Focus Fanatics - View Single Post - How To figure the compression ratio of your Zetec
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Old 05-23-2009, 08:13 AM   #12
guitarseth
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Join Date: May 2004
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Location: Wooster, OH, United States
What I Drive: 2003 French Blue Metallic ZX3

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I might have to pick up a junk head and see how much my machine shop would charge to cut 0.010" slices and cc the head after each slice. Take it out to 0.060"s, so we could have some idea. Its only ~$20 to get it milled, but I'm sure that would be per cut and ccing would be extra. I had him cut 0.020" on the head I'm having done.

Your calculations assume that 0.010" takes out a completely cylindrical section of combustion chamber, which may not necessarily be true (once you get past the first couple thousandths), but its definitely a good approximation. I ran through the math for my engine and will end up with around 11.3:1. That's with 10.6:1 pistons and 0.020" milled off the head. That's actually pretty close to what I was expecting it to be. Like Tom (and you) said, you really need to know the cc of the head.

Also as an additional note, its not necessarily clear in your post that to calculate the final compression ratio, you don't want to subtract the head gasket volume from the calculated combustion chamber volume. I kept subtracting head gasket volume and was at like 13.4:1. I knew that wasn't right.
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