Combustion Chamber Volume and Compression Ratio

While the formula is correct, some of the dimensions are wrong; this page will be rewritten soon.

First off here is what we have for combustion chamber volumes:

XV700
: 40.7cc
XV750
: 51.3cc
XV920: 63cc
1000 Virago: 63cc
TR1: 55cc
XV100SE: 67.1cc
XV1100: 68.6cc

The 920 and Virago 1000 I have measured directly with graduated beakers and syringes. The other models are best-guesses calculated from advertised compression ratio; stated bore and stroke; a presumed installed height of .6mm (.024") for stock head gaskets on those models that have non-recessed gaskets. If you have information to the contrary, click the error trap button and send us the details, please.

The volume of a right circular cylinder (the kind we're interested in) is (Pi) times the (radius squared) times the height. That's half the bore times itself, times Pi, times the stroke For example: a 920 cylinder is 92mm across, so its radius is 46mm. We square that, multiply it by Pi, then by the stroke (69.2mm) and get 460cc -- half of 920cc as it is a twin. If you are following along with a calculator (or in your head), you'll have to divide your total by 1000, as you are starting with (linear) millimeters in a single dimension and ending with (cubic) centimeters in three dimensions. Now, the gaskets are recessed into the cylinder on this model, so we can neglect their thickness and consider the cylinder and head to be in direct contact.

Compression ratio is reckoned as the volume of the cylinder plus the volume of the combustion chamber (plus any volume introduced by gasket thickness) divided by the volume of the combustion chamber (with gaskets). More simply, it's the volume of the cylinder and head at Bottom Dead Center (BDC) divided by the volume of the combustion chamber -- the space left at Top Dead Center (TDC) (on the compression stroke).

Back to the 920, that's 460cc plus 63cc, then that total (523cc) divided by 63cc giving a compression ratio of 8.3:1 -- exactly as claimed.

Say you're nuts for low-end and don't mind running smaller valves. You could slap 700 heads on top of your 920. You would get (460 + 40.7)cc divided by 40.7cc => 12.3:1 compression ratio! Impressive.

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