6-8 psi with a good tune is plenty safe, some run more and haven't had problems. It's all in the tune and good gas. A buddy of mine that's gonna help me did a 99 Cobra and he was well into the 4xxhp range and deep into the 11's with 6-8 psi, P1SC, 3 core IC, 03 Cobra fuel pump, 42lb injectors...