IceWater

I've had a lot of conflicting advice about appropriate street tire pressures for autocrosses. I'm running a stock 968 with 16" dunlop SP8000's. What pressure will put me in the right ballpark?

Thanks,
Tom

TomH

Last month's Excellence has an Autocross article with tips and tricks. They recommended starting out with tires inflated a few pounds higher than factory recommendations. (They say cars will usually handle a little better with higher pressure, and that its easier to lower pressure at the event than add pressure).

They talked about making your first run, and then adjusting your later runs accordingly. If the car understeers or pushes in a turn, lower rear tire presssure by 2psi. If the car oversteers with its back end stepping out in a turn, lower the front tires by 2psi.

AutoXdriver

Many years ago (80s) when I started autocrossing it was common place to put as much as 50-55 pounds of air in one's tires. The tires back then were not as low profile and did not have stiff sidewalls. These days the tires are much better. I still typically start out at about 40 pounds and decrease from there if/as necessary.

Z-man

Tom,
I have a stock 944S2 (with the M030 rear swaybar upgrade) and I also have 16" dunlop SP8000's. I usually run with 38 to 40 psi cold, the same pressure at all four corners.

I see tire scrub right to the edge of the tread pattern, so it seems to work. I don't have a pyrometer, so I cannot take any more exact measurements. But around 40psi works for me.

The SP8000 is a great tires: excellent traction, but the softer compound does wear pretty fast. (But who cares? hehe)

-Zoltan.

IceWater

Thanks for the advice, guys.

I've been running mostly in the mid 30's, but it sounds like I'd be better off at about 40. I'll give it a shot this weekend and see how it goes.

Thanks again,
Tom

Z-man

Forgot to mention this:
Higher PSI will increase the stiffness of your sidewalls, which will in turn give you crisper handling.

Good luck this weekend: let us know how you did!
-Zoltan.

Dave

Good advice, Zoltan, but don't forget that as the pressure increases the contact patch decreases, there's a happy medium to be found.

Good Excellence article, that #50 yellow and red cup car was NNJR's autocross chairman. IIRC Dom Miliano (The author of the article) is one of ours, too.

Eric D

I have re730 bridgestones and the PSI on those is 44max already. I run them at that pressure and it is sweet, alittle hard but but very grippy.

Dave

Logan, That MAX pressure is what the tire will hold without eventually blowing out, I hope you check it when hot to make sure your pressure doesn't exceed that level. I'm running the same tire and have found 31 front/32.5 rear good for the street, for autocross 36.5-37 front/35-37 rear depending on the course seems to work, above that they feel downright slippery. Above 37 PSI the tread round out and there just isn't enough tire touching the pavement to really push the car very hard.

Z-man

[quote]Originally posted by Dave:
<strong>Good advice, Zoltan, but don't forget that as the pressure increases the contact patch decreases, there's a happy medium to be found.</strong><hr></blockquote>
Yep, that's correct, and that's why I check tire scrub to make sure I'm using all of the tire tread.
[quote]<strong>
Good Excellence article, that #50 yellow and red cup car was NNJR's autocross chairman. IIRC Dom Miliano (The author of the article) is one of ours, too.</strong><hr></blockquote>
Love that shot of Scott airborne! Kinda stiff suspension, eh?

What I have seen is that some people prefer slightly higher pressures, while others prefer lower pressure. A lot of it depends on your driving style, your preference, and what you are comfortable with. Since I'm still in the learning category, I figured I'd rather work on my ability and just adjust the tire pressures by very little: that way I have a consistent benchmark to work off.

-Z.