Wheelspin

[TheRedSlantnose]

My track/street-modded '86 951 produces over 350 hp, and puts out 300 rwhp, which causes constant wheelspin when I floor it in 1st, 2nd, and 3rd gear, even with limited slip. The extra weight of the transaxle doesn't seem to provide much traction either, and last summer at a drag racing event at the Texas Motor Speedway, the wheelspin cost me, even against the easiest opponents.

BTW, I post here about my 951 instead of the Turbo thread since there isn't much user traffic there and I don't get answers as quickly as I do here.

Anyway, what can I do to control my 951's wheelspin?

Thanks,

[944 Ecology]

Get wider, stickier tires. If you are spinning only one tire, rebuild your limited slip.

While you're at it, put a chunk of change in the bank for when you break the ring and pinion. The stock 86 turbo transaxle is not strong enough to handle the power you say you are putting through it.

[TheRedSlantnose]

Originally Posted by 944 Ecology

Get wider, stickier tires. If you are spinning only one tire, rebuild your limited slip.

While you're at it, put a chunk of change in the bank for when you break the ring and pinion. The stock 86 turbo transaxle is not strong enough to handle the power you say you are putting through it.

What's a preferable width for the rear wheels on a 951? The ones I currently have are 18" x 9" with pretty big spacers, and I was once told by a tire shop that the spacers I'm running are too big and unsafe.

[944 Ecology]

If you have spacers that are "pretty big", you probably have late offset wheels on your early offset car. Without body modifications, I have seen 17x10 inch wheels on 86 turbos. With body flares, there are some 18x13 inch wheels on 951 racecars around here.

(P.S. That race car (550 bhp, track record holder in GT2S) breaks parts that I have never seen break on any other 951.)

[Rasta Monsta]

Originally Posted by TheRedSlantnose

Anyway, what can I do to control my 951's wheelspin?

Modulate the throttle?

[TheRedSlantnose]

Originally Posted by Rasta Monsta

Modulate the throttle?

Sure, if I didn't like to drive fast

[TheRedSlantnose]

Originally Posted by 944 Ecology

If you have spacers that are "pretty big", you probably have late offset wheels on your early offset car. Without body modifications, I have seen 17x10 inch wheels on 86 turbos. With body flares, there are some 18x13 inch wheels on 951 racecars around here.

(P.S. That race car (550 bhp, track record holder in GT2S) breaks parts that I have never seen break on any other 951.)

Well, the wheels I currently have on it are the 993 Turbo Twist wheels, and the wheels it came with when I bought the car were 17" Boxster wheels.

I don't think a 10" width would be enough for my car, but can I fit 11" width wheels under the stock fenders without the spacers? I've seen quite a few 944's with 996 Turbo wheels, so I think it's possible.

[fiat22turbo]

You could try increasing the spring and compression rate in the rear with some stiffer torsion bars and adjustable shocks.

Ultimately though, adding traction via stickier tires will help, but increase loads on your driveshafts, CV joints and transaxle.

I would look at going to more solid transaxle mount(s).

Adding a steel brace to the transaxle to reduce issues with the case cracking under heavy load: 016 and porsche LSD - GT40s.com

Finally, replacing the driveshafts and CV joints with stronger racing units.

Ultimately, something had to be the weakest link. Increasing the strength of the parts that fail or will fail, just moves that weakest link some place else.

Having your tires spin doesn't put much load on the drivetrain and you can always use your right foot to get the car to speed before using full boost.

Your choice. Good luck!

[flash968]

first off, due to the rear suspension design, this is the wrong car to drag race, but that's more a philosophical issue.

i have the same power range, and while i can force wheel spin, i can also control it. this may be as simple as learning the curve and working with it. if you're getting it off the line, don't dump the clutch so high. i shaved a full second off my 1/4 mile by learning to dump earlier.

generally speaking, stiffening the rear will actually increase wheel spin. the fastest 1/4 mile cars have pretty soft rear suspensions and soft rear tires.

at what rpm are you having wheel spin?
what are your camber angles?
what are your spring rates?
what shocks are you using, and where are they set?
what tire pressures are you running? on what size and model tire?

[TheRedSlantnose]

Originally Posted by flash968

first off, due to the rear suspension design, this is the wrong car to drag race, but that's more a philosophical issue.

i have the same power range, and while i can force wheel spin, i can also control it. this may be as simple as learning the curve and working with it. if you're getting it off the line, don't dump the clutch so high. i shaved a full second off my 1/4 mile by learning to dump earlier.

generally speaking, stiffening the rear will actually increase wheel spin. the fastest 1/4 mile cars have pretty soft rear suspensions and soft rear tires.

at what rpm are you having wheel spin?
what are your camber angles?
what are your spring rates?
what shocks are you using, and where are they set?
what tire pressures are you running? on what size and model tire?

I entered my 951 in the drag race because I wanted to see how it'd hold up, and I even entered my 930 Slant as well. My 930 did better, but I let my last opponent win because I didn't want to continue to the finals. However, when I was racing with my 951, the track officials did advise to lower the tire pressure, but that didn't seem to help as my wheels still spun.

-My wheels spin when I'm at full boost, which is somewhere past 3500 rpm, and it's only in the first three gears, on a DRY road.
-The rear tires on my car are actually different from each other. On the driver's side is a Triangle TR968 265/35R18 and on the passenger's side is a Pirelli P Zero 265/35R18, and they both have different tread patterns as well.

The other questions you're asking I'll need to look into some more to answer. My car didn't come with any build documents of any kind like my 930 Slant did, so I don't know that many technical details on it.

[jamess2]

You think you have traction problems wait until you have a 300whp front wheel drive... I had the same issue your having with your car with my 2012 mazdaspeed3 it has full bolt ons and tune and I was being out run by much less powerful cars to the point it was getting embarrassing.... I was running 20psi in front tires and spinning till the top of third gear feathering the throttle with traction control and a LSD... I upgraded to a softer sidewall and a stickier compound and it has helped considerably with my traction problems... But I still wouldn't drag a 951, these really aren't drag cars.

[TheRedSlantnose]

Originally Posted by jamess2

You think you have traction problems wait until you have a 300whp front wheel drive... I had the same issue your having with your car with my 2012 mazdaspeed3 it has full bolt ons and tune and I was being out run by much less powerful cars to the point it was getting embarrassing.... I was running 20psi in front tires and spinning till the top of third gear feathering the throttle with traction control and a LSD... I upgraded to a softer sidewall and a stickier compound and it has helped considerably with my traction problems... But I still wouldn't drag a 951, these really aren't drag cars.

As an old 951 article stated "For lunch it prefers Ferraris, although it has been known to snack on Corvettes." My car is very capable of outrunning Corvettes, but because of the wheelspin, I'd have Corvettes snacking on my 951 instead, oh the irony

Then I guess I'm lucky I didn't destroy my transmission, but there was also a 924 and a 996 Turbo at the drag race as well, and they appeared stock.

[ckelly78z]

What age are your tires ? There is a date code on the sidewalls up close to the rims. If you have tires near 5 years old (and especially a mis-mstched set like you have), it's time to invest in new soft compound tires to replace the old hard ones. The big problem is going to be when you finally do get some good grip, your transaxle will probably give you problems.

[v2rocket_aka944]

howabout some bladder bags in the rear cubbies/tire well filled with water to add weight?

[flash968]

first thing - match your tires. if you have a torque bias diff (as opposed to a clutch diff), the LSD will ultimately let one go first, and then you effectively only have a pegleg.

get me the rest of the data, and i can probably advise further. i spent a bit of time setting up some pretty unconventional cars for 1/4 mile, and had to play quite a bit (like a vw bug with a turbocharged 911 motor). like you, it was just to see what i could do.

[Lapkritis]

Easy to mistake a slipping clutch for wheel spin...

[TheRedSlantnose]

Originally Posted by Lapkritis

Easy to mistake a slipping clutch for wheel spin...

No, I'm positive my clutch isn't slipping when I accelerate because the rear slides whenever I floor it.

[TheRedSlantnose]

Originally Posted by ckelly78z

What age are your tires ? There is a date code on the sidewalls up close to the rims. If you have tires near 5 years old (and especially a mis-mstched set like you have), it's time to invest in new soft compound tires to replace the old hard ones. The big problem is going to be when you finally do get some good grip, your transaxle will probably give you problems.

I'll have to look at the tires to answer that, but what I can tell you now is that I bought my tires used.

[flash968]

neither one of those tires are particularly grippy. couple that with them being used to begin with, they are likely old and hard on top of that. the first thing i would do is get a new decent set of at least "max summer performance" tires on there.

[DannoXYZ]

Is the wheelspin a single tyre or both? For drag-racing you'd want to go with a locked-diff and forget about an LSD of any kind.

The real issue with drag-racing a 951 is that the torque-curve isn't linear. Going from 200 to 400 lb*ft torque within 1500rpms is what causes the tyres to break loose, the rate-of-increase is too much. You'll want an engine designed with more linear torque build-up and one that can hold that torque all the way to redline. I bet your 951 loses 20-25% of its max-torque by redline. So you've got a double-negative of too-fast torque increase, witih a narrow peak that drops too much. A car with less torque & power but a flatter curve would beat your ET.

What I've found to work well is electronic traction-control. Without replacing ECU you can get similar results with an electronic boost-controller that allows you to program a custom boost-curve per gear, like the Apexi AVC-R. I've found these settings to work best:

1st-gear: 15psi@3000rpm, 18psi@4000rpm, 20psi@5000rpm, 22psi@6000rpm
2nd-gear: 18psi@3000rpm, 20psi@4k, 22psi@5k, 24psi@6k
3rd-gear: 19psi@3500rpm, 22psi@4k, 24psi@5k, 25psi@6k
4th&5th-gear same as 3rd gear

This was with a TO4E-50 turbo and got about 380-390rwhp. The lower boost and more gradual built-up in 1st two gears put the tyres at the squeaking-edge of traction, but not squealing wheelspin. The rising boost-curve also combats the natural torque-drop of these engines (cam & exhaust tuned too much for low-RPMs).

With everything tweaked just right, I got high 11s @ +125-130mph using 17x11.5" 275/40-17 on the rear.