Basic newbie autocross question, Tire pressure Options

[jd66921]

Hi,

I recently started autocrossing my 914. I enjoy it a lot! The car is not really set
up for autcrossing, no sway bars, standard alignment, etc, but it is still fun to see
what I can do.

The tires I am running are Kuhmo street tires. I know they are not optimal,
but I am not ready to upgrade.

The issue is trying to optimize tire pressure. I know the 914 is a very light
car and therefore may not need a lot of pressure. I ran them originally at 35 psi.
That seemed okay. I ran them up to 40 psi, and while fun to slide around, it was
way too much! I am now running 32 psi, and I don't know if I should go lower or
not.

How does the pressure vs cornering curve go? I would think the cornering force
would increase with tire pressure up to a point, and then tail off. Any ideas what
the range should be? 25-35 psi? Higher? Lower?

I'm just looking for general guidelines. Tires are 195/50's on 5.5 inch fronts and
6 inch rears at the moment.

Thanks,

Jeff

PS: Autocross video

[Krieger]

I think what he meant by "stock pressure" was Porsche factory tire pressure specs for a 914. Obviously many differnt tires/wheels fit our cars. Every tire has a max pressure on it, but our cars still weigh roughly the same they did 30 years ago. I autocross my car on street tires. I run slightly less than stock factory pressures. 27 r, 25 fnt. factory stock I believe is 29r 27fr. Do what racer says and chalk your tires and do your runs. Also what I look at is on the sidewall of the tire. I think almost all manufacturers have some sort of "arrow" or triangle or some type of mark on the edge of the tread/side wall showing you where they think the tire should roll over to. If you are past it add air, if you not close let out air. 2lbs at a time. Get in the ball park and leave it alone. Check your cold tire pressures the next day so you have an idea where to start the next time before you leave your house. Do your best at improving yourself/ HAVING A GOOD TIME and don't mess around too much with tire pressure until you get good.