Lets talk about LSDs.

That's limited slip differentials for the curious and hopeful.

The Abomination has a 1987 RX7 GXL differential in it. It's getting pretty worn and spinning the inside tire coming out of some corners.

I have two options.

1 - Order a bunch of parts, pull the differential, and rebuild it. Cost between $100-$500 depending on parts needed.

2 - Order a OBX helical LSD and install it. Cost about $450.

3 - Order some other high quality LSD. Cost $900+

Any thoughts. Which are better LSDs for autocross. This is strictly a autocross car. I do not care about street manners, it only needs to hook up and not cause other handling problems.

Is the diff in there now a clutch type? If so, I'd rebuild it. It'll be more tuneable than any other option of similar cost.

Correct, the current diff is a clutch type.

I'm in the rebuild camp. If there is nothing else wrong, seals and clutches should be all you need.

If you get the OBX, you can probably sell the RX-7 differential and recoup some costs. You can sandwich one of them in a '94 Miata pumpkin, too.

I'm interested to know more about these OBX diffs. They make a really cheap one that may be adaptable to my Saab...

In reply to Robbie:

Quaiffe's Chinese cousin aka a Chinese knockoff of a Quaiffe LSD.

They work well when assembled properly, which many of the early ones weren't.

I'd rebuild what you have, but if you want to experiment, find some parts to tune it with and spend some time in the off season testing.

I will also advise a stock rebuild, but I'll add in something by asking if you can shim the clutch pack tighter to gain a tighter limiting effect? Not sure what your needs are though, an LSD packed tight will throttle oversteer like a moose, and understeer like a duck off throttle. (Are these analogies doing anything for ya?)

Yep, these are totally tunable stock. Add a shim, flip some around, use beer cans, whatever.

You can get almost whatever breakaway value you want.

OK, so rebuild it is. I'll have to pull it and see what parts I need.

The top-o-the-line these days is an OS Giken. They've figured out how to put springs to work against the ramps to make the lockup more gradual. If you can figure out how to add that to yours, you'll really have something! Otherwise you can play with the plates to tailor your ultimate lockup percentage. None of your other options will beat the cost/value of just rebuilding the one you have.

If you are rebuilding... give some thought to release and lockup ramp angles. Adjusting when it releases and locks can be the difference between a neutral car and one you have to fight with. Smoothness of engagement/release is important at the limit.

IMO, the release entering a corner is the part that matters most (to my driving anyway) - when I come sailing into a corner I don't want the diff dragging and causing understeer or suddenly releasing so I have to adjust. An 80/20 or 70/30 is about what I want and I want it to let go smooooooothly. I don't really care too much if the lock when I go back to the throttle is less smooth as long as it doesn't upset the car - but I was also driving a car that wasn't going to greatly overpower the tires so getting to full throttle didn't have a ton of drama anyway.

In both my IS car and the CM E30, I ran a rebuilt BMW unit with an extra clutch in the pack and a custom 30/70 ramp (40/60 was stock). Just that little bit was hugely beneficial to keeping more speed on entry and directly resulted in better lap times. IMO 80/20 would have been more betterer but I had to choose and I could only spend the money once. YMMV.