GM Compression ratio
12-12-2003, 07:01 PM
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GM Compression ratio
When GM advertises there comp. ratio on there crate motors it appears to me they use swept volume versus chamber volume.They do not appear to take deck height or gasket volume into effect.I have a new crate motor that I have decked and I was concerned about the commpression increase,but when I do the math I can't come up with the original 9.6:1 they are advertised at.Am I missing somthing.You do the math,4.47 bore,4 stroke 110cc heads,pistons .010 down hole,10 cc gasket volume,and ? cc dome(I have heard between 1 and 3cc)?
12-13-2003, 07:17 PM
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You can cc the piston (along with the piston-to-deck clearance) by running the piston exactly .500 down from TDC, sealing the piston-to-wall gap with light grease, placing a piece of plexiglass with a small hole in it over the bore, and using a buret or a large syringe (available from a pet store) filled with vegetable oil or light machine oil to measure the volume. When you know how many cc's of fluid you added you can subtract the .500 cylinder volume to get the cc volume of only the piston and deck clearance.
