The stock head and pistons produce a 9:1 compression ratio. This is a 626cc motorcycle engine, and therefore has a very small chamber, in the area of 17cc's. Redline is around 9700 rpm.
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Honda CB650 Forums:
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Originally Posted by pae
Combustion chambers are ~ 60mm diameter by 17mm deep, and pretty well spherical in shape. At a rough calculation that makes them about 20cc each, but I'll do that accurately later with a pippette.
The old head gasket measures 1.25mm.
I'll check the deck height for your (top of barrel to top of piston at TDC) so you know how much you might have to play with. The pistons are of course crowned so that'll reduce the volume, I'll check on that too.
An original brochure lists compression ratio (UK) at 9.0:1. So with bore/stroke of 59.8 x 55.8mm that gives 156.7cc per pot (627cc overall). If compression is 9:1 then the total combustion volume should be 17.4cc. With a head gasket of 1.25mm thick that's 3.5cc, leaving 13.9cc in the deck and combustion chamber combined.
If you want to get to 10:1 then you'll need to take about 1.73cc out, to reduce it to 15.7cc, or about half a head gasket thickness (~0.6mm).
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Originally Posted by pae
Hmmmm, did a 'fairly accurate' measure of the combustion chamber volumes at the weekend and they are about 26-27cc which is a bit more than I'd expected. Having looked at the pistons though they are quite crowned so I have to assume for the moment there's about 12cc of crown on them to bring the squish volume back down around 14cc (plus gasket at 3.5cc).
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Originally Posted by cb650
I took off about .o25" (inch) on a couple of mine with no probs. Some day going to make the kid play with one and do some clay to find out the valve clearance.
Terry
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