Also, the valves in the 452s are bigger, and the ports are bigger/higher flowing than what's in the -516 heads that were original to the '66. Adds up to lower port velocity, which probably reduces turbulence and makes for a less ideal burn.

I'd say that, coupled with the compression loss, is the reason. Assuming the -452 heads were in good condition, of course...

Parts I seek: driver doorpanel, 65 Sport Fury, prefer black, needs to be 7-8 on 10 scale, might buy set 16" x 6" Dodge truck wheel(s), from early 70s?, takes 9" dogdish - need for a research job so cheaper is better. 69-73 C-body caliper brackets and/or splashields Send a PM.