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Here you can see the results of porting the combustion chambers so the valves are "unshrouded". This can improve flow, increase the combustion chamber volume and lower the compression ratio. When the pistion is at its closest approach, the "squishing" action on the flat portions force a turbulent jet of mixture into the combustion chamber. The better mixture burns more efficiently and is more resistant to detonation. Many believe the piston should approach as close as practical in order to maximize this effect, but clearance is needed to allow for piston rock, thermal expansion, etc. The minimum is about .060" for this motor. The compression ratio can get too high unless the combustion chamber is made large enough. More volume can be made by cutting large valve relief pockets in the top of the pistons. In order to keep the C.R. down to 8.5, the clearance (deck height) will be 0.090". |