i was wondering how much compression is lost due to flycutting 0.100 on intake and exhaust.

It's not enough to worry about. You lose maybe 1.5 - 2 cc's, if that. Maybe .1-.2 loss.

Like said, not much to be too concerned about PTV clearance is more important.

+1

You will loose a lot more compression when Mr Valve says hello to Mr Piston.

I would not worry about the lost compression. Cut a deaper pocket, mill the head more and it should even out.

On a stock LS1 short-block with 243 (64cc) heads, by cutting a 2cc valve relief you stand to drop about .3 on your static compression. That's enough to plan for, IMO! While you've got it apart, mill the heads down a bit to get some compression back out of her, and make sure to cut enough to compensate!

I seem to recall reading an .080" deep intake notch will be about 1cc.

This of course varies on how large of diameter cut you are making too. Cutting a 2.125" dia notch for an intake valve will obviously remove more material than cutting for a 1.6" exhaust valve.

Since you are cutting both .100" deep, -2cc would be a reasonable estimate.


When I cut mine, I planned on a 2cc loss for compression calculation purposes. When I got to actually cutting, and checking p/v, it turned out I didn't need to cut for the exhaust, and didn't. So my 11.5 compression goal suddenly became 11.67.

Which has not been a problem.

sounds good i did mill the head .025 and used cometic .040 gaskets i had just been wondering what my compression ratio was approx i figured it would be arround 11:1. i have been tring to run down what my power loss down low is coming from. car has super slow take off not sure if i need a bigger stall, currently vig3200, i figured raising the compression would yeild more tq for take off not loose some.guess i need to mess with tune first.