An engine’s compression ratio is actually a theoretical number. This ratio compares the cylinder volume of the piston at bottom dead center (BDC) versus top dead center (TDC). So if we have a volume of 45 ci at BDC and 4.5 ci at TDC, then the compression ratio is 10:1 since the volume at BDC is 10 times the volume at TDC. While this is a useful number, it ignores one crucial variable. The amount of actual cylinder pressure at low >> engine speeds is determined by the intake closing (IC) point.
All
performance camshafts close the intake valve 50 to 60 degrees or more after bottom dead center (ABDC). The longer the duration of the camshaft, the later the intake valve closes. It should also be obvious that the engine cannot begin making cylinder pressure until the intake valve closes. Therefore, the distance that the piston travels up the cylinder at 60 degrees ABDC versus 52 degrees ABDC reduces the volume of the cylinder, reducing the cranking pressure.
Keep in mind that we are talking about a street engine here. Early closing intake valves (short-duration cams) tend to maximize cylinder filling at lower engine speeds, while late-closing intake valves (long duration cams) tend to move the peak
power point higher in the rpm range. With a given compression ratio, adding a longer-duration camshaft will decrease the cranking compression and therefore low-speed throttle response and power.
This is where the
compression gauge comes into play. By checking cranking cylinder pressure, you can use this as a tuning aid. We checked the cranking cylinder pressure on several modified small-blocks >> and discovered an interesting correlation between soggy low-speed performance and low-cranking cylinder pressure. This comes back to long-duration camshafts with late-closing intake valves combined with a relatively low static compression ratio.
It appears that building a performance street engine that combines a decent-duration camshaft with enough static compression ratio to create 175 to 190 psi, will reward you with a very snappy engine that is not only responsive and fun to drive, but makes decent power as well. For example, you can run 11:1 compression on pump gas if you use a long-duration cam with a late-closing intake to bleed off some of that low-speed cylinder pressure. This is the main reason why camshaft companies recommend higher static compression ratios with longer-duration camshafts. This is an attempt to improve power at low and midrange engine speeds by making up for the late-closing intake valve with more static compression.
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