The camshaft plays an important role in the SportTuning of your water-cooled VW. The correct application of camshaft design will ensure a proper balance between drivability and performance.
Race cams normally have rough idling characteristics due to the large overlap required for high rpm power. Street cams generally use a short duration and keep the lobe centers spread apart to prevent too much overlap. Too much overlap on fuel injected cars equipped with Lambda is especially bad. This can cause the airflow sensor to buffet up and down, forcing the system to overcompensate in either direction in an attempt to keep the CO in check.
Diagram #1 explains what happens to the fuel mixture when cam overlap causes both intake and exhaust valves to be open at the same time. Some of the mixture introduced into the combustion chamber is lost out of the tailpipe when the overlap is lengthy. The loss of partially burnt fuel increases hydrocarbon emissions without generating the low-end power desired for the street. However, a little overlap can help scavenge the mixture into the combusition chamber on normally aspirated vehicles. This is where the correct technology plays an important role in today's cam design.
The duration of a cam is measure in degrees of rotation on a circle when the valve is opened off its seat. An example would be a cam having 225 degrees of duration at .050". This is a portion of a 360 degree circle when the valve is lifted at least .050 inches off its seat. The longer the duration, the more chance of the intake and exhaust lobes both being open at the same time (overlap).
Another factor affecting overlap is the distance between adjacent lobe peaks. This is referred to as a lobe center. Simply stated, it is the measurement of distance in degrees of rotation from the peak of the intake lobe to the peak of the exhaust lobe. The closer the lobe centers are to one another, the more overlap. Fuel injected Volkswagen cams run best on 110 to 115 degree lobe centers. Racing cams are on the other end of the spectrum and run as close as 102 degree lobe centers, creating very rough idling characteristics.
The base circle of a cam is the diameter of the heel dimension. Base circles on cams may vary depending on certain design features in the cylinder head. Such things as valve stem length, valve seat height (how far down the seats are set in the combustion chamber), grind on the valves and seats, thickness of the lifter/cam follower, location of the bore in the cam towers, and the basic dimensions of the cylinder head casting are all factors. The base circle diameter of a factory solid lifter cam is between 1.496 and 1.500 inches. On the hydraulic lifter cam the base circle is between 1.331 and 1.336 inches. The top of the hydraulic lifter is approximately .0825 inches closer to the cam centerline than the solid lifter.
A smaller base circle can change the valve train dynamics during running. If a cam, such as a regrind, has a reduced base circle, the cam profile becomes more sharply pointed. This can lead to harsh opening and closing of the valve, and may even cause the lifter to skip off the cam surface during high rpm conditions. High wear or harmonic valve spring failure may result from these problems.
A quick and easy way to measure the lift of a short profile cam is to subtract the measurement of the base circle (B) from the measurement of the lobe height (A) using a set of vernier or dial calipers.
An example: (Line A) 1.919"
minus (Line B) 1.496"
Cam Lift =0.423"
Both intake and exhaust profiles should be checked because there may be a difference in the lift and duration of each respective lobe (referred to as an asymmetric cam). This "quick" method of measurement is not always accurate on racing cams. Long durations can cause inaccurate measurements at the narrowest point of lobe. In this case, the only accurate means of measurement is by using a dial indicator and a set of "V" blocks for rotating the cam, or a computerized cam measurement device.
Perhaps the most important thing to remember when searching for that "perfect camshaft" is this: Don't be mislead by the numbers! The lift and duration of a camshaft are not the only features that make a cam behave the way it does. Things like ramp profile, lobe center, and nose shape all affect a cam's behavior. It is virtually impossible to determine exactly how a cam will perform, or where it makes power, just by looking at the numbers. Ultimately, all of a camshaft's design aspects play a role in how it will perform.