how much compression is lost by flycutting
10-01-2009, 04:51 PM
#5
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!
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10-01-2009, 05:50 PM
#6
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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.
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.
10-02-2009, 11:13 AM
#7
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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.
