That's right. Once compression went beyond 7.5:1, then air flow became severly restricted, and there was a significant rise in combustion roughness. Also don't forget the other 331's claim to fame was the short stroke, which greatly reduced piston travel. This also had the side effect of exposing less flame to the cylinder wall, which resulted in more efficient combustion, and a smaller cooling system.
Plus, the slipper design of the pistons reduced reciprocating weight, and less stress on the crank and bearings and allowed for higher RPM with less stress.
OHV engines are inherently more efficient over L-Head engines. They have much higher volumetric efficiency by design of more effective breathing allowed by the heads. There are 2 main reasons why the L-head was the mainstay for so long. Fist, they were quieter by means of smaller less complicated valve trains that eliminated the clearances and gaps that made most OHV noisy. Second, they were much simpler, thereby cheaper to build.
The advent of efficient hydraulic lifters eliminated the first reason, and the need for higher compression and more efficient designs led to the demise of the second reason.
Mike
