Hey Folks, tough question because of the variables. I've been AXing for 15+ years in several different cars but never a 1999 Porsche 911 Cab. My question is what's a good starting point for the TP, higher or lower then normal TP and by how much? The rears are Yokohama S-drive, 265/35R 18 and the fronts are Bridgestone Potenza RE 760 Sport, 225/40R 18, front and rear have like new tread depth. Normal TP: front 36 psi and rear 44 psi. Any advice would be appreciated, my first AX this year is next weekend. Thanks, Larry

 

You may want to leave your tire pressure where it is until you heat up the tires after a few runs to see how your car handles. For the most part you'll probably want to lower your tp by a few lbs to get the most grip. I've seen some lower their tp so much that they broke the tire bead from the rim during hard turns. You can also talk to drivers with like vehicles, ie: rear engine, vehicle weight, weight distribution, etc, and see what pressures they're running.

 

Pors-Vette, Not many AXer's on here. But thanks for your input . . . I can't wait to try out the P-car on the AX course. I could have a 'handle' similar to yours, mine would be Pors-Cobra. I built a FFR roadster 10 years ago, scary fast and so much fun.
Thanks again, Larry

 

Why do you have different tires front and rear? I would think you would be better off with a matching set from the same manufacturer.

 

Hi FLA996TT, The main reason is that the car is new to me and I bought it sight unseen from a FL. dealer. I would agree that the norm is matching tires. However, when I remember my younger days, and even now, straight line racers many times had mismatched brands. I really can't see why it should make a difference (differing brands front and rear), assuming the performance level is similar. In fact there may be some minor advantages. If your current front tires 'push' perhaps a different brand would help cure that issue . . . along with all the other things that can be done to solve 'push'. In my case, I'm sure the dealer was just trying to get by cheaply. Larry

 

Actually there are a lot of us on here who autocross and race. Any engineer type that I have worked with (whether tire engineer or Porsche engineer) would highly advise to not drive a high-powered rear-engined car (or most others) with mis-matched tires especially in the wet.

The amount I decrease pressure in my tires depends on ambient temperature and whether I can heat up the tires prior to race time. Generally, when auto crossing, I start by lowering 5 pounds and adjust as the day heats or racing continues. Sometimes it depends on the surface being driven on as different surfaces have a different drag factors as do different tires at different humidity levels and road grades. Drive, evaluate, adjust...drive, evaluate, adjust....

Additionally, I would have insisted on the dealership changing the tires to new matched tires prior to driving it off the lot.

Edited to add from Porsche: It is recommended that only matching tires be used on Porsche vehicles. Since many Porsche vehicles are fitted with differently sized tires on their front and rear axles, this means matching the tire make, tire type and N-specification. If a vehicle was originally delivered with N-specification tires that have been discontinued and are no longer available, it is recommended to change all four tires to a higher numeric N-specification design appropriate for that vehicle. Mixed tire types are not permissible.