[steve25]

Here' s some test results I made and had locked up for years ( more like decades!) and forgot about.

The valve lift amounts stated here are for the springs attaining 320 psi of open pressure which is the highest you would want to run on a Hydro Cam lobe and get more then 20k miles out of the Cam and or lifters.
Who knows, maybe with today's oils you could go 30K miles or so, but do so at your own risk!

Naturally the average rpm the motor see's has a large effect here also!

320 psi of open pressure should allow any Pontiac motor with lifters set to close to zero lash and with the big 2.11" Intake valve to make / hit 6000 rpm.


Stock D port heads with a 1.586" installed height, .450" lift.

Stock D port heads with RA and or RA4 valves with a 1.720" installed height, .490" lift.

Stock RA4 heads with 1.812" installed height valves, .520" lift.

SD455 heads have less seat pressure so they can be used with more lift, however I have never tested out what exact pressure they would provide on the seat when reaching 320 psi.

My best guess is that .540" lift should be safe, but no promises!