Cam and compression ratio compatibility
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*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 '''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 '''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. | 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. | ||
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