Camshaft Basics

Not so long ago, the bigger is better philosophy reigned supreme regarding camshafts. The result was overcammed engines that sounded great and could crank serious top-end power, but were not very streetable and couldn't idle to save their lives.

But thanks to modern cam technology, you can come pretty darn close to the Holy Grail of street bumpsticks—cams that make high rpm power, have good low-end torque and drivability, decent vacuum, and that loping idle we all love. Camshaft theory is a complex subject that can take a book-length article to explain. We're going to concentrate on the basics you'll need to know to choose a good performance street cam.

Lift and Duration

Lift and duration are the primary factors that determine a cam's profile. Lift is the amount a cam lobe actually moves a valve off its seat, and is measured in fractions of an inch. Duration is the amount of time a cam keeps a valve off of its seat, measured in degrees of crank rotation.

Lift and duration combined determine total open valve area—the space available for air and fuel to flow into and out of the combustion chamber. The more valve area open to flow, the more power an engine can theoretically make. The trick is to "size" a cam to optimize valvetrain events for your particular engine combination.

Cam Sizing

Virtually every cam maker uses duration to rate camshafts. When someone talks about a "big" cam, they are referring to cams with longer duration. This keeps the valves open longer, increasing midrange and top-end power at the expense of low-end torque. A shorter duration cam does just the opposite. Because it doesn't keep the valves open as long, a smaller cam boosts low rpm torque and drivability. There are two ways to measure duration:

Advertised Duration is the figure you usually see in the cam ads and hear about at those late-night bench races. The problem with advertised duration is cam makers use various methods of measuring it, making it difficult to compare cams from different makers.

Duration at .050 measures duration at .050 inches of valve lift. Since all cam grinders use this measurement, it's a much more accurate way to make a comparison. Two cams may be very close in advertised duration, for example, but make peak power at different rpms.

Lobe Separation Angle

Lobe Separation Angle, or LSA, is the number of degrees that separate the peak lift points of the cam's intake and exhaust lobes. LSA helps determine the cam's behavior; you can take a given set of lift and duration figures, change the LSA, and get cams with vastly different characteristics. Generally, a cam with wider LSA (112-116 degrees) offers less overlap between intake and exhaust opening and closing events. That translates into a wider rpm range, better idle quality, and higher engine vacuum, but at the cost of less torque at low and midrange rpm. A cam with a narrow LSA (104-108 degrees) offers greater low and midrange torque production, but with a narrower operating range, a choppy idle, and less engine vacuum. For the street, typically you want a cam that offers a compromise--decent idle quality, and good overall power production. separation. Again, much depends on the overall engine combination and intended use, but as a general rule, cams with a 110 to 112 degree LSA offer good power and decent street manners.

Hydraulic or Solid?

Roller cams for overhead valve engines are available with hydraulic and mechanical lifters. Hydraulic lifters are self-adjusting; they use an oil-damped, spring-loaded plunger to help maintain valve lash (the distance between the valve stem and the rocker arm tip). Hydraulic lifter cams are quiet, require virtually no maintenance, and transmit less shock to the valvetrain. Their main drawback is a tendency to "pump-up" (overfill with oil) and cause the valves to float, or stay open too long, at high rpm. Valve float kills power, and can lead to engine damage if you maintain excessively high RPM's Mechanical, or solid, lifters are not self-adjusting. They rely on a properly set up, adjustable valvetrain to maintain proper valve lash. Because solid lifter cams are less susceptible to valve float at higher rpms, they are ideal for more radical street and racing profiles. The price of running solid lifters is periodic adjustment of valve lash and increased valvetrain noise.

Overhead Cam Considerations

Overhead cam engines, like the V Rod follow the same rules regarding cam selection as overhead valve engines. The primary difference is how valve lift is determined. Overhead cam engines don't use rocker arms, so there is no multiplication effect to increase valve lift (cam lift x rocker arm ratio = valve lift). Thus, cam lift and valve lift are the same. The only way to increase lift with an overhead cam is to reduce the diameter of its base circle (the rounded bottom portion of the lobes). Changing the base circle increases valve lash as well, requiring the use of taller lash caps on the valve stems or shims to maintain proper valve lash. This is a fairly involved process, which is a big reason why you'll see many street cams for overhead cam engines with various duration figures but the same lift number.

Vehicle Weight: You can run a bigger cam in a lightweight vehicle, motorcycle, because less low-end torque is necessary to get it moving. Heavy bikes need cams that emphasize low-end power.

Gear Ratio and Tire Size: If you have a bigger (numerically higher) gear ratio, you can use a bigger cam. Lower "economy" gears work better with a mild cam that makes power at low rpm. Tire height is important because it helps determine the final drive ratio.

The cam's powerband should match torque clutch "dump" rpm.

Engine Size and Compression: A cam's profile is affected by displacement. For example installing a cam in a 1000cc engine and then increasing the same engine's displacement to 1350cc (for example) and it will act like a milder grind. The more duration a cam has, the more compression is needed to maintain proper cylinder pressure at low rpm.

Airflow: Your cam needs to work within the airflow capabilities of the engine. The airflow characteristics of the cylinder heads (amount, intake/exhaust ratios, port work, etc.), induction system, and exhaust system are all factors.

Power Adders: Superchargers, turbos, and nitrous oxide require special cam profiles to take advantage of the extra power potential. In general, cams made for use with power adders are ground with wider lobe separation to take advantage of the extra cylinder pressure.

Rocker Arm Ratio: Going to a larger rocker arm ratio increases valve lift on overhead valve engines. The cam should be tailored to work with your specific ratio to avoid slapping valves into pistons or trashing valve springs. If a higher lift cam has already been installed and the rocker ratio is going to be increased, coil bind and valve to piston clearance must be checked.