Choosing a proper cam for stock cars

One of the most difficult tasks facing today’s racer is camshaft selection. The advertising in the camshaft industry is often misleading and contradictory. The important information about lobe designs is not available and if it were, few people would know what it means. Everyone wants the latest trick of the week and that trick seems to be whatever gets the most press. Have you ever read anything in a racing publication that was negative about an advertiser’s product? Not likely. Often an advertiser arranges the tests and you can bet it will put a favorable light on the featured item or you likely won’t read about it.

These things happen today in almost all advertising. It is left up to the customer to separate useful information from the hype. It isn’t easy, especially when you know nothing about the design of the product.

From reading tech articles and advertising it is easy to come to the conclusion that a short duration cam with a lot of lift is the hot ticket. This is usually true but what are the limits, and tradeoffs? What are the consequences of going too far? The more lift designed into a given duration, the less the nose radius. In other words the lobe gets more pointed or sharper. This makes for less desirable lobe wear. Even if it makes more power, to finish first you first have to finish. This poor wear characteristic is further aggravated by higher rocker ratios. When all information in print points to more and more lift with shorter and shorter duration, is it any wonder that cam companies respond with the desired product? After all it is too time consuming to educate the customer as to what is really appropriate so they just deliver what the customer asks for and hope for the best. Sometimes these radical profiles can be made to work with very expensive lifters and valve springs combined with regular replacement. The average racer can’t afford to buy $600.00 lifters with $300.00 valve springs AND replace them all frequently.

Most Stock Car racers have been faced with trying different cams in their cars and not seeing the results they think they should. Frequently this is followed by selling or giving the cam to a fellow racer who finds it is the best cam he ever ran. Why did this cam work so well for one racer and not for another? The reasons can be many. Improper installation and driver style are among the most important. Let me say here that if you are not degreeing in every cam you install then you should not be assembling engines. There is no way anyone can help you solve a problem if you don’t know exactly where the cam is installed. There are too many variables that affect the valve timing to leave anything to chance.

Here are some of my thoughts on cam selection:

To many racers the best cam is the one in the car when they get the chassis sorted out and the driver finally gets the track figured out.

Shorter duration cams with wider lobe separations usually yield much flatter torque curves
Longer rod motors prefer a shorter duration cam with wider lobe separation
Longer duration cams require tighter lobe separation to have any power off the corner. (Not usually a preferable combination in 2 bbl classes.)

Stock exhaust manifolds or a highly restricted exhaust usually respond well to shorter exhaust duration and wider lobe separation. Power increases are most evident at higher rpm where exhaust backpressure is greatest and reversion is most prevalent.

Most unported factory heads approach 90 to 95 % of peak flow at .400 to .450 lift and do not need or want a maximum valve lift over .540 to.555. Often a low cam lift with 1.65 or 1.7 ratio rockers is very helpful on the intake side as long as lift is kept to about .550. Exhaust is less critical with 1.5 or 1.55 being the most popular.
Dyno testing doesn’t test drivability or throttle response of the engine.

The important numbers on a dyno sheet are about a thousand RPM above and below peak torque and peak horsepower. Peak numbers are for bragging purposes and high peak numbers do not win races.

The benefit of high ratio rockers is faster valve movement and the added lift is frequently detrimental in unported heads. It often helps to utilize a lower cam lift with high ratio rockers to avoid excessive valve lift.
Changing the valve lash is a good way to get an indication of which way to go for your next cam change. You won’t hurt anything by going too tight but too loose will let the valves slam shut, causing damage to valves and seats. .004 to .006 loose is usually OK, at least for some testing.

Glowing exhaust pipes may be an indication of over scavenging by the exhaust. Shorter exhaust duration, smaller headers, or even a restrictor plate at the header may help. The problem is often mixture burning in the exhaust rather than in the cylinder. Many people think a lean mixture causes it. Be sure to ascertain which problem you have as the lean mixture is a much more serious problem and can cause quick meltdown.

Look at the Major Intensity numbers to get an idea as to how radical the profile is. (Major intensity is the difference between the .020 duration and the .050 duration.) Lower numbers are more radical but anything less than 27 or 28 degrees may be very hard on the valve train. Our 26-degree SXTL profiles are a notable exception to this.

When comparing different camshaft profiles it is critical that the checking heights are the same. .050 durations are directly comparable but advertised durations have no real standard. Mechanical profiles are commonly checked at .020 but not all designers use that point. Hydraulics are often checked at .004, .006 or .008. Other points are sometime used. If two cams are not checked at the same height then you can’t compare them. Our HCR profiles check seven degrees less at .006 than at .004. Our HMTs are about 4-5 degrees less. Here are some guidelines to camshaft comparisons.

Camshaft intensity is a measurement term coined by Harvey Crane to compare ramp characteristics of different camshafts.
• Hydraulic Intensity is the difference between the .004 duration and the .050 duration.
• Minor intensity is the difference between the .010 duration and the .050 duration.
• Major intensity is the difference between the .020 duration and the .050 duration

Lower numbers indicate more radical profiles but too low can be too radical and lead to noisy valve train and even to broken parts.

The average racer should rely on the manufacturer’s tech people to select a cam. They are generally familiar with the characteristics of their lobe designs and should be able to steer you in the right direction. You should find out which tech person specializes in your particular application. Some are better than others. Always try to give the tech consultant all the information he needs as well as the specs on previous cams you ran and which one worked best. Be wary of relying on your buddy’s cam selection unless his engine, car setup and driving style are the same as yours. It may help to find out what a few front runners are using just to be sure you get something similar. The consistent winners seldom win on engine alone. It takes a good driver, a good chassis setup and a competitive, well-tuned engine to be a consistent front runner.