Milling .010", .020" and .030" off a 3400 head

[SappySE107]

Compression ratio changes based on milling the heads has been guessed at quite a bit over the years. I remember testing .010" at once point and it didn't change compression much so I stopped caring. After being told by someone that their combustion chambers were 26cc and they took .030" off, I just had to know what the numbers looked like leading up to that. So, here you go. Note before I throw numbers out here, most stock 3400 heads have been 28.6cc chambers, or real close. Not 29.x.

Started with 29.4cc.
.010" off = 27.4cc
.020" off = 26.4cc
.030" off = 25.7cc

The chamber contours as you remove material so it isn't linear. This is 1 test on 1 combustion chamber.

What does that mean for compression?

Started with 9.53:1
.010" off = 9.8
.020" off = 9.94
.030" off = 10.04