Engineering’s a funny thing: there’s no such thing as “Best”, only “Best for a Purpose”. We’ll take The Ranchero’s cylinder head as a case study: I’ve got a good 200ci block bored .030″ over and a mystery head. I need to determine how much to mill off that head to ensure straightness and the proper compression ratio.
I wanted to do a whole tech write up on this process, but to be honest it’d just be recreating this diagram and repeating much of this Classic Inlines article about static Vs dynamic compression ratios. To summarize the first: there are many things that contribute to the ratio of displaced volume and leftover space at top-dead center. The second: gnarly cams effectively drop your compression ratio.
In my case, it’s a pretty common theme you’ll see: early-to-mid-’60s motors tend to have lousy cylinder heads with small ports and valves. Later 60s-70s-80s motors have better flow, but also larger combustion chambers. If you drop a later model head on an early motor, you’re likely to end up with a pathetically low compression ratio in the 7s-to-low-8s:1 range, so you’ve got to mill it down.
In my case, I measured the existing chamber volume using kitchen oil and a freebie drugstore syringe, then created my own Google docs sheet to do the math for how much milling gives how much leftover space.
With that in mind, what compression ratio would you target to eek a few more ponies out of a gutless motor without creating something that’ll detonate to oblivion on lap two?