Tire PSI for completely stock autocross?
 

I've started autocrossing my car and I was wondering what would be a good starting point for my tires? I'm going to keep running the stock tires until it's time for replacement. I ran 35 front/back last time and it seemed reasonable, but I have zero experience driving cars this hard, especially this one. Is 40/40 a better place to start?

there are too many unknown variables to account for, which makes it impossible to provide an exact psi. i'd start at 45/45, put chalk lines on the tires, and run them for a couple laps. check the chalk and release pressure as needed.

40/40 is a good starting point. My two autocrosses on the stock tires have been on slick asphalt lots, and 38 psi seemed fine. No roll over, etc. No grip either, but that's a feature of these tires.

Im assuming we're talking hot temps?

I was talking about cold temps but that's kind of irrelevant after the first few runs. I guess I need to see how much PSI I gain after a few laps as the tires heat up.

For autox, don't make it any harder than it has to be. Put in more air than you need and bleed down as you get a feel for handling and wear. 40psi cold and go from there. It's a heck of a lot easier to remove air in grid than to add it.

i dropped 2 seconds when i dropped my hot PSI from 40 to 33. maybe 33 is a bit low but 40 hot was too high

This weekend out here in Dallas, our member LoPro brought out his FR-S for his first autocross ever. I got a chance to talk to him afterwards and he mentioned he started at 40 and dropped down to 30 halfway through. I rode with him in one of the low PSI runs and it pushed a ton. I'd assume the sweet spot is probably 33-35. But, as others have stated, surface and temperatures will change this.

Take chalk. Mark where the tread meets the sidewall. If it is rubbing off after your runs try higher pressures.

If you have an infrared or contact thermometer you can measure tire temperature in the center tread versus the outer tread. Try to adjust pressure to balance them out. If the center is hot lower pressure, if not bump them up a hair.

Higher pressures can help stabilize softwalled tires but often at an unacceptable expense of grip. Try to make sure you are using all the tread you have and that the tires aren't rolling much. Beyond that don't worry while you are starting out.

I would try 38(f)-42(r) hot. My first autoz with the FR-S was basically just a screw around session so not much testing actually happen. I will say the car pushed a lot and would require a softer front.

This past Saturday we had another autoz. I co-drived my friends FR-S on toyo R888 215 on stock rims and we tested and about 38-43 felt the best, releaving some of the front push.

I haven't looked too much at the PSI I'm running in the awful OEM Michelins, but they don't roll over with anything over ~33PSI. I've found they seem "faster" at 42-44F and 40-42R, but I cannot wait to get rid of them and could care less about times until I get real tires.

Why are you running more pressures in the rear and less in the front? The FR-S/BRZ is a RWD car.

i found that OEM cold worked fine (38), i tried 43 cold and the car was too sensitive to throttle input, ie, dorifuto kingu

at 40 hot it seemed to do fine, next event i'll try 38 hot.

40 hot has worked pretty good for me with several tires including nitto nt01 r compound.

Last weekend me and a friend ran our BRZ and frs. We used chalk on our tires and monitored and adjusted pressures over the day.

His Frs with stock tires seem pretty good as far as grip and roll at around 36 hot all around.

My BRZ has 17x8 with 245/40 rs3's and 37 hot was about as low as I could go with out getting major roll, the rear how ever was very happy at 33 hot. The car still understeer initially but it would even out if you added a bit more throttle. Putting crash bolts on up front this week so I will see how much better it gets with a little camber up front.