an_juan

Jim, I like your style. As an engineer, I like to experiment with new ideas and designs and and relate with what you're doing. Screw the status quo, and do what you think is good. Good luck with the project!

jimlab

Or Super Street... I've seen riced Cavaliers in that rag.

Super Chevy was (and still is, as far as I know) considering doing an article on my engine build-up. We'll see.

jimlab

OK, time to revive this thread... I can already hear the groans.

Latest (revolting) developments: Talked at length with Ari Yallon about the 300M chromemoly axles that Rotary Performance sells and was told that they would not last. He said he's using 350M axles which they switched to after starting to break 300M axles when he was required to run DOT radials. He says the shock is much harder on the axles and they started to break the 300M axles again. We're talking about $1,600 for a pair.

Once they switched to 350M axles, the inner (green) cups which hold the "dogbone" joint of the axle and which have a short shaft that inserts into the differential were the next to go. They lasted only 2-3 passes, he said. They've now switched to cryo treating them, and they're lasting longer, but it's not a permanent solution.

And finally, we talked about the differential. The Kaaz differential will handle a lot of abuse, but the carrier is the next weak link. Ari said that the nose of the cast iron carrier will eventually crack and break as the pinion angle tries to walk forward under torque. The solution is to frankenstein it with some bracing welded to the carrier.

And after all this ($2,500+) it'll still be up in the air whether or not it'll hold the power.

I've started looking at alternatives, and here's one which uses a Ford 8.8" differential carrier, 3" or 3.5" aluminum axles (not shown) and a 5 on 4.5" (Mazda/Ford) bolt pattern. The fringe benefit, besides the strength, obviously, is that the shocks are mounted inboard, which gives even more clearance for wheel and tire width. I could run a 13" rim with a P335 with this setup without problems.

This setup is about $2,850 before the differential, gears, and axles.







Another option is a company that makes Ford 9" independent rear suspension conversions. The 9" is even stronger, also comes with 3.5" aluminum axles. Cost is $3,999 with steel axles, $350 more for aluminum, and a laundry list of other options.





This one does have outboard shocks, but inboard brakes. Reduces unsprung weight, but you'd have a limitation on wheel width.

Besides the cost of these kits, you'd have the headache of reconfiguring the exhaust, brake lines, parking brake cables, ABS, and a few other things that upgrading the stock IRS wouldn't require. There's also the consideration that none of these systems is tested in an RX-7.

So, anyone changed their vote about the Kaaz diff and axles? I'm leaning that way, but it's tempting to scrap the whole thing and do something different.

relvinnian

Oh my! You may as well have designed and built a mid-engine RWD sports car beast from the ground up. Okay maybe not, but it's quite an amazing undertaking. Almost a work of art I'd say. I were rich I'd be doing the same thing, I guess that's incentive to work hard!

Be different and go with a custom rear end. If Ari and his measly (in comparison of course) TQ curve, can break that re-enforced rear to pieces, I'm sure you wouldn't have much of a problem with it. Unless you want to be known as the next Darius, spinning through 5 gears?

j9fd3s

iTrader: (3)

so im confused, how are you going to be using this car?? are you launching at the strip?? are you going to be roadracing?? if its a straight line car you now have the opportinity to make the wheels not hop here.
option 1: put the kazz and axles into it and cross your fingers
2: do a fully custom rear suspension, might as well use a 9" 3rd member (the p type mazda is 9" btw), custom everything, tub it.
3: put a 9" pumpkin into the stock diff mount/ subframe.

one other thing we tried to put 335's on my friends car (it has 315's now) and you not only have to trim/roll the outer lip but bang out the inner wheel well too. it hits right in front of the inner wheel well.

mike

FastX7

DEFINITELY, I'd change my vote to that in a heartbeat. That's the best solution that I've seen. Not only would it work great for launching at the drag strip, it could be used for road racing as well. Best of both worlds in my opinion.

jimlab

I still need to figure out how to keep my ABS, if possible. I'd also like to have an operational parking brake.

The exhaust isn't really that big a deal. I may end up having a custom fabbed exhaust anyway, just to get a bigger pipe to the back of the car.

The two things I'm worried about are the delays that come with trying to custom fabricate something like this, and whether or not the performance is as good as the strength. The stock IRS functions very well, we know that already.

I'm going to use the car primarly for street driving with some drag racing. It'll probably never see a road course event, realistically speaking, and I don't autocross. I'm not even sure why I'm bothering to keep the IRS, except that it's a matter of pride and at the point that I lose the IRS, I might as well have just bought a Camaro to drop this motor in if you exclude body styling.

For what I want to do with it, upgrading the stock parts is probably just fine. I'm worried about the inner cup/shaft strength, and I'm worried about the carrier strength. Both will need to be addressed before I feel good about the power handling of the rear. The axles don't solve the problem all by themselves, nor does the Kaaz differential, unfortunately.

Still, I'll do some more research and see what I can find. I know that the guys who make the 8.8 Ford setup were looking at making a bolt-in IRS kit for Camaros based on input from several road racers, so they might have some good ideas for the RX-7.

FastX7

Jim,
I'll have to admit, I'm torn somewhere in the middle. Although yanked out the soul of the 7 I'm somewhat haplessly drawn to your project. Once I get the funds, I plan to do a project much like your's in the future, only involving a different car and components. I can't wait to see your PX-7 completed

jimlab

Personally, I almost wish that I'd done one that wasn't so drastic. I think that an RX-7 with a stock or lightly modified LS6 (405 horsepower/400 ft. lbs.) and an upgraded suspension and brakes would be a very, very well-rounded performance car.

Of course restraint has never been one of my strong points...

Dr Wankel

Well they say "Go big, or go home."

Jim Swantko

Jim,

Let me slap you back into reality... you have done it right!!! Keep at it my friend. As for the rear end.. if you want solid rear... go for it. Who cares if it will have some hop in the rear.

If you want to keep it IRS... I'd go with the modified Ford 9".... they are strong enough to survive a nuclear blast. you've done what you wanted to this point... it's your car man.

Hell if you wanted a good all 'rounder with an LS6 you should have just bought a Z06... oh... you already did that. Now keep this beast going! You already have the nice "cute little performance car" in the Z06... Now it's time for the "Fire-breathing... scare the **** out of anyone in a hundred yards ... kick-*** machine that you've started".

Trust me on this one... take it from someone with as much restraint as you... only not the finances.

jimlab

Personally, I always liked "if you're gonna be a bear, be a Grizzly!"

But the Polar Bear is actually more dangerous... can I upgrade?

Bacon

iTrader: (12)

im no expert in custom fabing, but couldnt u find someone to custom fab the *cups* outa titanium? (cnc?)
or whtever is in the area of strenth required...i mean its got to be able to get done..

but then again im prob way off ,,but for the sake of gettin the car goin soon and keepin the headaches as little as possible

but a custom irs witha for 9" would be the shiznet... and not a guess of will it hold

Wade

iTrader: (1)

Jim,

I just read the 4th page of this thread, and I wonder why you are even worried about this? If I remember correctly, you are going to use street tires, which tells me that you don't plan to drag race this car, that you mostly want it to cruise and occasionally embarrass someone on the interstate.

If this is the case, you shouldn't need more than a KAAZ and the upgraded axles. The only reason people are breaking those parts in the first place is because they are launching at redline with slicks. Even if you have twice the power, you shouldn't need more than the diff and axles if you are just driving it on the street.

Wade

jimlab

I've considered having the remaining "weak" parts fabricated, but keep in mind that just the 350M axles and differential are already $2,500, and custom fabrication is very expensive. At some point, it's no longer cost effective to try to keep the stock pieces. Especially when you start talking about custom fabricating titanium pieces.

jimlab

Well, there's a certain attractiveness to just upgrading some of the rear of the car and waiting to see what, if anything, breaks. But there's another philosophy which says you should fix what you can foresee being a weak link in the system while you're at it and already have the parts out of the car. Not having to fix things twice is always nice.

The car would only have to be drag raced once to break everything in the back if I didn't take precautions. A 200-250 shot of nitrous on top of a motor making 650 hp and 570+ ft. lbs. of torque is going to be hard on parts. I'd rather not have to trailer the car home from the first pass at the track, or find myself broken down somewhere because I didn't take the time to identify and address potential problem areas in the drivetrain while I had the chance.

It might be possible to get away with just a Kaaz diff and axles, and who knows, maybe it'd work fine. But I've learned the hard way that it's not fun finding that you can't get away with what you thought you could, because it will rarely (if ever) happen in a convenient situation.

LUV94RX7

iTrader: (3)

My opinion is don't drag race it and break it at the track. Just cost $$. Drive it on the street and have fun. Save your money and live with the street performance. It will be plenty good.

Ken

SPOautos

Jim, If you need some professional fab work done there is a vender that we use here at work. They have 30+ CNC machine stations and say they can and will make anything. They also specialize in doing it in small quantities so unlike most big cnc machine companies you dont have to buy a million of them to get a decent price. I'm sure it wont be cheap though.

If you want thier info email me at SJWhite2000@aol.com and I'll send you thier phone number.

Good luck,
STEPHEN

jimlab

Thanks Stephen.

j9fd3s

iTrader: (3)

i got an idea, why dont you make a beefy subframe, that uses the fd supension arms and geometry, but holds a 9" pumpkin. you need to take the best of both setups, the lightness and geometry of the stock suspension, and the unbreakableness of the big rear ends. you need to make hubs that will take bigger halfshafts.

mike

jimlab

Well, at some point you might as well scrap the stock suspension. Tubular control arms are even lighter than the forged aluminum arms in most cases.

Hanging a 9" Ford rear end in the center with inboard brakes gives strength and reduces unsprung weight in the rear of the car. Having a tubular control arms would allow a 3.5" aluminum (or carbon fiber) axle and would decrease weight further, and you could design the kit to use the stock shock mounting point and therefore work with both stock and aftermarket shock and spring kits.

The design of a hub is not a trivial matter, though, and it would be nice to use something already available. I don't think that you could adapt the stock rear knuckle to take advantage of a bigger axle without introducing a weak point into the system. At that point you might as well just design aftermarket axle ends for the stock system and stick with what you've got.

It would be interested to have a fabricated differential housing, though, to eliminate the weight of the cast iron stock unit and introduce strength where it's needed, to keep the pinion gear from walking out the front of the diff.

80-CU.IN.T

Hey Jim

It has been a while since I have read this tread. Took me 15 min. just to catch up.

Forget the 8.8 and go with the 9". It will be diffiult to destroy the housing or the center section The center section is very easy to work on. The hole peice comes out and sits on your bench. Makes life sweet.

So after you figure out how to keep your rear end in one peice, what are you going to do about your chassi. 6 or 8 point cage?

And you know what. I bet you do make it to a autocross. You are doing such a through job on your car it would be a shame not to.

Best of luck.

BLUE TII

iTrader: (9)

I couldn't help but think the same thing as the last person to post as I read this thread. Are you building a full cage that goes through the firewall to reinforce the front subframe mounts as well as the rear subframe mounts and seam welding the uni-body? I can't help but wonder if some of that 500+Lbs more weight a Corvette or Viper has is due to a chassis designed for higher torsional forces. I know big engine swaps in light cars in the past have resulted in twisted up cars; I would hate to see this happen to a beautiful 3rd gen.

FastX7

I'll have to go along the same lines. A guy that used to live down the street from me had a supercharged 350 cu. in. 69 nova on a 200hp shot of nitrous. The first time he ran MT slicks at the track, it twisted up the frame slightly. Enough to make it look like he was three-wheelin almost. Just curious as to what will cure that or have you addressed it yet?

jimlab

Agreed. Plus you have all the great aftermarket ring and pinion sets and other parts your heart desires.

The car has a 5 pt. (technically 4 pt.) bar in it right now. I haven't been willing to put door bars in it yet because it won't be a drag only car, and I barely fit in the stupid thing as it is. Making it unusable for the street is not one of my goals, and swing out door bars tend to end up messing up your door panels if you're not careful. I've considered a fully removable bar with a post on the floor and lynch pin and a clevis and pin on the bar itself, but haven't bothered yet. I'd do both sides while I was at it.

I was going to cage the car, but in order to keep a full interior, you pretty much have to remove the windshield in order to get the bars as close as possible to the A-pillars, and that means eliminating the A-pillar trim also, and cutting the dash. I didn't want a hacked-up car that looked like it lived at the drag strip. I wanted a full interior car where you can't even really tell that it has a bar or 6-pt. cage unless you looked hard.

Drag racing will be what, maybe 1-2% of its use?

You're right, it would be a shame. Ideally the car is built to be a good One Lap/Silver State Classic car. An excellent road course car, assuming I keep an IRS, and I will if at all possible. And it'd probably make a good autocross car, with lighter weight than the Z06 and far more low end torque plus a 1,500+ rpm higher redline. You could leave it in 1st gear almost.

Thanks for the support, I appreciate it.