POSI vs SPOOL
 

Is there any HP to be found by using a posi over a spool? 8 3/4 rear with a Stocker 318 2v. Stock axle over the HD units? Thanks

Re: POSI vs SPOOL
 

The performance gain will be in using the lightest weight parts you can put in the rear; from slick to slick. That includes the wheels and possibly using a lighter slick by going smaller in both width and diameter and adjusting the gear ratio as needed. And since there won't be much power, I would suggest a light-weight aluminum spool. Since this would be the lightest, that answer's the question on the posi unit. And I would find the lightest wheels available and no wider than 8". Personally I would be looking at 7" wheels with 8" slicks and running Cal-Track bars, springs & shocks. And you can also get away with a small diameter / thin wall / lite driveshaft.

Re: POSI vs SPOOL
 

I would not run a posi in a race car. You can never assure that equal power is being delivered to both wheels. I ran one years ago when I first started racing and the most common problem I had was unequal wheel speeds in the water box.

Re: POSI vs SPOOL
 

Way back in the real early 70's I ran a 69 Z-28 in stock. Remember this is with 7" tires. We noticed the et's would vary just a little with the posi. I think that was slip, but the posi always ran more MPH. So we didn't cheat with the spool. We did break a lot of spider gears and cross shafts until I shimmed the clutch pack extra tight and ran the service pack HD springs. The stock springs would come out and be bowed (if they werent chewed up) like the forces of RPM got to them. I think the extra MPH was due to the slip in the posi when there was un even tire growth. Most of the time I ran the Firestone tire on a skinny wheel and they grew a bunch. The only problem we had with axle's was the outer beraring going out and chewing up the hardened surface of the axle. We had a side gear or two break and chew up the axle splines. I think the Mopar axel bearings are not like the Chevy 12 bolt and you would not have that problem. You would not have the problem of the axels floating side to side and breaking the spiders and cross shaft. I had to do alot of work and buy alot of parts to keep the posi alive. With the rules today I agree with the statement about what weighs less...besides, a lite spool is trouble free.
Dwight, What did you guys do?

Re: POSI vs SPOOL
 

Ahh, the age old question, the spool for less rotating mass or the posi for less frictional losses. I'd love to see someone do a "back to back" test on them at the track. It's kind of hard to do with a GM rear.

Re: POSI vs SPOOL
 

Not allowed stock axles with a spool in NHRA.

Re: POSI vs SPOOL
 

We did it twice years ago and concluded that the posi was a bit faster, but no quicker.

When I first put my '67 Z28 together in 1980, I tried to run the posi and spent way too much time and $$ trying to make it live. Lot's of dollars to Tom's Differentials and heat treating facilities. Worth about 15 runs before something would break. Building your own is necessary to insure the clutches work correctly.

The best approach would be what Jeff says. Rifle-drilled or small diameter axles, aluminum spool, micro-ed gears, loose bearings, light wheels and tires. MHO

Re: POSI vs SPOOL
 

I also asked my chassis shop about spool vs posi and they said they just use spools in their race cars...

Re: POSI vs SPOOL
 

I guess what it all comes down to is if it's a high powered car go for all of the tricks. If it's a "dime rocket" with no power and an automatic (and I'm guessing that the combo that started the thread is) then there are lots of better places to spend your money like really light wheels and tires and the correct size header.

Re: POSI vs SPOOL
 

I did back-to-back Aluminium spool versus Strange light weight spool in my car with a 12 bolt rear, saw zero. Only lost a couple of lbs. Didn't pull it back out. Should have before Memphis nats in '08. Cleaned the pass side splines out on the starting line, 1st round. Won't run one again. Just my experience. Yours may vary.