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Author Topic:   getting to 9:1
posted November 03, 2002 10:06 AM
building a new motor, and want to know how to measure to get close to 9:1 comp. we get checked with a whistler. last motor I put in 9:1 pistons with fel-pro gaskets and pumped 8:2. how much would steel shim gaskets raise the comp.?

posted November 03, 2002 06:02 PM
How long is a piece of string? If you are building a new motor, you need to figure it out.
Using data from your new motor(in units of .001, like 4.030,etc.) do this:
Bore x Bore x Stroke x 12.87
Divide the answer by 8. Now you have the clearance volume to shoot for for 9:1, in cc's.
Clearance volume=
Volume due to piston deck height(+ if in the hole, - if out of the hole). Plus:
Volume of head gasket, plus:
Volume of piston notches(+ for notches, - for net dome), plus:
Volume of combustion chamber, plus:
The volume that exists between the piston and cylinder, above the top ring. Most people ignore this, or don't even realize it is there. If you can't handle the math, use 1.1 cc.
You have discovered the 'Piston Rating' deal that has tripped up many. A guy builds a "12:1" motor, and wonders why it runs like it does. Don't believe anything until you cipher it out yourself.

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