I have to say I like the way the roadster feels more.

Will

[Alvin]

Here is a GSL-SE for sale:
http://sfbay.craigslist.org/sby/cto/1404893845.html" onclick="window.open(this.href);return false;

[GoldHawg]

There are a couple of rearends on texas craigslists
http://austin.craigslist.org/pts/1362184003.html" onclick="window.open(this.href);return false;

http://houston.craigslist.org/pts/1401174672.html" onclick="window.open(this.href);return false;

Already pulled from car, 1st asking $300 for a bunch of stuff, 2nd asking $250.

Any update from Michael on how this is working long-term in test cars? When is conversion service offered? How much? Will this work well w/single leaf setup that others are working?

[GoldHawg]

"The Miata Torsen limited slip carrier can be used (which several of us are going to be using in our personal cars)..."

Well, I finally scored a 84 GSL-SE rear end in the pick and pull yesterday. It was missing caliper mounting brackets (at least the calipers were laying on the ground and didn't see to mount to anything on the axle). So I may have more rx7 parts to get depending on what Michael does w/rear brake/ebrake options.

Next on the list is getting a miata torsen unit. Anybody know what years I should look for? Search on the web shows the rx7 gsl-se LSD being a bolt in for 94-97 miatas (going the other direction), so I assume you can go from a 94-97 miata back to the rx7? Any other years that work? Do you just need the torsen LSD or also the whole unit to include the ring/pinion?

Thx for any info you can provide.

[spriso]

Goldhawg wrote:

Next on the list is getting a miata torsen unit. Anybody know what years I should look for? Search on the web shows the rx7 gsl-se LSD being a bolt in for 94-97 miatas (going the other direction), so I assume you can go from a 94-97 miata back to the rx7? Any other years that work? Do you just need the torsen LSD or also the whole unit to include the ring/pinion?

Everything you wanted to know about Miata Torsen LSDs can be found here:

http://www.miata.net/garage/vlsd.html

Torsen LSD into a GSL-SE Rear End:

http://www.jimrothe.com/mazda/torsen.html

http://www.nellump.net/peri/epi/firstGe ... enLSD.html

Let me know if you have any other questions!

Michael Spreadbury
spriso motorsports

[GoldHawg]

Michael,
In the 2nd link, it has the following:

The following comments relate to a GSL-SE only. I have no experience with any other generation, although I suspect the units are more of a direct drop-in for the 2-gens.

Either Torsen will fit directly into the differential housing, but they are a little smaller than the stock LSDs. Therefore, the axle splines do not engage fully. The OEM spline contact is 25mm. Contact with the Torsen is reduced to about 15mm. Since my car has an aftermarket turbo, we didn't think it was a good idea leave it that way, although Mazda Comp said one guy was racing with a stock engine and a Torsen. To deal with the problem, the axles must be moved inward. This is accomplished by removing the spacers adjacent to the wheel bearings, which results in spline contact of 23mm.

Unfortunately, this little trick results in the axle hubs at the other end also moving inward, which takes the brake disks with them, so the disks need to be shimmed outward to recenter themselves in the caliper. We used cut-down rear disks from a junked car, plus some washers around the studs for fine tuning. And, since the brakes are now farther out on the wheel studs, you don't have enough threads left to secure the wheels, so you need longer studs, and for that matter, longer fasteners to hold the brake disks to the hubs when the wheels are off.

Given you are shortening the axles, can you compensate for this by just shortening them slightly less? Or have you noticed this issue in your installs?

[spriso]

Given you are shortening the axles, can you compensate for this by just shortening them slightly less? Or have you noticed this issue in your installs?

Yes, we just adjust the axle length depending on what center section that you are using.

Michael

[GoldHawg]

Pulled the differential and axles out of my housing last nite--wow the housing is light w/o that, don't think shipping to OR will be too expensive. The gear oil was extremely clean, and the ring/pinion showed no signs of wear. The car I pulled it out of was in exceptionally nice shape (don't know why it was in the yard) and only showed 105k. I'm rethinking whether I shouldn't just bolt the differential back in once the housing is modified. While the torsen is undoubtedly better, it sure would reduce total costs to stick w/the stock LSD. Anybody have experience/knowledge of how long the mazda LSD should last? Or how I could check the health of the LSD (again, ring/pinion look great w/no wear). Thoughts?

[SR311DET]

Just a heads up as when I was building my project (1969 S13 SR20DET Project) I decided to use the stock clutch pack LSD because of the cost and availability at the time. I deffinatly beleive the torsen is a better unit but they are hard to find and then the you will add some time into sorting out the ring and pinion gear ratio. I have a set of mazdaspeed HD clutches to rebuild my center section if it every gives out. But I drove the car this summer hard and had no issues or weird noises.

When it comes to wear and tear on the factory LSD I hope the R&P has now signs of ware. The only place of normal ware and tare should be in the clutches. With the factory LSD you could rebuild the unit and get the Mazda speed clutches and then you should be good for a very long time as long as you don't abuse it. One thing to consider is the car it came out of made no torque, so for the most part the clutches are in fine condition.

Now all that being said you only want to do this once. This is not something you decide to change later down the road as your axles will need to be longer.

Either way you will be happy with the result. No more one wheel wonders.

Hope this helps
Eric

[blueridgespeed]

If not already known, I'd be interested to know the weight comparison of the RX7 rear end versus the Roadster's.

To reduce variables, I guess we'd be comparing a shortened RX7 beam with hubs and rotors and calipers attached to a (removed for the swap!!??!!) roadster rear beam with hubs, drums and shoes

I'd guess they're probably pretty similar, but when it comes to unsprung weight, every little bit counts!