Editing Cam and compression ratio compatibility (section)
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===Static compression ratio vs. Dynamic compression ratio=== {{Note1}} *The '''static compression ratio''' (SCR) is computed using the full swept volume of the cylinder from BDC to TDC compared to the volume of the combustion chamber, valve reliefs (or dome), and all the other volumes not occupied by the piston stroke. *The '''dynamic compression ratio''' (DCR) is computed from the position of the piston at the point of IVC to TDC compared to the volume of the combustion chamber, etc. (instead of using BDC to determine the swept volume of the cylinder). The relationship between SCR and DCR when different IVC points are used can be seen [http://www.crankshaftcoalition.com/wiki/Dynamic_compression_ratio '''here''']. Most factory production cams are considered to be short duration cams. This means that the intake valve closes much sooner ABDC than a high performance cam might, and thus less of the intake charge is pushed (reversed, thus the term ''reversion'') past the intake valve back into the intake tract. Since less of the intake charge is lost to reversion, more of it stays in the cylinder, so the compression stroke can yield a satisfactory DCR with a relatively low SCR. An engine having a lower SCR and an early IVC figure will produce power in the lower RPM range. Since there is little reversion, these engines idle very smoothly (no lope) and will have high idle vacuum around 20 in/Hg. If you were to put a compression tester on these engines you would see something in the 150-190 PSI range. Performance cams with longer durations begin to exhibit the opposite traits of a short duration cam. At low RPM more of the intake charge is lost to reversion because the intake valve stays open longer ABDC. Hence there is less cylinder pressure built up on the compression stroke. To compensate for this, a higher SCR is used. As the RPM climbs, so does the intake charge velocity. Now the later closing intake valve will allow more air to cram into the cylinder. At higher RPM this type of cam is able to more completely fill the cylinder, so will make more power. An engine having a high performance cam (later IVC point) needs a higher SCR in order to keep the DCR within an optimum range. Because the late IVC point can cause reversion (along with an increase in overlap and/or a tighter LSA), this type of tune can cause the engine to idle rough (have a lot of lope). Idle vacuum will be lower and if you were to put a compression tester on these engines you would see something in the 125-150 PSI range. ====Estimating intake closing point==== If the intake closing (IC) point isn't known, it can be calculated: # Divide the intake duration by 2 # Add the results to the lobe separation angle (LSA) # Subtract any ground-in advance # Subtract 180
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