Quote:
Originally Posted by joelster
I noticed a big difference in tire pressures on the 660's. The sweet spot for me is 30lbs. I ran them for 2 events at 31-32lbs. On the 3rd event I had a decent lead so I tried lowering them. OMG what a difference. Took them to 30 and the front end was noticeably better. Lowered my time by 1.1 seconds. Went to 4th event and ran them at 30 for the whole event and set top pax. I run 19x11, 19x11.5, and 315/30/19 square. Car is roughly 3650lbs.
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That's interesting because I just did an autocross test and tune with my RT660's with the sole-purpose of playing with tire pressures and found the opposite. I am running 275/30 front and 315/30 rear on 19x10/19x11 wheels.
Day was about 55-60F, sunny, dry day. Surface is some weird blacktop. I used a tire pyrometer and digital tire pressure gauge (Longacre stuff). I started at 29.6 psi cold and ended up adding pressure after two or three test runs at that setting. Hot pressure coming in from the runs would only get to about 32psi and the tire temps showed the same temp middle and inside with the outsides being about 2-3F cooler (front and rear). My notes indicate that running at those pressures the tires were not coming in quickly [to grip] and the rear was squirrely. I actually ended up adding pressure the rest of my trials. I can't comment what the true "cold" psi is (I can go check them in the garage as I haven't touched them), but the pre-run psi I ended at was around 34psi front, with the rear having just a touch less psi and rendering a 2-3F spread across the outside, center and inside of the tread (i.e. 92-94-97). My notes indicate the tires came in a lot quicker and the car had a good balance of grip. Times dropped at least 1-second.
But, again, I'm on 275/315 tires on 10"/11" wheels, maybe your ambient temp was a lot hotter and/or your track surface was a lot more grippy.