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

hard to say, different tires like different pressures ...
and i'm not familiar with your tires ...

just as a reference, my goodyear slicks work best at about 17lbs of pressure. funny thing, they really start feeling greasy at 25lbs or so, but in fact hook up better than at higher pressures.

at 17lbs, they don't feel very stable, but the times show that they have a lot more "glueage".
i just needed to get used to the greasy feeling ...


Andy

I would run those Kumhos about 36 to 38PSI. Because the side wall is soft you need the pressure to keep the tire carcass in shape.
Otherwise you will be rolling over to the sidewall. (not good)

Plus with no sway bars your body roll is probably significant and you are getting lots of Postive camber on the outside wheels.

Lowering the pressure will allow a street tire to "roll over" and loose grip just as yoiu are really getting through the corner quickly and leaning on the tire. Higher pressure will make the loss of grip a bit more linear and easier to control.

Once you figure out what what you can tune the pressures to achieve more or less grip at the desired end.

Massive grip at both ends is not always the hot ticket. I would bet that this is what Andy is really experiencing with low pressures.
AX cars need to rotate with the rear end quickly.

Only way to find the optimum pressure is to have a G meter and a skid pad.

Best we could get on my car was sustained 1.5 G's with 24 PSI in my slicks.

We found the car would not rotate and we backed off a bit and got the rear end to slide a bit more. Then it got real fast with a bit less ultimate grip in back. And we take temps after every run.

Stock pressure, under "heavy load" is 29F/R. In my old 914, I ran 30/32 or 30/30 F/R depending on course layout.

You might start by "chalking" the tire. Mark where the Tread meets the shoulder. If after the run all the chalk is worn off, add pressure. If its all still there, lower pressure. You should have some type of wear mark on the tire to help show how much "rollover" the tire is doing.

Realize too that pressures will change a bit with temp. Once you find a pressure that works for you, you may need to "add some" in the cold morning but let some out as your runs progress.

Hope this helps!

Stock pressure? It veries by tire. DOT recomends 32 PSI for the street, but when racing there is no stock pressure. My RA-1's run at a very different pressure than my V-710s. Joe's advice was very good. Follow what he said and keep a close eye on how far the tire is rolling to the side.You want a little but too much can be very bad.

Thanks for the replies!!

I ran 32/32 psi this week. The car seemed good, although not fast (need more
power!). It rotated well. If I kept my foot in it, I could get the rear to slide around
nicely. The hardest part is to keep my foot in it! Wear seems to be fine. I chalked
the tires and there was wear down through the treaded area but not beyond that, so
I guess I'm okay.

I wonder if I should drop the pressure more. In general, would grip increase or
decrease? I expect the curve should be bell shaped, pressure vs grip. I wish I
knew where the peak point is!!

Thanks!

Jeff

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.

When you get comfortable with the car you will find tat your car will like to have you put your foot down as the back end s getting out of shape. What it will do is transfer weight to the rear wheels and make them grip.

I wa riding with a new guy that kept lifting his foot mid apex because the car was sliding the tail.

I holllered "GAS GAS" he gassed it the car stuck and scooted right in the intended direction. He was like wow that was cool.

Which leads to throttle lift oversteer Nuff of that as I am rambling.

Yes there is a bell curve for grip vs air pressure. but it is also dependant upon suspension and body roll.