by John Hildreth
People replace cams for many reasons, but it's usually to enhance an engine's performance. When your cam choice works with the parts you're using and the goals you've set, the results can be impressive. However, when the cam doesn't get along with the rest of your package, there's a noticeable loss of performance.
"So, how do I choose a cam?" One way is what I call the “brother-in-law” or “best friend” approach relying on gossip or word of mouth from family and friends. There’s also the Ouiji Board or “Psychic Friend’s Network” approach picking a cam out of thin air, or, worse, based solely on the inflated advertisement claims of a manufacturer. The only reliable approach is to use a knowledgeable engine builder and competent computer-modeling programs. Today’s sophisticated and accurate modeling software makes the complicated job of cam selection easier and far more efficient than in the past, saving you time and unnecessary expense.
As always the case when building a motor, seriously evaluate your performance goals, and, sometimes even more importantly, determine what you can afford before deciding on a cam. To help you make an educated choice, here are some cam vocabulary definitions:
DURATION. Duration is the time the valve is opening and closing. Because the intake valve is closing later and high duration cams require higher compression ratios, compression ratios need to be taken into account when choosing a cam’s duration. The more duration you have, the higher the RPM requirements are to achieve peak torque and horsepower. This means that at low RPMs, the engine looses some of its volumetric efficiency allowing a portion of the cylinder fill to flow back toward the carburetor before the intake valve closes (reversion.) More duration equals a lower corrected compression ratio, and a low corrected compression equals a loss of throttle response, horsepower, and torque in the lower RPM ranges. To compensate for this, you must raise the static compression ratio of the motor.
LIFT. Lift is how far the valve opens. To correctly choose lift, you need to know the flow characteristics of your heads. More lift does not necessarily give you more torque or horsepower. For instance, I’ve tested heads that flowed very well at .500 lift, but when the valve opened to .600, there was a significant loss of flow and a corresponding loss of horsepower. Lift and port flow should be matched to achieve the desired peak horsepower.
OVERLAP. Overlap is the period of time when both the intake and exhaust valves are open, or off their seats. It can also be described as the period of time when the exhaust valve is ending its closing phase and the intake is beginning to open. More overlap benefits high RPM performance, but can cause major problems at low RPMs. Cams carrying a lot of overlap result in a motor that is more particular about the exhaust system you choose. With the added overlap, the exhaust flow has a longer ‘window of time’ for a reversion pulse of exhaust gases to pollute the new cylinder charge. Most people feel this window as a “flat spot” in the power band, and it readily shows up on graphs during dyno runs. Ideally, you want smaller overlap numbers in order to keep that reversion window small.
CORRECTED COMPRESSION RATIO (CCR). For Harley-Davidsons, the CCR is the compression the engine actually experiences at approximately 2000 RPM. As you continue to hold the intake valve open for longer periods of time, the CCR will go down, allowing you to run a higher static (the usually advertised) compression ratio. This increases performance at both low and high RPM. A good corrected compression ratio for the street is 9:1, while race motors run significantly higher ratios.
Ported heads give you the additional opportunity to take advantage of a wider variety of cams. Mild cams and ported heads, as part of a well-planned package, are more than capable of producing impressive horsepower and torque numbers while prolonging engine life by being gentle on the valve train. An added advantage is that a conservative cam normally yields more rider satisfaction than a radical choice for street riding.
Both computer modeling and flow bench evaluation are available through HILDRETH PERFORMANCE. In addition, all head porting is computer matched to cam specs. For more information, contact John at firstname.lastname@example.org.
To choose a cam, compare manufacturers’ specification numbers. While there might be slight differences between how different manufacturers advertise their cams, here are some typical charts from parts catalogs. While all the numbers are important, let’s just take a look at timing, duration, and lift.
Other manufacturers may use a different format:
Stack the intake numbers over the exhaust numbers and a comparison is easier:
When comparing cams, first check to see that lift numbers are similar. There is no value in comparing a cam listed with a .490 lift and one that lists at .550 lift. Next remember that stock heads need more duration for maximum performance. Longer duration means peak horsepower won’t come on until late in the RPM range. Good flowing, ported heads can best utilize shorter duration timing and still give better performance than stock heads with hotter cams. Here, Cam “X” would work well with ported heads, while stock heads need Cam “Y.”
Finally, if you take the first number of the intake opening timing and add it to the last number of the exhaust closing timing, you’ll get the number of degrees of overlap (i.e. Cam “X”: 10 + 8 = 18 degrees. Cam “Y”: 27 degrees). In this particular comparison, if all other components were the same, Cam “Y” would be more particular about which exhaust system you use to achieve a wide power band. The additional overlap makes this cam extra sensitive. Consult your engine builder for additional suggestions.
Last Updated on 06/14/2004