Thursday, May 14, 2009

Figuring Compression Ratio

Compression ratio is the volume of the cylinder when the piston is at the bottom of the cylinder compared to where it is when the piston is at TDC. A 100 cubic inch cylinder would have its volume squeezed into 10 cubic inches with a 10:1 ratio. The easiest way to keep track of it is to think of every thing as volumes that are stacked on top of one another. The factors that stack up are the displacement of the cylinder (bore and stroke), the volume of the deck clearance (getting back to the zero-deck issue from before), the volume of the gasket (which is basically a short-round cylinder), and the volume of the combustion chamber. The net combustion chamber volume means you must subtract dome volume add the dish volume. Use the formula (bore x bore x stroke x .7854 x 16.4) to get the volume for a cylinder in cc's. Stack up the cylinder + the deck volume + the gasket volume + the net chamber volume---take this number and call it A…Stack up the deck, the gasket, and the net chamber volume and call it B….. Take the big number A and divide it by the small number B and it will give you the compression ratio.

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