Alloy Dommie Head CC's

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Jan 27, 2015
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Hi All,

Trying to work out how much my compression ratio will increase when i change heads over to fix a spindle issue. New head has had 6thou taken off the deck. I have a calculator to plug the numbers in but need to know the cc amount for the head??
Anyone ever measured this for 500cc dommie.
Cheers
 
Here is a formula that I worked out for calculating the compression ratio change when adding a base shim. This should should work for you, without the need to know CC volume. In your case, the "shim" thickness is minus 0.006 inches. Plug that in to the formula with the stroke in inches.

Here is a formula which can be used to either calculate the thickness required to achieve a desired CR, or to determine the CR that will result from a given thickness:

h/s = (CR1 - CR2)/((CR1 - 1)(CR2 - 1))

h = thickness of shim
s = stroke of engine
CR1 = known compression ratio before shimming
CR2 = compression ratio after shimming

Note: the formula is dimensionless - this means thickness of shim and stroke can be in inches, mm, or furlongs....just be consistant! Also consider that one gasket at cylinder base is the norm and using a shim will require two gaskets....the compressed thickness of the second gasket should be considered part of the overall shim thickness.

Slick

PS: I measured the CC volume in my Atlas head. I leveled the head both ways, then added water with a syringe calibrated in ml until the meniscus formed on the deck lip. If you have a spigot, be sure to subtract the spigot volume. I think this method is accurate to +/ - 2-3 ml.
 
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