Cam and compression ratio compatibility
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Intake valves close after the piston travels '''past''' 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 it 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. | Intake valves close after the piston travels '''past''' 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 it 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 | + | 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 XE274H cam's intake valve closes 64º ABDC. |
Factory cams are low duration cams, this means that the intake valve closes much sooner ABDC and thus less of the intake charge is pushed (reversed) out of the intake valve. Since less is reversed, then the majority of it stays in the cylinder, and as such, the compression stroke will yield statisfactory DCR compression with 8.5:1 SCR. This type of engine will have a low SCR and a high DCR. These engines produce power in the lower RPM ranges because they cannot utilize the effects of DCR. Since there is little reversion, these engines idle very smoothly (no lope). If you were to put a compression tester on these engines you would see something in the 150-190 PSI range. | Factory cams are low duration cams, this means that the intake valve closes much sooner ABDC and thus less of the intake charge is pushed (reversed) out of the intake valve. Since less is reversed, then the majority of it stays in the cylinder, and as such, the compression stroke will yield statisfactory DCR compression with 8.5:1 SCR. This type of engine will have a low SCR and a high DCR. These engines produce power in the lower RPM ranges because they cannot utilize the effects of DCR. Since there is little reversion, these engines idle very smoothly (no lope). If you were to put a compression tester on these engines you would see something in the 150-190 PSI range. | ||
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