A big portion of moving the powerband around is done through the adjustment of overlap. Overlap is the amount of time during which the intake and exhaust valves are simultaneously open. It is during the sliver of time that overlap occurs that attention must be paid, because it is here that power, gas mileage, emissions, and driveability are all affected. As the exhaust gas rushes out the open exhaust valves, the vacuum it creates helps draw in some of the intake mixture through the open intake valves. A balance must be struck though, taking care to manage the movement of gases in order to avoid an overly lean or rich situation.

By advancing or retarding intake and exhaust cam timing we can adjust overlap and its resulting effects on power and torque. In general, retarding an exhaust cam or advancing an intake cam will increase overlap and advancing an exhaust cam or retarding an intake cam will decrease overlap. The amount of overlap and the proper settings for each cam gear depends on each car, but be aware, it is during overlap that valve-to-piston interference can occur. Possessing an inverse relationship, by decreasing overlap there is an increase in both valve-to-valve and piston-to-valve clearance, while increasing overlap will decrease both.

So, needless to say, try not to advance your intake or retard your exhaust cams too much. Both will decrease your valuable clearances and run closer to the risk of engine damage. Horsepower is good, but pistons hitting valves is definitely not. The best settings to achieve peak power and reliability are different for every car, but the recommended way to find out which settings are best for you is on the dyno.

If you have no knowledge of how your engine will respond, you'll have to try various combinations of advancing and retarding the intake and exhaust gears and note the results of your powerband. When you are satisfied with those results, you can fine-tune your powerband throughout the entire rpm range by adjusting both gears the same amount. In general, advancing timing for both intake and exhaust will increase low-end power and decrease high-end power, while retarding timing for both will shift the power to the top-end.

Also, all adjustable cam gears are not made equal. Due to their adjustable nature, many cam gears on the market make use of small bolts to hold their settings in place after installation. Cheaper models have bolts with a tendency to slip, leading to disastrous results. Attention must also be paid to the machining of the cam gear teeth. Lower quality machining of brittle metals can lead to gears with poor grip that prematurely wear both the timing belt and their own teeth.

Skunk2 CNC machines all of their gears from billet aluminum and utilizes a six-bolt design to ensure the gear will never slip out of place. Their Pro Series cam gears also possess the benefit of being some of the lightest on the market, which brings to mind another good point. Anytime you are able to free up some unwanted reciprocating mass in the valvetrain you and your engine will be very happy.

SOURCE
Skunk2 Racing
http://www.skunk2.com