Hi guys,

I've got an FD with a 408ci V8 pushing nearly 650hp. The car is used for autocross and a lot of drags with slicks. What diff do I use? Ford 9"? Or what other diffs are used in FD's with 600+ hp and strong launches?

Help and advice is appreciated.
Thanks.

 

Cobra rear end can be used. You can do a search for it

 

A Ford 9" isn't really a diff, it's a whole rear end. People use the T2 diffs, Kazz, ext. You'll also be looking for a support to support the factory casting...

 

Solidify the torsen with new Bushings, and Pillow *****, eliminate wheel hop, and for my LS1 it's done just fine... (1.3 Years of track days, SCCA, and street driving) but I'm not pushing 600 HP

 

I am currently making a custom stock diffrential brace. What it will do is back up the diff support and tie it all into the PPF bottom two studs. I think it will really beef the housing up to keep it from spreading upon a torque load. I am almost done with the prototype which is being installed on my white FD. Pics will be taken later on.

later

 

There are a couple of Ford 9" setups that work with an IRS. But they're really big and I'm not sure that they'll fit in the stock rear subframe.

The 8.8 (Cobra in particular) has been adapted. It works well but won't support the massive torque that a 9" can take. But the 8.8 is far better than anything that bolts into an RX-7.

Dave

 

Thanks guys, so can I say that the Ford 9" rear end is the best option, but it probably wont fit into the rear subframe?

How much does the sub frame need to be modified? A lot? Only a little?
Who knows of anyone who has done it or uses it?

 

I put a ford 9" in my 85 gslse

 

I don't know of anyone using the 9" in an FD. I didn't say it was difficult, I just said I don't know if it will fit because it's definitely bigger.

Anderdick, how did your install go? Did you fit it as IRS or solid beam?

James, are you talking IRS or solid beam? For autox you really want IRS, and for drag you will prefer solid - so what's your priority?

Dave

 

How about a Kaaz diff with Driveshaftshop 30mm chromolly axles?
Do u think that will handle 650hp?

AutoX is my priority but really don't feel like replacing broken parts due to drags every now and then, so must be able to handle powerful launches.

 

Look into the diff cradle from PFS. That will support the case.

 

Ernie is using the Kaaz and chrome-molly axles. If you're doing auto-x I wouldn't use the Kaaz. The stock axles should be fine for the auto-x aswell. If they go it's just because of the fatigue. I say this as you're continuously moving not really providing harsh off the line engagements. Everyones a critic though...

 

Interesting comment, the two fastest AutoCross RX7's in the country both use a Kaaz. Why would you recommend not using it?

 

^I was just about to ask the same thing.

 

I always thought of the Kaaz as a harsh engaging diff. I'll bud out then...

 

Thanks guys.
I apreciate all feedback, anyone know of any pros/cons to the Kaaz diff and chrome-molly axles setup?

 

Ive heard that the Kaaz units are rather loud. I dunno if you would hear it over that V8 though. Cusco are supposed to be very good and quiet.

 

https://www.rx7club.com/showthread.php?p=6538599

 

ever thought about a nissan R200 diff? Ive never heard of anyone blowing these and they use em in the 2JZ swapped S2000s....

 

Here is what I was talking about above. This is my version of a diffrential "girdle" that beefs up the casing and resists spreading. Resembles the girdle I installed on my Z06.

Another mod from deep in the garage...

It includes new 10mm studs to replace 6 case bolts. If you check you will see that the stock bolt that attaches the aluminum cover only utilizes half of the case thread boss. These studs penetrate to the bottom. The cover plate is 1/4" cold rolled and the brace and strapping to the PPF bolts is 3/16" steel. Also I replaced the 14mm ppf studs with longer ones so I can incorporate my diffrential brace along with this girdle strapping.

Later

 

what engine and trans do you have in the car?

 

BadddRx7, that brace looks pretty good. I'm waiting on the PFS cradle i'll post pics once I recieve it...

Jay7...

 

To be honest, I'm sure it would hold up for a while, but I would bet that it will eventually break at that power (doing drag launches).

Unfortunately, there is no middle ground. You either have a very good drag car and a not so good AutoX car, or vice versa. If you intend on doing drag launches, I would just go ahead with the Cobra cradle:

http://www.v8rx7forum.com/v8-rx7-tec...naly-here.html

If you are going to be doing AutoX more, go with the Kaaz but I wouldn't use slicks for drags.

 

pg. 8 of new jimlab thread. A little progressin "other engine conversions"

 

One of the FDs here run a 9" in IRS configuration - requires a custom cradle - believe it has an alloy or magnesium case. Coincidently, cradle has cracked just recently.

Was used in a local V8 racing series here in live axle mode with similar power output you are seeking and also standing starts with slicks....although probably much harder compound than any drag slick.

From memory, circa $Aud7000 with GST though! Google Harrop engineering.

A few other drag FDs have done complete 9" live rear end swaps too for the cheap alternative.