jmills

My wife and I taking our new 997s (our first Porsche) to it's first autocross tomorrow. The car has PS's on them (295/30/19 rear and 235/35/19 front). The manual says pressures should be 44 rear and 37 front. What do you guys recommend as a starting point for pressures? I plan on chalking the sidewalls to gauge them. However there is a chance that my wife and I may not be in the same run group (I am running the Mazdaspeed Miata) and she doesn't want to fool with monitoring the tires.

Thanks and can't wait to see how the car performs.
Joe

Mussl Kar

If it is her first event then I would drop the pressure maybe by 2 pounds cold, front and rear.

Driver8

+ with Mussl Kar, or you can even leave them at the stock pressures. If this is going her first autocross she will be me much more concerned with learning the course and car control rather then tire pressures.

jmills

Thanks for the input. We just went with stock pressures as a starting point, which seemed to work well. She attended the Evolution school last year, so this was not her first autox, but the first one in the Porsche. I got to drive the 997 in a couple fun runs and all I can say is wow! I drove our slightly modified Mazdaspeed Miata at the event, but the Porsche is a completely different animal. The amount of grip is amazing and it feels incredibly stable. I know my wife is looking forward to driving it at future events, but I may only get to drive it during the fun runs.

Thanks again for the input.
Joe

lolayay

Stock pressure is good for AX. AX is not intense long enough to heat up the tires. On the other hand, there are lots of extreme turning and rolling with the tires. So you don't want to keep the pressure too low to roll into the sidewall. In fact, I add 2+ in the front on every AX event (rear stay stock).

sjfehr

This REALLY depends on the tires. For autocross, you need to be adding pressure on race day, not reducing it, regardless of experience, even for a Porsche. The best thing to do is experiment on a skid pad. If that's not possible, consider that the tire grip curve flattens out somewhat after the peak- if you're going to err, it's far better to err with overinflation than underinflation. Also, if you're using chalk/shoe polish, it's a whole lot easier to bleed air out between runs than to add more!

There's no reason to ever be underinflating tires for autocross, you're giving up traction of you do that. The weight becomes disproportionately spread and the tire deforms, and it gives up far more traction than the increased contact patch gives you. PS2s aren't low-pressure drag racing tires, you have to account for the sidewall.

HarryD

Hi,

I do not know wht pressure you ned but a simpe test is to put a chalk mark on the shoulder of each tire where the thread meets the side walll. Depending on how it is rubbed off, you can decide if you rolled into the side wall or did not use the entire thread width.

How do you tell?

For Michelin tires, you will see tiny Michelin men (other tire makers use triangles or other symbols) in the tire side wall, where the thread meets the side wall. Put some chalk (or white shoe polish) on him. At the end of the run, your goal is to have his upraised fist still covered witht he chalk and the area immediately above it, the chalk will be rubbed off.

If there is chalk above his fist, you are overinflated and remove 3-5 psi.
If there chalk is removed from his arm or body, you are underinflated, and add 3-5 psi.

Good luck. For my Yoko ES-100's, I add 5 psi.

sjfehr

From the OP, it sounds like you're experienced, but for the sake of others reading this thread who may be doing their first ever autocross with no experience:

There is a sweet spot in the tire pressure curve. Adding more pressure will continue to increase traction up to the point where the tire bows from the pressure under dynamic loading (it may turn a bit convex under static loading which is bad for street driving [why most people raise pressures only for racing] but this deformation under static load is OK for autocross). You lose less traction being 2psi over than you do being 2psi under, so it's better to slightly overinflate than underinflate. Also, your tires will probably heat up 2-3psi during the event; that's why you'll see guys running around bleeding air from their tires after each run.

During racing, weight transfer can double (more or less) the amount of weight carried on a particular tire. I have photos of my stock 986S picking one wheel off the ground in a corner- the weight transfer is very significant. I was fortunate enough to have my dad & his DSLR at an event, and he caught a good photo of this for me. Here's what happens under that force (this tire was inflated 8psi over spec, btw:



And that wasn't even a worst-case turn, just one where he got a good photo. I raise the pressure on my 986S (17") from 29F/35R to 37F/38Rpsi for autocross.These tires don't so much roll over as crush under the weight transfer; you need to have enough pressure to keep the car from rolling over onto the sidewall. What you can do is put white chalk on the corners of the tread, where it rolls down to the sidewalls. I like to chalk at 3 places on each tire, and then check after the run; there should be a little chalk showing where the tread meets the sidewall. If all the chalk is gone, then your tire pressure is too low. If too much chalk is showing, you either weren't driving as hard as you could be, or your tires are overinflated.

Ask the guys at the autocross to help you, they're all old-had at this. FYI, here's a diagram of about where the chalk marks were after my first autocross. My rear tire pressures were good, but I had to increase the front to 36psi after that race. I had to increase then further another 1psi per tire as I got better and began transferring weight properly You have different tires, so your pressure will be different than mine, but the techniques are the same:



All you guys that don't change pressure between autocross and street... you really should be. If not, you're either giving up (free!) traction and destroying the corners of your tires in the process, or if you drive with race pressures every day, you're destroying the life of your tires. Same with camber and toe, really, though it's easier to adjust tire pressures for race day than camber and toe Have you ever tried chalking your tires to see what's actually happening? Are your tires wearing out quickly in the middle from overinflation?

HarryD

sjfehr:

Well said. Thanks for giving the "whys" to my "hows".

FWIW, you know you have too much weight transfer when your car does this at every turn:

roman944

that looks perfectly fine to me