Valve float, where the valve fails to close fully, is more likely to occur at high RPMs with weaker springs. This can lead to poor engine performance and even damage.

One downfall, however, is their ability to rev. Pushrod engines tend to have rather low redlines. You won't find a GM small-block V-8 revving to 9,000 rpm, but there's a reason for it all.

Near redline, the valves may bounce off their lobes, and valves and pistons can collide. Stiffer valve springs can alleviate this "valve float," but they bring friction and noise.