Going to the strip(Union Grove Wi) Friday for test and tune.Trying to get into the 12's.We are running Firestone Firehawk Wide Oval street tires 275x40x17 with 34 psi.Is there any advantage to lowering the air pressure and if so to what psi?Thanks

 

If you're already running ~34 psi, try pumping them UP to about ~40 . At least "try it" first. Do the fronts too (they'll roll easier) but for the rears, by deflating them, they'll "cup" in the middle, and you may actually LOSE traction because of it. I know I dropped my stock tires (Eagle RSA's, 245/50/16's) from ~30 to ~25, then to ~20 psi, and my 60' times suffered (2.1's up to ~2.3's!) ... whereas on street tires pumped up to around ~35-36 psi (16's still) I've cut into the 1.9x's!

 

Each wheel tire combination is different and how the weight transfers front to back, or not... will have a significant impact on the best tire pressure.

The goal is to have a completely even contact patch to maximize traction. Back when my car was basically stock with just headers, cold air, and drag radials I had the greatest success at 24 pounds in the rears and 40 pounds in the fronts.

When the weight transfers front to back if the rear tires have too little air they will cup in the middle and lose their grip. If they have too much air the outside of the contact area will not hold effectively. You need to consider the effect of additional weight on the rear when estimating the proper tire pressure. It will take a few tries to get it right.

If the contact area is greater than the rim or less than the rim that will also have a significant effect on the amount of air pressure. The goal is if you have an eight inch rim (for example) to have a tire that has eight inches of tread. This will provide the best scenario in terms of weight distribution on the sidewalls.

Since street tires do not have much flex and the sidewalls are stiff, the benefit of very low pressures does not exist and if often counter productive. However all tires seem to have a sweet spot and my experience tells me it is generally between 22-30 pounds. Keep the fronts nice and hard to decrease rolling resistance and provide as much diameter as possible to increase the rollout distance at the start.

 

The wheels are 9.5'' and the tread width is 9.5'' max tire psi is 51.I'm thinking I probably should be running 40 psi any how all the time.

 

No... you should probably not be running 40psi all the time. You should be running something close to the tire pressure sticker on your door. I've run anywhere from 30-36psi on the various tires I've had on the car.... 245/50's at 34-36, 275/40's at 32-34psi, and 30-32psi in the 315/35 BFG Drag Radials on the street. The "max pressure" on the sidewall of the tire is the pressure required for the tire to reach its max load capacity. The tires on your car have nowhere near max weight load on them. But, the way to check is to measure tread wear from normal driving, watching for excessive wear at the center of the tread with 40psi. If you are after street traction, do a few hard launches and see if you have a nice, wide, uniform black patch on each side of the car.

The answer on the track is also to try a few launches and adjust the pressure until you get the best 60-ft results. I'd be surprised if you got good results at 40psi.