Quote:
Originally Posted by Old Mechanic
Compression ratios are calculated by considering the bore and stroke as well as the volume of the combustion chamber area that is still present at TDC.
That represents the theoretical maximum compression ratio. In reality any restriction present that reduces the amount of air that is actually in the cylinder at BDC reduces the usable (I call it effective) compression of the engine. In some cases cam timing can allow the atmospheric pressure to actually push more air in the cylinder than the measured theoretical compression.
When you have a manifold reading of 14 inches, you are only allowing the remaining inches of measured air into the cylinders, so to say that the same amount of air always passes through and engine is incorrect. You can confirm this by just considering the exhaust exiting the tailpipe. At idle it is a very small amount, Rev the engine up and the amount vastly increases.
Lean burn, when properly designed and utilized compensates for engine operation states where manifold vacuum is fairly high, probably above 40% of atmospheric pressure. Effective compression is much lower than the potential maximum.
Since higher effective compression creates more pressure during combustion, lean burn allows for the compression to be slightly higher, while preventing the power from being more than is necessary.
regards
Mech
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The compresion ratio is = volume at BDC/volume at TDC [ie 10:1]
What the throatle plate does is control the MASS of air entering the cylinder.
Because the mass of air is reduced, at part throatle, when the intake valve closes the pressure in the cylinder is reduced, therefor what changes at Top Dead Center is the Pressure. The compresion ratio is fixed as is the vollume, and does not change. What changes is the compression pressure.
I make these comments for better clarity I hope it helps.
