I'm guessing they spun on you? You're probably pushing more power than me, with that 3.2 pulley. Just me personally, I probably wouldn't want to go lower than 14psi, but that might just be me.
Basically what you need to do is find the sweet spot where they no longer spin. Sounds simple enough I know.. but it is different for everyone and no one is going to be able to tell you where you need to be with your car. They can only give you a general idea based on tire size and rear wheel horsepower. Pick a starting place with air pressure, go launch. If you don't spin, raise it 1psi. If you do spin, drop it 1psi. Just keep going until you find that "sweet spot" for your car.
I don't have any problem cutting low 1.7s at 18 psi with 285/60-15s
My main concern is that I didn't want to go TOO low. I made two passes at 15 psi and pulled a 1.821 and a 1.834. Was hoping for high 60's-low 70's. I'm not the most experienced driver either.
How were you launching?? slipping the clutch or dump? did the tires spin? motor bog?
maybe try some higher revs?