On four stroke engines, it is important to realize that the cam rotates once for every two rotations of the crankshaft.

Volumetric efficiency is based on cylinder fill. If a 2.0L engine is filled with 2.0L of an air/fuel mixture, we say its volumetric efficiency is 100%. If a 2.0L engine fills with 3.0L of an air/fuel mixture, we say its volumetric efficiency is 150%. A forced induction engine will have a larger than 100% volumetric efficiency since the intake charge and combustion chamber are being pressurized. 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.

The art of designing camshaft profiles is meant to increase the volumetric efficiency in the RPM range that the customer requires. 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 four strokes of the engine are:
Exhaust
Intake
Compression
Combustion
**The “start” is not important because it’s a CYCLE, meaning it repeats**

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 intake starts to open, just as the exhaust is closing, piston goes down, and the intake valve closes. Then both valves stay closed for the compression and combustion strokes. This means that the first lobe to come through the rotation will be the exhaust lobe, immediately followed by the intake lobe.

Overlap is the point where the exhaust valve is closing, and the intake valve is just opening.

To increase overlap, you have to RETARD the EXHAUST, and/or ADVANCE the INTAKE.
To reduce overlap, you have to ADVANCE the EXHAUST, and/or RETARD the INTAKE.

Simple cam tuning rules for NATURALLY ASPIRATED engines:
Advancing both cams => more low-RPM power, less high-RPM power
Retarding both cams => more high-RPM power, less low-RPM power
Less overlap => more low-RPM power, less high-RPM power
More overlap => more high-RPM power, less low-RPM power

In a naturally aspirated engine, the extra overlap is called "scavenging". Scavenging is using the out-flowing exhaust to help draw in the next intake charge (partially causing lumpy idle).

Simple cam tuning rules for BOOSTED engines:
Advance intake and exhaust => more low-RPM power, less high-RPM power
Retard intake and exhaust => more high-RPM power, less low-RPM power
Less overlap => lower EGTs, faster turbo spool, less fuel
More overlap => higher EGTs, slower turbo spool, more fuel

Boosted engines don’t like overlap. The incoming cold air and fuel cools down the outgoing exhaust charge, condensing the exhaust gasses. This is VERY counter-productive in a turbo application since the engine needs no help from scavenging to fill the cylinder. I've heard this being called "turbo chill".

Cool, condensed gasses in the same space push less hard on the turbo, causing lag. HOT gasses are better at spooling the turbo, thus the advanced exhaust timing to open the valve sooner in the power stroke. This steals some of those hot, expanding exhaust gasses to help spin the turbo a little faster. When the piston is near the bottom of the bore, hardly any energy is going into rotating the crank anyway, so stealing expanding gasses won’t hurt anything. The retarded intake just helps cut down the overlap further.

Retarding overall cam timing:
Retarding overall cam timing is better for high-RPM power. This is because the valves are closing later. The intake valve is closing AFTER the piston has started to travel back up the bore for the start of compression stroke. This is terrible at low RPM because the intake air velocity is low, and air that was once in the cylinder is now being pushed back into the intake manifold and causing turbulence.

At high-RPM, the rules change. Air has weight, and thanks to Sir Issac Newton, we know that once it is moving, it doesn’t want to stop moving. This means that the air can continue to flow into and fill the cylinder, EVEN AFTER the piston has begun to travel UP the cylinder bore. This can allow an engine to exceed 100% volumetric efficiency, if even by a small amount.

Advancing overall cam timing:
Advancing overall cam timing is better for low-RPM power. This is because the valves are closing a little sooner. The intake valve is closing AT or NEAR when the piston is at the bottom of the bore for the start of the compression stroke. This is great at low RPM because the intake air velocity is low and easily affected by changes in the direction of piston movement in the engine. Almost as soon as the piston gets to the bottom of the bore on the intake stroke, the valve gets slammed shut so no air can escape as the piston begins to travel back up the cylinder on the compression cycle.

At high-RPM, this may become a restriction since the air has inertia and responds a little slower to pressure changes, potentially choking the air flow to the engine a little.

Conclusion:
This information is aimed at allowing tuners to understand what happens when cam timing is altered. When a larger duration camshaft is being installed, unless the lobe centerlines have been changed, the overlap will be increased. If installing larger camshafts in a turbo application, advancing the exhaust and retarding the intake will reduce the inherent increase in overlap caused by upgrading to a larger profile. Most cam grinders, especially regrinders, put the new profile in the same position as the old profile because it is easier, or the only way possible. This has to be changed when the cams are installed in an engine to attain the desired result.

A forced-induction engine should idle smooth when properly tuned, and a naturally aspirated engine should be “lumpy” and have a lope if it is tuned aggressively towards the high-RPM range. If a forced induction engine is loping at idle, fuel is being wasted, turbo spool time is being increased, and power is being lost.