My experience is at road courses, no autoX. To get to the quite low hot pressures strat is talking about, 27-28psi hot, I'd have to go out ~18psi cold. I seriously doubt that would give any more grip, it's likely well into a range where increased rolling resistance = slower, and likely getting into the realm of *dangerously* low...
I don't agree with the notion that less pressure => more grip is an absolute truth. Not for a radial tire at autoX/road course on appropriate radial tires anyway. As opposed to a dragstrip with a very tacky launch pad and drag tires, which is a very different scenario...
When I get to work later today I'll look at lap times over a session from Watkins Glen (in Cayman, not BRZ) last week, with cold pressures 24/26psi and hot up to 36/37psi (outside rear saw 40 one session early in the day) and see how my best lap times compare early to mid-session. Very hard to get reliable data, though, as there's traffic to contend with. Also later in the session, 200tw street tires can be overheating so fall-off in laptimes can be due to that rather than "too much pressure".
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Originally Posted by strat61caster
When I said 29 that's usually my target and I'll bleed a pound out of whatever end needs more grip so really 28 hot is my 'optimum' as above lots of factors, maybe you've got a gauge that reads different to mine,a longer haul between course and grid, etc. 27 psi is close enough to what I run that I doubt you're leaving much on the table.
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How do you know 28psi is optimal? What is the fall-off in time above and below that pressure? You'd need to be able to test isolating pressure from other effects, which would involve many runs over the same course, adjusting pressure first in one direction and then in the other, multiple times, to account for other factors changing throughout the day.
I seem to be able hit lap times within a few tenths over a fairly broad range of pressures. But it's impossible for me to test lap times vs. pressure in isolation, without a ton of other variables coming into play, which makes drawing hard conclusions impossible. But from what I've experienced I don't think there's a whole lot of difference in lap time potential for me from 30psi to 36psi.
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(For OP this is a 245/40/17 on 17x9, I'll be surprised if you can get away with much lower than 30psi but could be wrong you're not pinching like most do)
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FWIW I'm on 245/40-17 RE71R on 17x9 on the BRZ, -3 front -2.5 rear camber. Which should be tolerant of lower pressures. But I never experienced anything suggesting that running less than 34 hot was buying me anything in terms of lap times at the track. Or that inadvertently running more than 36 hot was hurting me all that much.
For OP, with limited camber and 215s on 7" wheels, I doubt that running 28psi hot vs. 34-36psi hot is going to be worth anything. But again, I don't autoX... You're likely totally fine at the 32/33psi hot pressures for autoX.