2nd gen RX-7 rally car - brainstorming anyway

Hokay, I figure why not make a thread on this. I've been kicking this around in my head for quite some time and think I've got it figured out how to even do it as both a rally and roadrace car. I'll explain my thoughts and reasoning and shit, because why not? Also, yeah, this is going to be a while off no matter what - I have other financial priorities right now, and a car ain't going to trump them. I do have fab ability, tools, shop, welder, benders, etc etc etc - done roadrace crap before so I do have a pretty good idea what's involved in car prep.

And I'm posting this as a thread rather than calling John because it's going to be a little while off, as I said, and it feels kind of douchey to take up people's time when I'm not at the point of paying them what their time's worth.

Basic car is 2nd gen RX-7 - figure it's pretty much the same sort of thing as the Xratty in terms of suspension. Advantage is that it's a lot more available around here for replacement body bits. Also I'm a rotary perv. Wheelbase is shorter than the Xratty, but oh well.

Engine-wise, Group 2 requires a non-ported rotary. The RX-8 engine with a standalone seems like a good candidate; dynos have it doing about 200hp at the wheels with a tune, with a nice broad torque curve (90% of peak torque from like, 4.5-7.5k RPM, tapering off gradually to a 9k redline), I figure with a standalone and no restrictions other than a "cat" somewhere in the system that's easily doable. Also easily ebayable for new cores, and I know how to rebuild them. Upgrades for Group 5 duty are easy - peripheral port 13B, 13B with turbo, whatever.

Gears are pretty easy - Miata 5-speed with an RX-7 front cover is almost free as far as these things go and goes between 3.136 and 0.814, so about the same as the T5. Upgrade options down the line if there's too much cash sitting around, Quaife has synchro gearsets running either 2.564 to 0.794, or 2.35 to 0.881 for a couple grand, or I have a friend who started a business doing Hewland Mk5 gears into an RX-7 case in any ratio they're available in... but not sure on longevity there. Rear diff is the obvious Supra one, and I think I figured 5.71:1 would work.

Brakes... I think I've seen the Mazda brakes swapped onto crap as upgrades, rotors are like 11", there's good compounds available in the shape (at least, places that make them) and they fit under the OEM 15" wheels. Upgrade-wise, it can't be rocket surgery to get roundy-round stuff under the wheels, Wilwood or whoever.

Suspension isn't rocket surgery either - front struts have the classic mounting ear that bolts to the strut, rears would be coilovers and probably with the top mount moved up a bit. I figure Bilstein based is pretty much a given; we know who makes those.

Question in my mind is tires - whether there's any advantage to running something on the order of the 14/62-15 as opposed to 17/65-15 (or insert whoever's sizes - 175/70-15 vs 205/65-15 for Dmack, etc) other than the obvious gearing advantage... and with 5.71:1 final drive ratio, well. Or is narrower better on loose surfaces?

Roadrace trim would see something like SCCA's STU, which is a street tire class. That would see the obvious rally stuff taken off and wheels/struts/coilovers swapped with different units but otherwise the build should work with both. Supra diff isn't technically legal; possibly some hybrid of truck centersection and Mazda housing could work and be legal but that's a bit of messing around. With the rally tube sizes, it might be a bit overweight for the class, but I'm looking at it more in the sense of "I can run on track" rather than "I'm building something that will be a contender for national championships"

Anyway, anything obvious I'm forgetting or overlooking that makes this a dumb idea? And yes, I have a pretty good idea what the costs and time investment would be; I'm not interested in half-assing something that I'd have to transport 9 hours each way.

Don't feel bad, talking early makes later things easier.

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John Vanlandingham
Sleezattle, WA, USA

Vive le Prole-le-ralliat

www.rallyrace.net/jvab
CALL +1 206 431-9696
Remember! Pacific Standard Time
is 3 hours behind Eastern Standard Time.

You have friends at Mazdaspeed for OEM parts and contingencies.
JT

Oh yeah, well familiar with Mazdaspeed. Dad and I have been running roadrace '7s for something like ten years now... club level stuff but fun, and the Mazdaspeed program is great.

I built arguably the most successful 2nd Gen. RX-7 in North America (Group 5 national champ in 2002) so if you have any questions feel free to send me a PM or something. It had it's challenges but it handled well and was a good platform. Certainly better than a Gen. 1 RX-7.

Okay, more thoughts and questions, some of which are probably applicable to general stuff so I'll just sort of shotgun them out here.

Brakes - front and rear rotors are 10.9 and 10.75", so about the same... front pistons for the turbo calipers are 35mm/1.38" and there's four of them, rear calipers are, as far as I can tell from googling (I don't feel like taking them off the car right now, it's cold out) one 35mm sliding piston. Seems like that would give about a 66-70% front torque bias, is that in the ballpark or is more front good? Definitely going dual masters, so options easily exist. I saw 75-80% front being talked about in Subaru MC sizing thread, but that's AWD and probably a lot more nose-heavy than an RX-7 would be.

On the topic of brakes, any reason not to upgrade from the OEM 4-pots to Wilwood Dyna-whatsit jobbies? Rebuilt RX-7 calipers seem to be about the same price as new Wilwood narrow dynapros, and I have to imagine pads are cheaper for the Wilwoods, but is it worth screwing with? Also, compound-wise, I've used and like Raybestos ST43s on the roadrace car - Raybestos says they're suitable for rally/hillclimb/etc too so the temp range is probably broad enough, and they're available in the right shape, but anyone have experience with 'em on this sort of thing? Finally, I saw mention about RA being against brake reservoirs in the cockpit - wtf? Is that just a matter of running the suckers out past the firewall and having tin around the hoses, or braided hoses, or what?

Hoses now... in an ideal world, should everything be inside the cockpit, fuel, brake, etc? The car's pretty low and scraping off brake or fuel lines sounds kinda bad. Also, seems to be common consensus in rally circles that at this sort of budget level fuel cells are more trouble than they're worth, but is there any concrete reason not to? If it's going to be a dual purpose car like I'd like it to be, it looks like a cell's required.

Suspension-wise... enh, have an idea about turreting rears, and vague guesses on spring rates, and I bet John has better guesses on details and valving, so won't go into that part... what about sway barss? I know the really expensive blue Subarus have sways, but I think they're also running really soft spring rates because of unobtanium dampers... and I'm not building a blue Subaru of any sort anyway.

Bacon?

I thought I already answered some of these questions for you? There's nothing wrong with the stock Turbo II brakes setup, it works great. You can't do much better than the Mazda 4 pot aluminum calipers, it's what I'll be using on my Opel. There is no advantage to going to the trouble of converting to the Wilwoods. The Mazda caliper is better, quite frankly.

If you are replumbing then running the fuel and brake lines inside the cockpit is best. However, I didn't replumb anything on Dave Hintz's car and with full underbody plastic protecting it there has never been a problem in 10 or 12 years of rallying.

John will tell you to turret the rear shocks because he's all about the turrets but check the travel of the rear suspension, it could be that a longer shock would not give any more travel anyway. Then again, turrets may be necessary with the length of shock he has available, you'll just have to check it. We did not run any sway bars on Hintz's car. Especially on the rear you want to keep the wheels on the ground as much as possible for best grip.

You did answer a lot of them; just thought I'd throw out a few more that seemed a little less specific to this-specific-car. Or maybe I had some time to kill at work... in either case, it may be that I think too much, heh heh. Thank you; I'm going to just stop worrying about the damn calipers... it's just weird to me that stock bits are actually worth a damn.

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mekilljoydammit
You did answer a lot of them; just thought I'd throw out a few more that seemed a little less specific to this-specific-car. Or maybe I had some time to kill at work... in either case, it may be that I think too much, heh heh. Thank you; I'm going to just stop worrying about the damn calipers... it's just weird to me that stock bits are actually worth a damn.

The 4 pots are made made by Sumitomo--under a licence from Lockheed GB
about when AP was about to spin off...
With a booster, they're gooder-er than the little Dyna-lites.
Without a booster the pistons are too small. They would want to be minimum 4 x 38mm or 1.5"---oddly enough what everybody used long ago..
good pads are available.
Toyota pickup OEM sky blue pads--in the alluminum Maz-dog is what Rod Millen said he used a hundred years ago when I first saw those on his car.
He said " I don't know what they are but they work amazingly well and are kind to the discs and they're cheap"

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John Vanlandingham
Sleezattle, WA, USA

Vive le Prole-le-ralliat

www.rallyrace.net/jvab
CALL +1 206 431-9696
Remember! Pacific Standard Time
is 3 hours behind Eastern Standard Time.

Oh, interesting! Never realized where the suckers came from, historically speaking, and kinda neat that it's that far back. Hm; would a 5/8ths master work for a dual cylinder setup? The 1.38 to 1.5 change on caliper piston size is only an extra 18% area, so going down on the master seems like it should get the clamping force up. I know roadrace guys have the dual master setups work, but those are different pads, and I'm trying not to carry assumptions over without at least checking them.

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mekilljoydammit
it's just weird to me that stock bits are actually worth a damn.

2nd-gen RX-7s are a collection of nice parts that are in an unfortunate chassis.

Nice engine (for a starting point), nice oil cooler, nice brakes, and the front suspension is nice if you have an older car, but the rest?

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Pete Remner
Cleveland, Ohio

1984 RX-7 (rallycross thing)
1978
Silence is golden, but duct tape is silver.

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Pete

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mekilljoydammit
it's just weird to me that stock bits are actually worth a damn.

2nd-gen RX-7s are a collection of nice parts that are in an unfortunate chassis.

Nice engine (for a starting point), nice oil cooler, nice brakes, and the front suspension is nice if you have an older car, but the rest?

All of it is better than the 1st gen RX-7. Nothing particularly wrong with the rear suspension but you do tend to get uneven tire wear due to camber change.

thats why you shove it all into the 1st gen! (1.5 gen?)

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First Rally: 2010 First RallyX: 2004 (a bunch)
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Last Updated, Apr 9, 2023

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Doivi Clarkinen

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Pete

Quote
mekilljoydammit
it's just weird to me that stock bits are actually worth a damn.

2nd-gen RX-7s are a collection of nice parts that are in an unfortunate chassis.

Nice engine (for a starting point), nice oil cooler, nice brakes, and the front suspension is nice if you have an older car, but the rest?

All of it is better than the 1st gen RX-7. Nothing particularly wrong with the rear suspension but you do tend to get uneven tire wear due to camber change.

Short arms. Short like old BMWs..
Sumpin to be said for longers arms.

And agree the 2nd gen nothing basically bad --if you like short cars..

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John Vanlandingham
Sleezattle, WA, USA

Vive le Prole-le-ralliat

www.rallyrace.net/jvab
CALL +1 206 431-9696
Remember! Pacific Standard Time
is 3 hours behind Eastern Standard Time.

Originally when I was planning this crap out I was going to do a 1st gen. I have more spare shells, like them, have been screwing with roadrace versions of them for a decade or so, have bodywork molds, etc.

Problem is, with everything I know is wrong with the suckers on a fundamental level, I figure I'd have probably 3x the man hours into the car. 2nd gen front subframe swap or custom control arms, subframe, and rack and pinion... adapted over 2nd gen or Wilwood brakes... equal length 4-link rear with Escort style tunnels for the links, or 3 link... non-Mazda diff... etc etc ad nauseum. I know I can do all that crap, but it seems like a lot of effort for a first car when the 2nd gen is a lot closer to "fawkin' good enough" in most ways.

I can always go back later and build a Group 5 1st gen.