A 55-62 motor can take more then 4000 rpm. Heck, the mechanical advance is not all in until 3500 rpm but yes, the springs would be a limiting factor. With good springs, or double springs, a 55-62 235 motor can go to at least 5200-5500 and I have one motor still in the back of my garage that I ran into those numbers on a regular basis on the street and race track, and once or twice into the 6000 range for 25 years. The issue in those rpm ranges is that the head really does not breath much past 4800 or so even in modified form, and probably not that deep in pure stock form, and it gets a little squirrelly when you are in the 5500 and up range with those rods. That and the harmonics start to play a part over 5000 rpm (read that loose flywheel bolts for example). So really your best bet is to keep rpms under 5000 rpm, but yes, no real problem 4000-4500.
Last edited by Dragsix; 02/10/2019 7:27 PM.
