[Excaliber]

I think the primary reason for a high pressure cap is to make sure your not blowing water out the overflow at high rpm, when pressures and turbulence can get pretty high. My 427 doesn't blow water at high rpm with a 15 psi cap so I'm OK, but you might not be. Of course when you do loose water, for whatever reason, THEN your going to overheat for sure! So the advice of a higher pressure cap is generally a safe call that covers all the bases, but not necessarily called for in all cases.