SCCA Street Modified RX-8
05-21-2018, 06:05 AM
#1
SCCA Street Modified RX-8
I guess this is what happens when I get bored... I decide to post threads. I've been (slowly) working on developing an '05 RX-8 (base!) for SCCA Autocross, in the Street Modified class. For the past six years I've been running a BMW 330Ci in D Street Prepared, but years ago ('05-'07) I ran an RX-8 in what was then B Stock. I have a lot of great memories of those days and wanted to get back into an RX-8. Street Mod seemed like the perfect place. Lest you think I'm not a rotary guy, I owned an '87 RX-7 TII from 1994 through 2001, and have done several rotary engine rebuilds over the years. I ran a '90 RX-7 GTUs in STS for a couple years. So while my RX-8 won't have a rotary... I do have some fond memories of them.
But instead of starting with the details of the engine swap, I'm going to start ... at the back. One of the challenges with Street Modified is power potential and tire fitment are essentially unlimited. With big power and big stick comes potential for breakage. Knowing that the stock RX-8 diff and transmission aren't known for being bulletproof, I decided a swap was in order, so I teamed up with the guys at Ronin Speed Works (Ronin Speedworks) for one of their Ford 8.8 IRS kits. The 31-spline 8.8 is a very strong differential and has been used for years in big-power drag race RX-7s. There are tons of gear ratios available. And they're plentiful. I pulled mine from an '08 Ford Explorer. Figuring it would be really heavy, I had to weigh it against the RX-8 diff.
82 lbs for the stocker.
68lbs. Not a huge difference, but for a much stronger differential it's a nice side benefit. I got this from a salvage yard, with the axles, for $275.
But instead of starting with the details of the engine swap, I'm going to start ... at the back. One of the challenges with Street Modified is power potential and tire fitment are essentially unlimited. With big power and big stick comes potential for breakage. Knowing that the stock RX-8 diff and transmission aren't known for being bulletproof, I decided a swap was in order, so I teamed up with the guys at Ronin Speed Works (Ronin Speedworks) for one of their Ford 8.8 IRS kits. The 31-spline 8.8 is a very strong differential and has been used for years in big-power drag race RX-7s. There are tons of gear ratios available. And they're plentiful. I pulled mine from an '08 Ford Explorer. Figuring it would be really heavy, I had to weigh it against the RX-8 diff.
82 lbs for the stocker.
68lbs. Not a huge difference, but for a much stronger differential it's a nice side benefit. I got this from a salvage yard, with the axles, for $275.
Last edited by John V; 08-06-2019 at 10:36 AM.
05-21-2018, 06:06 AM
#2
Anyway, onto the Ronin kit.
The kit is really well packed and comes with everything. I'll go into what "everything" is as I go along.
The first piece in the kit is this rear mount. This will bolt to the rear cover of the final drive unit, and the ears will accept bushings and then fit to the rear mount on the RX-8 subframe without modification.
The welds on this piece are really well done. It's made of 3/8" thick steel plate which has been laser cut and it's gorgeous.
This plate will bolt around the rear cover of the diff. But first I need to cut the factory Ford rear mounts off of the rear cover. I pulled the cover off to do this and broke out my angle grinder.
The kit is really well packed and comes with everything. I'll go into what "everything" is as I go along.
The first piece in the kit is this rear mount. This will bolt to the rear cover of the final drive unit, and the ears will accept bushings and then fit to the rear mount on the RX-8 subframe without modification.
The welds on this piece are really well done. It's made of 3/8" thick steel plate which has been laser cut and it's gorgeous.
This plate will bolt around the rear cover of the diff. But first I need to cut the factory Ford rear mounts off of the rear cover. I pulled the cover off to do this and broke out my angle grinder.
05-21-2018, 06:07 AM
#3
The final drive is upside down in this picture. The inside houses an open differential with 3.73 gears. I think that's the gear ratio I'll start off with, but I may need to go shorter in the future and rev the engine higher. We'll see. Either way, the ring and pinion look to be in good shape. The Explorer this came out of only had 70,000 miles and I got the axles from it as well, which will be important later.
I didn't get a great picture of the rear cover with the ears cut off, but you can kind of see it in the top of this photo. I also had to grind down the rear cover at the pry points. They were proud of the mating surface for the rear cover plate, so 1/8" needed to come off of them. This wasn't in the instructions but it clearly didn't fit well without grinding them down.
You can see how the plate fits flush against the rear diff cover with the bosses ground down a bit.
I didn't get a great picture of the rear cover with the ears cut off, but you can kind of see it in the top of this photo. I also had to grind down the rear cover at the pry points. They were proud of the mating surface for the rear cover plate, so 1/8" needed to come off of them. This wasn't in the instructions but it clearly didn't fit well without grinding them down.
You can see how the plate fits flush against the rear diff cover with the bosses ground down a bit.
05-21-2018, 06:08 AM
#4
I'm at a bit of a standstill until I make the bushings for the front and rear mounts of the diff. The Ronin guys expect you to use the factory Ford (rubber) front mount and a stock RX-8 rear mount, but I'm going to machine solid bushings for both. Anyway... the next thing that needs to happen is to assemble the custom axles. Obviously an RX-8 rear half-shaft is not going to work with the Ford diff, and an Explorer axle will not work with the RX-8 rear hubs. So the Ronin guys give you new custom 4340 axles which are designed to mate to the Explorer CV joint on the inside and the RX-8 CV joint on the outside. Here are the axle shafts they send. Beefy!
Here's an Explorer half-shaft.
I only need this end of it... the inner CV joint.
To disassemble it, the band clamps holding the boots on get opened up. The boot slides up the shaft and the inner CV cup just comes off - the only thing holding it on is the boot which seems common with American cars. Once the cup is off, the CV joint is exposed. There is a small circlip holding this on, which comes off easily with a circlip pliers.
The circlip, which gets re-used.
A three-jaw puller extracts the CV from the Explorer axle shaft.
All the bits I need from the Explorer axles. The rest hits the bin.
The boots slide onto the axle shafts, then theb CV joints install using a 32mm socket and a deadblow, followed by the circlip. I'm going to stop here on the inboard side and move to the RX-8 axles.
The kit uses the outer CV joints and stubs from a 6-speed RX-8. They're very beefy and hold up to a lot of abuse. Disassembly is largely the same as the Ford axles in the beginning. Remove the boot, clean off the grease.
The difference is the RX-8 outer CV joint is held in place with a blind circlip. To get the outer CV off, you need to either pull it out or strike it off. I put the axle in my vise, put a block of wood on the CV joint and smacked it with my 10lb mini-sledge. It came off no problem.
The RX-8 CV joint looks a bit different than the Ford, and is fairly common for Japanese and European cars.
Here's that little bugger of a circlip, now installed on the new Ronin axles.
Ready to install on the new axle shafts. It's a little fiddly to get them on, because that circlip needs to be closed up with a pliers enough to allow the CV splines to engage. Once it clears the chamfer, it slides in with a little help from a deadblow.
After greasing everything up, the new boots can slide on and the retaining rings can be clamped into place. It's important to try to get the excess air out of these before clamping them down. Here the retaining clips are in place but not crimped.
I actually bought the correct tool to clamp these, back when I was working on Matt's car. It makes this job a snap.
And that's where I'm going to leave it for now. The Ford CV shells use a weird rubber adapter that the boot clamps around. New ones didn't come with the boot kit that the Ronin guys sent, and the ones from the Explorer were a little dry rotted so I'm going to order new ones before wrapping up the axles.
I designed the bushings I need to make and ordered materials, so the next thing will be installing the bushings. Then I can get the diff mounted in the subframe using the rear mounts, and prepare to weld in the front cradle. In our next installment...
Here's an Explorer half-shaft.
I only need this end of it... the inner CV joint.
To disassemble it, the band clamps holding the boots on get opened up. The boot slides up the shaft and the inner CV cup just comes off - the only thing holding it on is the boot which seems common with American cars. Once the cup is off, the CV joint is exposed. There is a small circlip holding this on, which comes off easily with a circlip pliers.
The circlip, which gets re-used.
A three-jaw puller extracts the CV from the Explorer axle shaft.
All the bits I need from the Explorer axles. The rest hits the bin.
The boots slide onto the axle shafts, then theb CV joints install using a 32mm socket and a deadblow, followed by the circlip. I'm going to stop here on the inboard side and move to the RX-8 axles.
The kit uses the outer CV joints and stubs from a 6-speed RX-8. They're very beefy and hold up to a lot of abuse. Disassembly is largely the same as the Ford axles in the beginning. Remove the boot, clean off the grease.
The difference is the RX-8 outer CV joint is held in place with a blind circlip. To get the outer CV off, you need to either pull it out or strike it off. I put the axle in my vise, put a block of wood on the CV joint and smacked it with my 10lb mini-sledge. It came off no problem.
The RX-8 CV joint looks a bit different than the Ford, and is fairly common for Japanese and European cars.
Here's that little bugger of a circlip, now installed on the new Ronin axles.
Ready to install on the new axle shafts. It's a little fiddly to get them on, because that circlip needs to be closed up with a pliers enough to allow the CV splines to engage. Once it clears the chamfer, it slides in with a little help from a deadblow.
After greasing everything up, the new boots can slide on and the retaining rings can be clamped into place. It's important to try to get the excess air out of these before clamping them down. Here the retaining clips are in place but not crimped.
I actually bought the correct tool to clamp these, back when I was working on Matt's car. It makes this job a snap.
And that's where I'm going to leave it for now. The Ford CV shells use a weird rubber adapter that the boot clamps around. New ones didn't come with the boot kit that the Ronin guys sent, and the ones from the Explorer were a little dry rotted so I'm going to order new ones before wrapping up the axles.
I designed the bushings I need to make and ordered materials, so the next thing will be installing the bushings. Then I can get the diff mounted in the subframe using the rear mounts, and prepare to weld in the front cradle. In our next installment...
Last edited by John V; 05-21-2018 at 06:11 AM.
05-25-2018, 06:03 AM
#5
I found time this week to make the bushings for the rear mount on the lathe. The bushing ears that Ronin fabricated are designed to accept a factory or aftermarket rear isolator for the RX-8. I chose to make my own out of black delrin with a 7075 center sleeve since I had the materials already and the aftermarket delrin ones are on the pricey side for what they are. Here's the rear mount bolted to the modified rear cover of the 8.8.
Next up is making the front bushings for the 8.8 which will be a little more complicated
Next up is making the front bushings for the 8.8 which will be a little more complicated
05-25-2018, 08:33 AM
#9
More fun racy parts courtesy of Fusion Works Fabrication. New series 2 front lower control arms with spherical bearings. I wish I could claim some sweet fab skills for these, but all I did was provide a credit card number. Definitely well beyond my fabrication skill set. Now, if only I had a subframe on which to install them.
05-25-2018, 08:51 AM
#10
What am I doing here?
Join Date: Mar 2006
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I'm not seeing it so I'm just going to make something up...
3.7L Cyclone V6 from a Mazda 6/CX-9 mated to a Mustang gearbox with all the 3.5L EcoBoost go-fast bits that will bolt on?
3.7L Cyclone V6 from a Mazda 6/CX-9 mated to a Mustang gearbox with all the 3.5L EcoBoost go-fast bits that will bolt on?
05-26-2018, 08:26 PM
#15
Time to make the bushings for the front of the final drive. Bit of a weird configuration, so I made the bushing as two pieces, a complex shape that presses into the front ears of the diff, and a disc to serve as the lower isolator. The Ronin guys sent me a link to a guy that makes something like this, but I thought the version I came up with was a little better designed and didn't rely solely on the press-fit to hold the diff in.
Sorry the pics are so dark.
With the front cradle mocked up. Everything fits surprisingly well. The Ronin guys did a really nice job with the front piece. I think just a little minor grinding will be necessary to tighten up the gaps for a nice weld. I have a bit more work to do before this is ready to weld but it's mostly just prep.
This is a shot of the rear bushings I made, fitted to the Ronin plate, bolted to the RX-8 subframe.
This is the front mount, with the bottom isolator bushing installed. You can see the gap from the Ronin front plate to the subframe is just a bit on the large side, probably okay for a MIG weld but a bit too large for a TIG.
Sorry the pics are so dark.
With the front cradle mocked up. Everything fits surprisingly well. The Ronin guys did a really nice job with the front piece. I think just a little minor grinding will be necessary to tighten up the gaps for a nice weld. I have a bit more work to do before this is ready to weld but it's mostly just prep.
This is a shot of the rear bushings I made, fitted to the Ronin plate, bolted to the RX-8 subframe.
This is the front mount, with the bottom isolator bushing installed. You can see the gap from the Ronin front plate to the subframe is just a bit on the large side, probably okay for a MIG weld but a bit too large for a TIG.
06-05-2018, 03:35 PM
#17
Registered
I was recently looking at the NC tubular subframe and wondering if it would be an easy button for putting the MZR in the RX8
https://v8roadsters.com/product/mzr-...ight-subframe/
John, with the NC subframe is the MZR hitting the firewall on the RX8?
https://v8roadsters.com/product/mzr-...ight-subframe/
John, with the NC subframe is the MZR hitting the firewall on the RX8?
06-06-2018, 09:01 AM
#18
There is no easy button for putting an MZR (of any flavor) into the RX-8. The V8 roadsters subframe makes moving the engine mount location easier as compared to the stamped and welded stock MX-5 subframe, but regardless you're going to run into clearance issues with the RX-8 firewall, cowl, and whatever steering rack you choose to use. With the engine mounts in the stock MX-5 location, the engine wants to be about 4 inches inboard of the RX-8 firewall, and the VVT sensor wants to live inside the cowl (where the wiper mechanism is).
I'm still working through those challenges with mine, as soon as I have them figured out I'll share it. The engine fits without modifying the firewall, but it requires a lot of work to do it within the SM rules.
I'm still working through those challenges with mine, as soon as I have them figured out I'll share it. The engine fits without modifying the firewall, but it requires a lot of work to do it within the SM rules.
06-07-2018, 06:36 AM
#19
Registered
Subscribed. If I ever give up the Rotary in the RX-8, I have long been thinking about a NC Miata Swap with a Flying Miata Supercharger kit on it for a great daily driver of an RX-8.
06-15-2018, 03:03 PM
#20
Continuing work on the 8.8 swap. Final test fit before grinding everything clean and welding.
Good fitment to the front of the subframe, TIG welding this was not a problem at all.
We also put a bead on the underside, where the rear of the front plate meets the RX-8 subframe.
Cleaned, primed, and rattle-canned black.
Here's a close-up of the lower "legs" of the front plate welded to the subframe.
Another close-up of the welding on the front plate / subframe interface, as well as the front bushings, with the final drive installed. There were no fitment issues after welding.
Top down view with the final drive all bolted up. Looks like it was meant to go there! The Ronin kit is awesome...
Good fitment to the front of the subframe, TIG welding this was not a problem at all.
We also put a bead on the underside, where the rear of the front plate meets the RX-8 subframe.
Cleaned, primed, and rattle-canned black.
Here's a close-up of the lower "legs" of the front plate welded to the subframe.
Another close-up of the welding on the front plate / subframe interface, as well as the front bushings, with the final drive installed. There were no fitment issues after welding.
Top down view with the final drive all bolted up. Looks like it was meant to go there! The Ronin kit is awesome...
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