jimlab

I've been doing a lot of thinking now that my engine is nearing completion (well, within the next couple months... not bad for so many custom parts) and I've now reached the conclusion that there's not a chance in hell that the RX-7 differential and axles, upgraded or not, are going to hold up to this kind of power.

I knew the stock components wouldn't handle the power from day one, but my plan was to replace the differential with a KAAZ unit, and use Rotary Performance's chromemoly axle replacements. Now I'm not so sure that it wouldn't just be wasting $1,500+ (not including labor) to put those parts on the car, because the engine will likely just snap the axles like matchsticks the first good hard launch, regardless of what they're made of. I'm also not entirely sure that they'll stand up to repeated power shifting. Keep in mind that stroker motors which make less power than mine tear the hell out of Chevy 10-bolt differentials right and left, twist drivelines in half, and just generally "tear **** up".

Paying for the parts and then breaking them is not the smartest idea, obviously, not to mention what kind of damage could happen to the car if an axle lets go at speed, or the differential comes apart and welds itself together, so now I have a dillema.

After giving it some thought, I don't know why I'm bothering to keep the IRS at all. It's not as if I'm going to use the car for track events, and everyone pretty much knows that I'm a drag racer at heart, so what do I need IRS for? Initially I was trying to prove that you could keep the balance, weight, and the handling of the car as good or better than stock, even with an iron block in the front, but I realized that it really doesn't matter if the car isn't set up for what *I* want to do with it. Let someone else prove that handling doesn't suffer.

So I thought I'd set up a poll, since there are other V8 conversions in progress, and people possibly considering one, and see what everyone thought was the best choice for handling the power of a V8 was as far as the rear suspension is concerned. A stock or mildly modified LT1 isn't going to cause major havoc, but an engine that makes 400+ ft. lbs. at just off idle and just climbs from there will eventually put the hurt to everything behind it, so although I was already planning on beefing up the back of the car, now I'm at the point where I should start deciding how much "beef" is necessary.

As you can see, the options above outline the possible choices. Let me know what *you'd* do if it was your car, even if you don't necessarily agree with V8 conversions, and think that it consitutes rotary blasphemy.

At this point, would you do whatever it took to maintain *an* independent rear suspension in the car? Trust to the KAAZ differential and chromemoly axles in the stock setup? Say screw it and hang a narrowed 9" fabricated differential in a 4-link style arrangement and mini-tub the car for P345s?

Thanks!

relvinnian

Wow Jim I'm touched, you actually care what we THINK? Ahh, didn't think so. It's all about what makes you happy. Honestly I think for pure drag you'd be better off with a twin turbo V8, but all motor is beautiful as well. You might as well go with a Ford 9, solid rear. In my opinion though, a C4 IRS would just rock =).

-Brian

jimlab

Not the best picture in the world, but all I could find on short notice...



The fringe benefit of the C4 Corvette ('84-'96) independent rear suspension is that everything except the rotors, including the carrier, is aluminum.

And look at those half-shafts... they're the diameter of an RX-7 driveline.

jimlab

From Rotary Performance's web site (www.rx7.com)

"For the serious drag racer, we now offer state-of-the-art axles which are an EXACT replacement for the stock axles. Under serious racing conditions, the stock axles are well known to twist and snap like a twig. We've been known to bring 3-4 PAIRS of axles to an event for spares before we began using this new, high-tech axles. For you material engineers out there, our axles start life as 4340 Chromoly Steel which is then vacuum melted argon degassed. This new material, called 300M, is the same material Indy car axles are made from using the very same technique. The axles are then polished for maximum tensile strength and come delivered to you ready to install."

RP Chromemoly 300M axles (pair) - $795

KAAZ 1.5way LSD - $880

Looks like they raised the price on the Kaaz diff since the last time I checked.

dclin

iTrader: (4)

Hmmm, Unobtanium Axles.... Oh just tub it out and sell me all the shiny stuff, will ya?

LT1-7

I am in the same situation your in Jim:p I'm probably going to go with whatever you go with, that's after you've tested your setup out 1st of course

LUV94RX7

iTrader: (3)

When you get your engine finally completed, I'd dyno it and get the real numbers it produces. I'd talk to the Kaaz and RP guys and see what they say. My guess is with a strong launch with real sticky tires that combination will break. Then check with the guys that run the Corvette combination and see if that can handle it. My guess is maybe it can. You seem to have unlimited
$$$ to spend on this dream car. If the Corvette stuff does not work go to the next step. Can always run with tires that will go up in smoke and not put that much strain on the driveline. I'm sure the RX7.com racing team with there souped up FDs and dumping NOS in at the launch are putting down some really big power numbers to get their 1.5 60' times. I'd check with them first.

Ken

jbontke

Jim,

The Mega Monster Miata uses a ford thunderbird rearend to hold the power to the ground. Have you looked ino this for an option. From what I understand the Miata and RX7 share a relativly similar rear end design.

Johnny

C4 OR WHATEVER...I agree...kaaz and even rp axle are not going to do it.

Mykl

Wow, drag racing. How exciting.

SPOautos

What rear does Chrisspeed use??? He's putting down right at 700rwhp but its a track only car so I dont know if its anything you'd want to use. Pluto is also putting down in the neighborhood of 600rwhp and he drives on the street all the time. I think he is using the upgraded axles from rp but could be wrong. I saw a thread a month or so ago that said they were coming out with a new set of axles that will be even better than the current ones but prob cost over $1000 and maybe close to $1500. I think Pluto was the one I saw talking about thier new axle thats coming out so you might want to ask him about it.

You could always just have the cryogenics (sp?) treatment done to them. I think it add in the neighborhood of 500% strength and should cost considerably less that those upgraded axles.

What do you plan on doing? Are you going to be driving on the street or just driving to the track and back??? What situation is the car going to be in?

Later,

jimlab

Far ahead of ya, Ken. Part of the huge bill I just got is for dyno tuning the motor with the SEFI-8LO. The price is subject to change, since that only includes a single cam shaft, and it may require more than one to find just the "perfect" combination and get every last bit of power out of the engine.

There are quite a few street driven and drag raced 9 and 10 second C4s with the stock IRS, including the "1,000 horsepower" nitrous injected 421 which was on the cover of Vette magazine about a year ago. They're not bulletproof, of course, but close enough.

jimlab

Thanks for the info. The car will be street driven with occasional trips to the track. It'll be running P315 drag radials 24/7, if I can find a way to get them to fit (already have coil-overs and the M2 Performance trailing arms) and possibly running 10.5" slicks at the track, although I'm not a big fan of slicks and IRS.

Steve's car makes a hell of a lot of power, obviously, but you have to remember that the rotary (even with nitrous at the launch) makes the bulk of its power at higher rpm. Not to say I won't also, since horsepower will peak around 7,600-7,800 rpm, but you have to keep in mind that I'll be making 400+ ft. lbs. of torque at just off idle, and by 3,000 rpm I'll probably be making more torque (and continuously) than Steve makes at peak. One of the advantages (or disadvantages?) of having 6.5 liters of displacement. It's the low rpm torque that I'm worried about. When the car is in motion, it's less of an issue unless you downshift under power. But from a dead stop, something has to give. Hopefully it'll be the tires, but I'm not in a big hurry to find out if eventually it'll be the axles. I'm not a big fan of walking home either.

LUV94RX7

iTrader: (3)

It looks like you are an experienced "drag racer". I know my nephew is a good drag racer. He has to do a lot of tricky clutch slipping to get his 190rwhp "busa" to get low 9s and 140++mph in the 1/4 mile. Trouble is he goes thru clutches, but that is the only way to get good launch, else dumping the clutch the tires just spin.

Aren't there tricky drag racing clutches to put the power down without dumping the clutch at launch?

Ken

jimlab

Well, if I were dead serious about drag racing, I'd simply back-half the car and would have gone with a Liberty or Jerico transmission instead of the BW T56, but primarily this will be a street driven car, and I wasn't willing to compromise that and build a trailered track-only car.

My clutch is a McLeod aluminum Street Twin twin disc that will hold more power than I'll be making, so once it bites, something has to give. Either the car moves, the tires spin, or something twists or breaks. Eventually the power will fatigue the rear components even if they don't break immediately, and I don't feel like replacing parts on a regular basis, so I want to build for strength from the start.

jimlab

I'm surprised, Mykl, usually you're more open minded.

Not everyone has a track in their back yard, and I've never had anything against orange cones that'd make me want to kill a bunch of them in a parking lot with a bunch of other weekend warrior pseudo-racers. Ouch!

I have no interest in autocrossing and never have, and the only local road course is an ill-kept death trap where an alarming number of friends and local boys have lost or damaged cars, including an F40 burning to the ground.

I'm my only sponsor, so I'll stick to the stuff which I like and leave the track driving to people who like carrying a spare set of tires around, swapping brake pads, enjoy off-course driving, and want to drive long distances to do it. California is a long drive. I'm on the wrong coast and too busy for the One Lap. The Silver State Classic is more my style, but again, I don't have the time, and once again, it's a long drive from Washington.

Besides, I have a new 2002 Z06 that'll be showing up in a week or so that'll do just fine if I ever change my mind about turning corners.

RX-Midget

"If I were you"...
I'd use a 9" with a 4 link and panhard rod. Connect all the link mounts on the car to the roll cage and brace the rear. Make the mounts on the car adjustable so you can change the way it launchs and use coil-overs.

reason - a 9" can be built to take the power. The independent is pretty, but I don't think it will help you at all - you would have to get rid of all the camber correction to keep the wide tires planted square for max traction.

My $0.02

jimlab

Thanks. A Moser 9" might be the ticket.

I heard a rumor that there was a company that made bolt-in rear subframes for 1st gen. RX-7s for 4-link conversion, but I have not found any information on them yet.

A bolt-in solution that replaced the rear subframe in the FD might be something that others could take advantage of. Not having a coil-over to the inside of the tire or a trailing arm in the way might mean that it'd be much easier to fit a P315 or P345 without mini-tubbing the car or rolling the fenders.

I always wanted a set of 13" wide rims.

Mykl

Naw man, it's not that.

I'm just beginning to wonder why you chose an RX-7 for this. You take a beautiful car, that has a killer chassis, and drop a V-8 in it. Not a big deal to me, a V-8 is merely a different means to an end.

Honestly, the idea of having such a powerful high reving V-8 under the hood of an RX-7 sounds kinda neat to me. All that power tied in with the already wonderful chassis would be insane. Originally, I thought that was what you were after. The car would totally dominate anything on a road course, or a mountain pass.

I guess this just comes down to one thing I believe that the RX-7 is, and what it is not. I believe that the car truely shines as a sports car, it lives on curvy roads. I do not believe that the RX-7 is a drag car. To take an RX-7 and handicap it's ability to destroy anything on a curvy road is difficult for me to grasp. There are a lot of people who disagree with me on that.

Not to say that I'm not in complete awe of what you're doing with your car. It's just that I would beef up the chassis and suspension to handle the power and make no major changes to it, because it's already among the best at what it was designed to do.

Mykl

Oh, and I'm not saying that you're wrong for doing what you're doing, just that I would do it differently.

I'm not going to sit here and talk smack and enter a flame war, because in the end my opinion doesn't matter and it would be childish and stupid to try.

I can only voice my opinion, in the most respectful way possible.

DamonB

Christ, Jim. You anglin' for an exorcism? With what you are going to do with the car, I think your idea of completely replacing the rear end is a good one; both financially and reliability wise. I don't have any C4 experience so I can't help there. I think a solid axle would be best for the drags; no camber problems with squat. And I would imagine cheaper and stronger than any IRS.

Nice of you to rub it in that the Z06 is on the way:p

95R2-89TII Ground Zero

Man, drag racing is fun as hell.

relvinnian

Just letting you know... Crispeed uses a solid Mopar rear end. He also lauches bouncing off the 9k revlimiter, boosting around 20 psi. He's probably making about 500 ft/lbs of TQ right off the line. Those factors combined make for those spectacular wheel stands he does =). I'd use a Ford 9 rear, mainly because they are strong, easier to rebuild and swap than the mopar rear ends, and the ring and pinions can can be swapped very easiely. What kind of gear you using back there? 6-speed right? I believe the 5th and 6th gears in the T56 are overdrive gears. You could probably run 4.11's and still hit 200mph :p

-Brian

jimlab

I didn't chose an RX-7 for this... the RX-7 chose me.

It's not like I specifically bought an RX-7 with this in mind. I went through two engines, built a 3rd ported motor with 3mm ceramic seals, sold it unused, and even went so far as buying a 20B (which is a lot farther than most 20B conversion afficionados get) before being stalled by problems with a certain vendor. At that point I decided to stop and rethink what I wanted from the car, and sold the 20B. People ask me why I didn't just buy a Camaro or Corvette instead. Besides the fact that I just did buy a Corvette, I had the RX-7 sitting around with no motor. What else was I going to do with it?

Originally (and still) I was after a car that would dominate anywhere. The bottom line is, though, that in order to do that, I'm probably going to have to switch to a different solution for the rear of the car, or hope and pray that I don't twist the axles like rubber bands. If I thought that the 300M axles stood a chance, I'd go that route and keep the rear of the car "stock". A Kaaz differential would complete the "bulletproofing", or at least make the rear as bulletproof as the FD rear can get without major modification. But I'm not sure that it'll hold up, and it's a lot of money to gamble on something you have very little confidence in. And bottom line, it's not the money, since you have to know money isn't an issue by now, it's that I'm getting too damn old to want to walk home after I break my car again.

Chances are good that I will exhaust all options to keep the rear of the car "stock" before I go to the extreme of modifying it to accept another manufacturer's equipment, and I would go to that extreme rather than hang a 9" solid axle in the back, because I too believe that something will be lost if I go to that extreme. I am not a track driver, really, but I'm not a drag racer to the exception of all else, either. I'm not really trying to build a drag car here. I wanted something that someone else (with better driving skills) could (maybe) mop up One Lap with. I wanted something that would be a high speed contender for the Silver State Classic, even though it's unlikely that the car will ever see competition there either. In other words, I wanted an all-around car capable of doing anything that was asked of it.

I still do, I'm just having second thoughts about the Kaaz diff and 300M axles being up to handling the power I've created, which is a very real possibility. The car is too nice to have things snapping and breaking under the rear of the car, so I want to avoid it at all costs. If that means swapping in someone else's IRS, so be it. But trust that I won't put a solid axle under the back of the car unless there are no other options left.

Thanks Mykl, I value your opinion.

Magnificent 7

Whew, that's good to hear. I'm following your projects as closely as I dare, and with more interest than ability to act...right now.

What always sold me on the conversion was the "bolt on" aspect. After all the trouble you went through to fabricate the engine cradle and other pieces required to make it a "bolt on" conversion, it seems a shame if you have to tear up the rear of the car just to equip it with something that can handle all the torque. I'm sure you would devise the most well thought out answer, and I'm just as sure it would be best suited for the broadest range of possibilities, but if it involves mangling sheet metal in any way, I'd have to draw the line there.

Call me unrealistic or stupid or whatever, but whatever I do to the car, I want it to be "un-doable" so that I can return the car to its stock condition some day if I choose. I realize that may never happen, and the car may never be a Concours show winner, but I have a feeling that these cars are going to be as highly sought after ten years from now, as 60's muscle cars are today.

What would be the limit on torque and HP that I should observe if I want to keep the factory rear end, what about if I upgraded to the KAAZ/300M combo?

Have you developed a PPF that attaches the T56 and the stock IRS yet?

Will you be able to keep the factory instrument cluster and accesories (A/C, cruise control, etc) or have you delved into that can of worms yet?

Is the Silver State Classic a NORRA type race? I don't know much about it at all, but there is a part of me that wants to pilot a vehicle (preferably the one in my garage) going 200 or 200+ mph (I'm probably gonna need to buy a helmet to do that right?).

That's one of the my dark secrets that I don't tell anyone because they just look at you with a blank stare as if you asked them to borrow one of their children for a midnight human sacrafice/cannabilism ritual. Sheesh...some people....

I guess my answer is: either go with the modified stock set-up, or develop a way to make the C4 rear end a "bolt on" unit. Oh, what are the odds that you're working on a one-of-a-kind "jimlab special" rear end, completely designed and built under your supervision?

Thanks Jim. Good luck with it.