I have never posted in this section so let me now if this is appropriate.

I have a complete rear end for an s4 auto transition. I need to do more research too but I thought I would put out some feelers to the race people here.

My goal eventually is to be at 170hp or so with my GXL. I would like to get competition 5 speed gears or maybe some other custom setup. Maybe a four speed Miata setup with the GXL 5th and as close to a 5.12 rear gear as possible. Or a custom setup with 6 speeds, 5 competition gears and an overdrive gear for highway driving and close to a 5.12 rear gear. Maybe a 4.77 and shorter tires on 15" wheels to get to 5.12? I honestly don't know if my final HP is measured at the wheel or at the engine though?? I read that 170hp is about right for 5.12 rears and 5 speed competition gears. I did some testing on racing games with five comp gears and 170hp, the car is a lot of fun for track use. On the highway, it would be almost unusable. Even at 60mph and 5 comp gears it is a pretty high rpm. I would prefer to keep it a road car, too.

Can I start with this 3.9 rear end as a base or is a T2 rear needed? Also, I do understand that the amount of heat generated with 5.12 rear it pretty significant. I would need to work out some cooling system, preferably air cooled. There are solutions for the speedometer so, I am not to concerned or.... should I be? Maybe my whole idea is garbage? Let me know.

Thanks!!

 

What are your motorsports goals, specifically?

With a solid axle, and the kind of competition I do, I have tried 4.44 to 5.43 gears and settled on 4.87 gears to be the sweet spot for using 2nd gear. I run 205/70-15 tires on the street to cut driveshaft RPM, which is more important than engine speed IMO. Best fuel economy seems to be if the engine is spinning at 4500rpm or so - I've seen 29mpg with the bridge port and 32 with a stockish 12A, running those kind of RPM at highway cruise. Sadly the 9" that I have now sucks a lot of power compared to a Mazda rear so fuel economy ain't what it used to be, but at least I don't break axles anymore.

Something to note is that all Mazda gearsets, no matter if FC or FB/B2200 or Miata or even RX-8, have a 60% ratio difference between 1st and 2nd gears,. So if you are mainly interested in low speed activities, you won't gain anything at all from changing gearsets once you have 2nd gear where you want it speed-wise.

The RX-8 trans is a neat piece. 2nd-6th gears have almost the same gear ratios as a five speed Mazda Comp box from the 80s and then there's a First gear below that. That and the 3.9 would be a nice fit.

 

Wow, what a great post, thank you!! You did some great R and D on this stuff. This is a really great community here!

I considered an RX8 trans but not with a 5.12 rear, first gear would be hard to use I suspect. I saw a RX8 manual install on an FC here but it didn't look complete. It moves the shift lever toward the dash some?, if I remember correctly. Mashing part of my hard or my knuckles on the dash every time I shift into third or fifth would not be very fun. As a daily, I would need a good rubber boot to fit and clean up reasonably on the interior. This idea is great so far, it is easy on the pocket book. too.

Parts needed beyond the rear end, an engine and exhaust:
6 speed RX8 gearbox
Custom mounts fabricated/welded
T2 clutch and flywheel
Custom Drive shaft?
A way for the speedometer to work.
I am sure to be missing something?? maybe different axle shafts. I will need to look this up.

As for you question about motorsports..... I have plans to do some volunteer work with SCCA, maybe work in the tech shack passing out radios?? or taking tickets? Then I can get a feel for what is going on and meet people. My main goal is friends and community so, going for a few laps in training may never happen. Also,, I am 6+ ft tall, 48 years old and own a GXL. So, having a plan without knowing if I would even fit in the car might just be like peeing into the wind.

I do have a track close to my house that hosts both SCCA and the other one? and a second track a little farther into Ohio.We have 2 or more dirt tracks close by and even more a few hours radius, a drag strip within an hour and and more in a three hour radius and at least one dirt bike track at High Point that isn't terribly far. So, you don't have to dig far to find a race fan here. I would really enjoy the community and friendships. Perhaps my place is in the audience? or the pit crew? or passing out radios? and not driving. Based on my driving in video games, I am average at best right now. ... time will tell. if I did drive, safety for me and everyone else comes first.

As to why I posted this now and not later. I have had this rear end and many other parts in my garage for more than 20 years. It is time to figure out what I need and clear out the rest. Right now it is a five year plan to get this car mechanically where I want it with mods. I would like the option to track it without getting completely smoked by Miata's and Hondas and without going turbo. It could take longer as I was defrauded my life savings that included my retirement and an additional personal loan about 1 year ago. Honestly, it hasn't slowed me down as much as I expected. However, fully pulling apart the car to get it painted is a very long way off, I have an aging roof on my house and a 20 year old daily driver.

This will be my third summer working on the GXL just to get it to factory spec or close to spec mechanically and get a state inspection. Hopefully, I will finish or finish good enough for now..... very soon.

Edit: I searched and here are enough threads on here to attempt the trans swap. Hopefully some kits are still available. I was somewhat close with needed parts.

 

I'm 47 and 6'3ish depending on how compressed my back is feeling that day. (LOVE the heated seats in my Volvo) I've been competing with 1st-gens since 2001.

Not sure if he posts here anymore, but fidelity101 stage rallied an FC and he's taller than me, and stage rally requires some significant roll cage action. He did the RX-8 trans swap and loved it. Sadly he had a really bad off last year that basically totaled the car because of how much it damaged the cage, I'm not sure where he is at with respect to plans to rebuild or start with another RX-7 or just moving to a chassis that is easier to find parts for.

You're near BeaverRun, err PittRace. I used to autocross there when it was called BeaveRun, but with a late 80s Golf not my RX-7. By that point in my life the RX-7 was for dirt and I preferred front drive hot hatches for asphalt, and the Golf was a load of fun to autocross.

Definitely reach out to your local SCCA region, maybe show up to their next monthly meeting, and explain your case. I'm more of a cone dodger than a track rat, but judging from the e-mail I keep getting from the various local regions, road race series are ALWAYS looking for volunteers and if you are willing to learn to work flags you can attend every race you can make it to. I think NASA used to have, maybe still does have, a program where volunteering for X number of events gets you a free entry. One of my friends way back used to use this to his advantage to juggle college, starting a family, and affording his motorsports habit.

 

i think after the second one, at least on the west coast. my friends and i did it for years.

in the group we have a mostly stock 87 GXL, the ABS turns the tires octagonal

 

If your plan is to do track days, you can basically do whatever mods you want to the car in terms of power and gearing.

If you plan is to race, then you need to read the rule book, figure out what class you want to race in and understand what changes are allowed. If it doesn't say it is allowed, then it is not allowed.

It is highly unlikely that you will ever hit 170 mph on any race track in the US in a normally aspirated 13B so I don't see the point in using that metric...

 

Yeah, I commented on that one, it is rather unfortunate about the wreck. The navigator got the short end of the stick on that one. Looking at the damage I was very happy to see they both walked away. I wasn't sure about how well an FC would dirt race, I was very surprised and had loads of fun on the video game with 5.12' rear and comp gears.

Yes, in Beaver County near the turnpike. I can hear the track from my parents house. I am not sure how good it is but it's about a half hour down the pike away from me.

Thanks for the heads up. I need to get my car finished and get a new steering rack in my daily first. It could be late in the season.

 

It is highly unlikely I will even get out of fourth gear as the car sits. You would need a very long track....

I did actually write 170HP. Sorry I didn't capitalize it or something. I apologize for the confusion. Also, 170mph is way to fast for me. I was shotgun in a 12 cylinder Jag at 160mph many years ago on the highway. It felt very smooth but wasn't something I would want to repeat. Speed is addicting though so......

I chose 170hp because some drivers here said that it works well with 5.12 rear gear. I will have to do some testing to see how well a car will pull with 3.9 rear and the first 5 gears of the RX8 box. That 170 could change now. Considering that I don't want a super loud exhaust unless it is highly tuned and no turbo, I will have my work cut out for me.
Edit: As a side note the 170HP fits within the recommended high rage Hp that the diff and gear box can handle, for the most part.

Thanks for bringing this up. I need to look through the rules some more, they are kind of complex. I might just build what I like and hope for the best. I am certainly open to small changes with limits. If my build falls outside of the rules then.... maybe it wasn't meant to be. I will gladly knock over some cones instead and find an open track day somewhere. We'll see how it goes... keeping my hopes high though for a great experience whichever way it goes.

 

I run the S1 Rx8 6 speed with a 4:44 on my street driven rx3. I actually love it. Power is good thru all the gears. I run a big street ported 4 port 13b with a RB Holley and free flowing exhaust.

 

Thanks for the feedback.Best wishes to you and any of your family/friends if you were impacted by the floods.

 

Just a few obersavations about the RX8 Gen. 1 and comp gear boxes from very un scientific "testing" at 170HP on asphalt.

RX8 Gen 1 box is slightly faster on short courses the 3.9 gears than the comp box with 5.12 gears. Less shifting equals better times on short tracks.

About the same on both for medium courses maybe a slight edge to the RX8.

On courses with log straight aways, the RX8/3.90 set is best.

The RX8 Gen. 1 has a long 4th gear for whatever reason. The shorter 4th of the 99 Miata is about a perfect ratio for 4th.

The RX8 Gen.2 box seemed lackluster? From reading on here they are more durable for higher HP builds though.

Now, for street driving, the RX8 and 3.9 are a great combo as mentioned earlier unlike the comp with 5.12.

Thanks for the help with this everyone. I now have a plan forward and I will keep the 3.9 differential for future use.