Quote Originally Posted by allmotoronly
if lowering the compression is your goal, it would be a lot easier just to get some lower compression pistons. Swapping from a P72 head to a P30 head would not lower the compression that much. The P72 head has a 41.6cc combustion chamber, and the P30 head has 42.7cc combustion chamber. thats about a 2.5% difference. Now according to you, you shaved .012 off of the head. That means you lowered the piston to deck height from .030 to .018, which is pretty significant. According to my calculations, you removed another 1.709cc's from the combustion chamber when it was shaved. so now it is down to about 39.991 CC's, which is about 4% less CC's than stock, and about 6% less than a B16A2 (P30) head. All in all, you are still not going to get that much of a drop by switching to a P30 head.
When I say lower the compression, i'm speaking in specific terms. I want to run ctr pistons in a gsr block. That setup with a gsr head would put my compression ratio at about 12.5:1. Now, with the shaved head, just to be safe, ima say it'll be around 12.6:1.
After rigerous research, a gsr block, with a b16 and ctr pistons should yeaild around 12.2-12.3:1. Now, that may not seem like a big difference to the un-aided eye, but it is a big deal when dealing with detenation. At around 12.5:1 compression, 93 octane gas reached it's limits, and thats a battle ground I do not want to be on. So I figured 12.3 is much safer that 12.6.