fastpig

I'm looking to have a 12 bolt conversion done to a stock C3 rear-end. Right now I'm doing the drag race thing but in a year or two I plan to dedicate the car to road racing / autocross. When I have this rear end built I'm trying to decide wether to go with a spool or posi. I was going to go the posi because of the future road race plans, but then someone told me that spool rear ends are also used in road racing / autocross. I didn't think a car with a spool would handle as well with the axles locked together. I was told the spool would keep the car more stable entering the turns ? The car currently runs 9.90 @ 139 mph in the 1/4 mile that's why I was going to go to the 12 bolt conversion.
Any input would be appreciated !

90Z51

I've always been taught that spools are for drag use only. I can't imagine a benefit for road race use, you're dragging one tire along any time you turn. Maybe I'm totally wrong though.

STEVEN13

I use to run an RX7 (an X SCCA ITA car)which had the spiders welded to simulate a spool. It had a tendency to push/understeer at low speed. At high speed it was ok. It did also have an adjustable front sway-bar.

I would recomend a LSD (Posi) rear.

Steven

Solofast

It depends a lot on how you drive.

With a spool and/or a really tight diff, you have to brake really late and hard to unload the back tires and then you start your turn in with the back end still unloaded. That minimizes the locked rear effect and actually the tight diff helps keep the car fron getting loose on turn in. That kind of driving is best done with more "rubbery" tires, Kumho's come to mind here. On really stiff tires, like Goodyears or the later Hoosiers, it is harder to pull off.

Once you get the car rotating with a spool you can lay into the power, the the really loaded rear tire will help push the car around and you exit the corner like you were shot from a gun.

Another drawback is that if you overpower the back tires the back end can break loose and result in a lot of snap oversteer, it is not as forgiving as a limited slip.

I would recommend the limited slip, and build it really tight, and that is probalbly the best way to start. It give you better corner entry stability, but still lets the car rotate more easily than with a spool.

It also effect setup, as others have alluded to above. With a tight diff, you will want to set the car up with less front bar, otherwise you will see more push than you might otherwise like.

fastpig

Thanks for the advice, It definately sounds like the posi is the way to go. Tom's Diffentials say that they have a couple guys running in the 8's with posi's which is faster than I'll go ! This way I'll only have to do it once and have the rear end ready for road racing when I'm ready to switch.

Miaugi

Sounds like a good plan.

BTW, impressive numbers!