Cam and compression ratio compatibility
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===Advanced Reading and Engine Theory=== | ===Advanced Reading and Engine Theory=== | ||
| − | Intake valves close after the piston travels ''' | + | Intake valves close after the piston travels '''after''' bottom dead center (ABDC). This sounds counter-intuitive, but this design aspect was a break through in modern engines and has allowed them to make more power as well as run at higher RPM's. The reason why is all about the Dynamic Compression Ratio (DCR). Air is a gas and as such is compressible (unlike liquids or solids). When air enters the engine is does so at a high velocity, and when the piston reaches bottom dead center, due to momentum, the air wants keeps moving in and starts to compress. In short, the longer the intake valve stays open, the more air can be packed into the cylinder. |
The extent to which intake valves stay open is measured in degrees. IVC (Intake Valve Closing), is one of, if not the most important determining factor in how the cam shaft impacts engine performance. Manufacturers will give this value in degrees ABDC (after bottom dead center). As an example, the Comp Cams XE275H cam's intake valve closes 64º ABDC. | The extent to which intake valves stay open is measured in degrees. IVC (Intake Valve Closing), is one of, if not the most important determining factor in how the cam shaft impacts engine performance. Manufacturers will give this value in degrees ABDC (after bottom dead center). As an example, the Comp Cams XE275H cam's intake valve closes 64º ABDC. | ||
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Performance cams with longer durations, are exactly the opposite. At low RPM's since the intake valve stays open longer ABDC, more of the intake charge is reversed. Hence there is less pressure build up on the compression stroke. To compensate for this, a higher SCR is used. As the RPM's climb, so does the intake charge velocity, thus more air is able to cram itself into the cylinder. Even though the piston is moving up ABDC, the air is compressing upon itself, and able to move into the cylinder because the intake valve is still open. As you can see, at higher RPM's these cams are able to more completely fill the cylinder with air and are able to make more power. These engines have a high SCR and low DCR. They make power at higher RPM's. Since there is a lot of reversion, these engines idle very rough (have a lot of lope). If you were to put a compression tester on these engines you would see something in the 125-150 PSI range. | Performance cams with longer durations, are exactly the opposite. At low RPM's since the intake valve stays open longer ABDC, more of the intake charge is reversed. Hence there is less pressure build up on the compression stroke. To compensate for this, a higher SCR is used. As the RPM's climb, so does the intake charge velocity, thus more air is able to cram itself into the cylinder. Even though the piston is moving up ABDC, the air is compressing upon itself, and able to move into the cylinder because the intake valve is still open. As you can see, at higher RPM's these cams are able to more completely fill the cylinder with air and are able to make more power. These engines have a high SCR and low DCR. They make power at higher RPM's. Since there is a lot of reversion, these engines idle very rough (have a lot of lope). If you were to put a compression tester on these engines you would see something in the 125-150 PSI range. | ||
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===Dynamic compression ratio calculators=== | ===Dynamic compression ratio calculators=== | ||
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