What to do with the diff?
 

The rear of my FD is stock manual. I don't want to put anthing sticky in the rear because the last time I did, two of the gears in the diff split the long way and wedged in between the ring and pinion. Splitting the diff housing in two.

Do I:
Carbonetic diff and DSS axles ~$1200

or

Cobra 8.8 with DSS stuff ~$2200 This could be cheaper but I want to do it once using the cobra pumpkin.
diff $600
axle $599
DS $479
Samberg frame $670

Obviously I'd prefer the cobra rear but money is important.

I want to drop the torsen diff either way. I've had a couple situations where the inside tire came off the ground and nearly caused me to spin out.

My concern aside from price is power. If I go either route does the weakness become the outer hubs? I am also going to the Netherlands for a few years in August and I think a 3.73 gear might be better for the mileage. :-)

The suspension is fully done up. Including a Samberg diff brace and bushings.

Thanks folks.

How much power and torque are you putting to the wheels?

440/410 currently. I don't expect it to change too much for a while.

Do you track, autocross, drag or just spirited driving? There is a guy that is in the high 10's with drag radials and an s4 TII diff. This would be a much cheaper route for you.

You mentioned that your inside tire came loose. Is this because of the stock diff or maybe another issue in your suspension? I am thinking of a future swap and am aiming for 400 whp and was planning on keeping the stock diff. It will mainly be a street car with maybe occasional track and autocross, but I do not want it to be a safety issue. Your input would be appreciated.

You might be thinking of me. I am running an S4 TII diff inside my 3.90 FD pumpkin. I am running about 435rwhp/390 ft-lb and have been driving it pretty hard (50+ drag passes 1.5X 60's and high 10's) for 2 years now and it has held up just fine so far. It's the ~$300 option.

Hey Matt,

Have you upgraded the half shafts? If not I think that's the route I'll go. I'm sure someone would be willing to swap my 4.10's for a set of 3.90's. :-) Why the S4 instead of the S5? Cheaper with no real gain?

I do a little bit of everything. Auto X and drag mostly. I'll be in Europe by August so I'll probably add more track time.

As for the inside tire. It was a VERY tight uphill hairpin. Not too common to find anywhere but it was enough to make me want to go for a clutch style diff.

If you want longevity id say a rear-end swap is the best route. You can do the swap with other more available rear ends (thunderbird i think?) for much cheaper.

OEM halfshafts and I went S4 over S5 because it's a clutch style diff.... S5 is torsional style and IMHO not much of an upgrade (if any) over the stock FD diff unit. I do lots of autocross too. There is a member "Daniel" over on norotors that has a really good writeup on swapping S4 internals into an FD pumpkin. It's in his build thread... PM me if you can't find it.

It's still really expensive to go this route even with starting with a Tbird diff.

S5 is a viscous LSD and won't work with FD axles. (the FC stub shafts are different lengths) Nor do you want it...

I probably knew that in 2004.....been a while since I looked that stuff up. lol Thanks. I'll start looking for an S4 TII diff unless something cobra jumps out at me for the cheap. I'd still really love to run 3.72's in europe.

Matt,

Are you using 5th with a 129 trap or a big tire like the 26" ET street? My 4th tops out at 117 and I am definitely getting there before the 1/4 is up.

Looks like a 26" tire, 3.9 rear and 6600rpm redline will net you ~130 in 4th.

To answer your question, my car has the rev limiter set to 6,800 and I always stay in 4th. Your calculations are correct, since I run a 26X10.5 M/T ET Street I usually pass thru the traps at about 6,600 RPM.

There is actually a way to run 3.73's in an FD pumpkin, in fact if I can find some time, I might be running 3.73's this season! The secret is an uber-rare Mazda MPV: It's got be a 2WD 5-sp which is seriously a unicorn... it took me 2 years of searching to land a diff from one. They share the short pinion length of the FD and have a 3.73 gearset. The only difference is that the pinion diameter is 5mm less... people who are currently running 3.73's usually use a pinion sleeve to make up for the diameter.

Here's a picture of the MPV Pinion (left) and an FD pinion (right).

Attachment 675570

And a picture of the MPV diff still assembled. The tooth count is 41 on the Ring and 11 on the Pinion.

Attachment 675571

I had totally forgotten about the MPV....Sourcing that thing will be a pain....I bet it would be easier to find in Europe. They hate Auto's. Hrmmmm. I'll try while I'm still here.

So you just make a sleeve for the bearing and that's it huh? Are the companion flange splines the same too?

Can you find me one too? If I don't find one soon I'm forced to go with a cobra rear end.

so what is the best route for a rear diff if you want to use IRS?