* Advance intake and exhaust => more low-RPM power, less high-RPM power Simple cam tuning rules for BOOSTED engines: Scavenging is using the out-flowing exhaust to help draw in the next intake charge (partially causing lumpy idle). In a naturally aspirated engine, the extra overlap is called 'scavenging'. * More overlap => more high-RPM power, less low-RPM power * Less overlap => more low-RPM power, less high-RPM power * Retarding both cams => more high-RPM power, less low-RPM power * Advancing both cams => more low-RPM power, less high-RPM power Simple cam tuning rules for NATURALLY ASPIRATED engines: To reduce overlap, you have to ADVANCE the EXHAUST, and/or RETARD the INTAKE. To increase overlap, you have to RETARD the EXHAUST, and/or ADVANCE the INTAKE. Overlap is the point where the exhaust valve is closing, and the intake valve is just opening. This means that the first lobe to come through the rotation will be the exhaust lobe, immediately followed by the intake lobe. Then both valves stay closed for the compression and combustion strokes. The intake starts to open, just as the exhaust is closing, piston goes down, and the intake valve closes. Looking at a camshaft, the sequence would be as follows: The exhaust lobe pushes open the exhaust valve and the piston comes up to push the exhaust out, then starts to close. **The “start” is not important because it’s a CYCLE, meaning it repeats** Camshafts don’t make magical horsepower from nowhere, they simply move the powerband around by changing the volumetric efficiency to attain the desired results. The art of designing camshaft profiles is meant to increase the volumetric efficiency in the RPM range that the customer requires.

A naturally aspirated engine can also have a slightly larger than 100% volumetric efficiency, but it will only happen for a short duration, and is usually only in the peak of the powerband. A forced induction engine will have a larger than 100% volumetric efficiency since the intake charge and combustion chamber are being pressurized. If a 2.0L engine fills with 3.0L of an air/fuel mixture, we say its volumetric efficiency is 150%. If a 2.0L engine is filled with 2.0L of an air/fuel mixture, we say its volumetric efficiency is 100%. Volumetric efficiency is based on cylinder fill. On four stroke engines, it is important to realize that the cam rotates once for every two rotations of the crankshaft.

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