Some Things To Consider
Not everyone’s bike or riding style is the same. Camshaft selection must be based on what you’re looking for as well as the component parts and accessories being used on the motorcycle. The installation of the right cam can produce both performance and reliability. Here are the key factors to consider:
1. Riding Style
Is the bike for weekend use or your main form of transportation? Is the bike being used for short trips around town “from light to light”, or for touring long distances? What terrain do you ride? Is high performance or racing your primary concern?
2. Weight Of The Bike
What does it weigh? Is it light, or weighted down with accessories? Do you tow a trailer or have a side-car? Do you ride solo or tandem?
3. Operating RPM Range
What gearing does the bike have? How many speeds to the transmission? Does the bike have “International” gearing? What rpm range are you most likely to be operating in?
4. Engine Displacement
And Compression Ratio
What cubic inch is your engine and what compression ratio does it have?
5. Engine Modifications
And Engine Accessories
To obtain the maximum potential from your bike it is important to have engine components that are compatible. The components, accessories and modifications to consider are:
a. Air Cleaner Filter, Jetting
b. Exhaust System
c. Ignition System
d. Carb Size, Style, or F.I.
e. Compression Ratio
f. Cylinder Head Modifications
1. Performance Valve Springs
2. Enlarged Valve Diameters
3. Porting, Polishing, Milling
g. Aftermarket Cylinder Heads
h. Nitrous Oxide
i. Supercharger or Turbo
The Heart Of The Matter
An engine is a giant air pump and the more efficiently the air moves through it the more power the engine makes. The camshaft opens and closes the valves allowing the air to enter and exit. Camshafts differ in design, but the two main factors are the amount of “duration” and “valve lift”. The “duration” is the time the valve is open (measured in degrees of crankshaft rotation). Duration determines the “rpm potential” of the engine and power band. The longer the duration period the higher the RPM the power band will operate at. The “lift” is the distance the valve opens (it is measured in thousandths of an inch of travel). The more lift the more air/fuel mixture can enter and exit the engine, thereby producing more torque.
Bigger’s Not Always Better
The camshaft provides an “rpm power band” that is approximately 3,000 rpm wide. This rpm power band can be produced in either the Lower Range (from off idle to 3,500 rpm), in the Mid Range (from 2,000 to 5,000 rpm) or in the High Range (3,500 to 6,500 rpm). Select the camshaft that will perform the best in the particular rpm band that the bike will be operating in. The amount of valve lift that the engine can accept is often determined by valve to piston clearance, valve to valve clearance, and the amount of travel the valve spring can handle before its coils touch one another and bind up. Big lift camshafts could require modified cylinder heads or pistons and performance valve springs with additional travel.
Power vs Weight
The “horsepower to weight ratio” is very important. You must produce the right amount of power at the correct rpm range. A heavy bike normally needs a cam that will provide good lower rpm power (torque) to get the bike moving. Generally, a cam with less duration would work the best in a heavy bike. A lighter bike requires less low end power, therefore a cam that provides power in the higher rpm range would be better suited. But of course there are always the exceptions. A rider with a heavy Dresser who rides the open highway might not care how long it takes for the bike to wind-up to speed. He likes to ride at 65 mph and wants to pass up to 90 mph! In this case, a cam with increased duration and rpm potential would work best.