Not as much of a compression loss as one would suppose. When a guy has to choose between slightly lower compression with more flow, or the reverse, flow always wins. It's why I've stated here many times that I've NEVER put 360 heads on a 318 and lost power. I can tell you from the experience of driving them, the factory 318 Thermoquad (when up to snuff) will kick the living gizzard out of a 318 2bbl.