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Anyone can easily answer that the compression ratio on an SV is 11.5:1. Is that significant? Sort of. Maybe. With supercharging you can have more than static compression ratio while running. If the intake isn't forced (supercharging or turbocharging) the running compression ratio is always lower than the static ratio.

Static compression ratio is just a number, (the total volume of the combustion chamber + the cylinder with the piston at bottom dead center) divided by (the volume of the combustion chamber with the piston at top dead center). It's stated that way because the top of the piston isn't flat on all engines.

So when your engine is running is that the compression ratio? Nope. Some of the volume is missing because of pumping efficiency. That is, the piston may go to the bottom of the stroke, but there isn't enough time for the volume to fill to atmospheric pressure. So the charge starts below atmospheric pressure.

There's also the complication of cam overlap. Both the exhaust and intake valves are open at the top of the exhaust stroke. This is to promote scavenging, the use of the inertia of the exhaust gases to pull more spent charge out and get the new charge started. At higher RPMs the intake valve stays open past bottom dead center. This gives a bit of extra time for the flowing gases to pack into the cylinder by inertia. At lower RPMs this extra packing is less efficient because the intake charge is traveling slower, so it doesn't have as much inertia.

Even with efficient scavenging and inertia + sonic intake charge packing the fresh charge is always at a lower pressure (so lower density) than it would be under static conditions.
 

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So are you volunteering to make a variable runner length intake? I'll be happy to test it out. Or perhaps some variable valve timing to get us closer to that static compression ratio? Sign me up.
 

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Has it ever been posed to give andyauger his own forum? He will randomly come out with these in depth and amazing explanations, it would be kind of convenient to have them all in one place for people to browse over.
 

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Yea but if he had that room we'd never hear from him again.
 
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