Originally posted by mrlucretius
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To explain this fully, it would be a long post and in a place not many people would see in the bottom of a thread. Do some reading up on dynamic compression to get a good grasp on it. Just be aware some of the info out there is pretty dated, but it really helps visualizing what I meant by matching.
In short the cam overlap (both valves open simultaneously) can help or hurt depending the the intentions of the engine. A lot of duration/overlap (can be lower duration and tighter center lines too) hangs the pair valves open longer, allowing intake gasses to pass into exhaust without combustion, and also exhaust pulses to revert to the intake.
Now if you have a really high CR with a low overlap cam, you are trapping more mass (increasing dynamic CR) and it gets to a point where the fuel mix will pre-ignite from pressure no matter how little timing advance. In a lower compression you have the opposite. Too much mix is being expelled and will get to a point, the engine won't even start - the flame front will just go out the exhaust instead of pushing the piston down - path of least resistance.
So, in a running engine things change as RPM increases (dynamic CR) due to all kinds of variables. Such as back pressure in the exhaust (high RPM, running condition), a long duration cam won't bleed as much as if cranking to start, there's already pressure in the exhaust helping it stay in the chamber.
With that in mind, you can see how cam specs will kind of indicate the temperament of your engine. Low compression takes more RPM for a big cam to reach the point where it's most efficient. Many times people will over cam the engine and all the real power it "could" make is outside of usable range. When we design custom cams for the domestic makes, we try and walk that fine line of too much trapped mass (dynamic compression) so that as much of the combustion/flame is pushing on the piston as possible (maximizing VE). Over cam-ing them sure does make a cool idle though lol - just don't expect to have any brake assist or crankcase ventilation since vacuum won't be there. We actually sell a "thumper" cam people like because has a crazy sounding idle, but doesn't make as much power as the others since the peak power is above the redline.
It wasn't an m20 and has a fair bit better combustion design, but we had a client with 13.8:1 compression running pump fuel (93 octane) with a matching cam (stock engine was 9.6:1). He is in FL, so we don't have any emission standards and never tested his, but based on remote tuning for other clients with a little less CR, his could be made to pass (specially if EGR was retained).
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