Diff questions

[DaveKern]

I'm trying to plan out what rear diff to use on my rally project and could use a little help answering a few questions.

First, a little bit about the intended useage of the car. It will see mainly competition in the dirt, so the diff will be working hard (lots of slip limiting), but probably not seeing large shock loads like you'd see at the drag strip or on a road race course.

Second, a bit of info on the rest of the powertrain. I've just sourced a 1995 S50 engine and transmission, and my plan is to go with a diff in the 4.xx range, as rally top speeds are rarely over 115mph. I'm trying to be a bit weight consious, but if reliability is going to be a concern with the small diff, out it comes.

Finally, on to the questions:

1) what's the recommended HP/torque rating on the factory (small) diff?
2) anybody have weights of the small and medium diffs?

Thanks!

Dave

[cooljess76]

Small case diffs are good for around 150hp. Anything over that you really should consider a medium case diff. As for weight, I happen to have a few diffs sitting in my garage and would be happy to weigh them for you tomorrow. You have to consider halfshaft weights though. Medium case diffs will use shorter/beefier halfshafts than small case diffs. I know it's obvious, but a heavier diff will more than likely be stronger. The weight isn't really a problem since it's supported by the chassis and has a low center of gravity. For off-road rally style driving, you'll want weight on the rear axle anyway. If you have the cash, I'd highly recommend a Quaife diff. They're a little expensive, but it'll probably be the last diff you'll ever buy for the car, provided you practice good maintenance and change the fluid regularly.

[DaveKern]

Cool, thanks for the info. Looks like a medium case is going to be the way to go. The engine I picked up already has an intake & chip, so I'm under the assumption it'll be north of 250hp at the crank.

For rallying, alot of guys like to just pack extra clutch packs into a diff. I may have my names mixed up here, but IIRC, the Quaife's are great for road racing, but they suffer from "open syndrome" when you have a wheel off the ground, which happens a bunch when you're bouncing around on a rally.

I've used Cusco and Carbonetics on the Evo, but for costs sake, I may try the extra clutch packs this time around. I've also had a buddy that recommended looking at having a Toyota Supra rear diff fitted, as lots of rock crawler guys ensure that there are plenty of high ratios to choose from. Still not sold on any option, but I like the idea of keeping things simple (and as inexpensive as is reasonable).

Dave

[cooljess76]

I'm sure you already know, but regular e36 diffs won't fit the Ti. You'll need a Z3, Mcoupe, e24, e28 or e30 diff. The Z3's and Mcoupes came with Torsen style LSD's while the e24, e28 and e30's came with clutch style LSD's.

[pdxmotorhead]

At the power to weight ratio your going to be in, the weight of the diff is good or at least no big deal. Its weight in the right place. I'd always give up a little weight for reliability.

Coil over shocks will drop 5 lbes each in the front.

I'd get 2 diffs, a quaif for all weather and a spool for dry hardpack.
Just have someone tig the spiders together and go....

Dave

[DaveKern]

Originally Posted by cooljess76

I'm sure you already know, but regular e36 diffs won't fit the Ti. You'll need a Z3, Mcoupe, e24, e28 or e30 diff. The Z3's and Mcoupes came with Torsen style LSD's while the e24, e28 and e30's came with clutch style LSD's.

Yep, I did catch that I'll need to look for something from a 6-cyl E30. There's good info floating around on this forum.

Now that you mentioned Torsen diffs....I think those are the ones I was thinking about that go open on wheel lift...but maybe its that Quiafes are based on the Torsen design so suffer the same issue?

Dave