Originally Posted by Burky
Is there a formula for calculating compression ratio knowing only bore,stroke, and the volume of the combustion chamber when the piston is at TDC?
The volumes contained by the gasket, the space if any between the top of the piston and the block's deck, any volume contained in the piston crown which can vary from 4 to 7 ccs for a flat top with valve reliefs to well over 20 ccs with a dish, to a negative number with a dome. So these are numbers you need to be accurate, if not in the ball park as these numbers can become a large fraction of the combustion chamber volume.
You can force the on line calculators to give you an SCR (Static Compression Ratio) without this information. Some will accept zero as an input; others an innocuous number like 1 or .1. But the computed ratio can be very far off the mark since the gasket, deck clearance, and piston crown volumes aren't known.
An example; a Chevy 355 (350 with a .030 overbore) with a 64 cc head, a .053 inch thick head gasket, .025 inch clearance from the piston crown to block deck, and 6ccs worth of valve relief in a flat top piston generates an accurate SCR of 9.4 to 1. However, if you take the gasket, clearance and valve relief data out and recompute only with swept volume divided by combustion chamber volume it gives an SCR of 12.4 which is 3 ratios off. That's a big miss, it's the difference between running 92/93 octane pump grade premium or having to run 110 leaded racing fuel.