This isn’t always your fault, but this is often followed by the racer selling the cam to a fellow racer who thinks it’s 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. The following are some tips for selecting the proper cam for circle track applications.
• Longer duration cams require tighter lobe separation to have any power off the corner (not usually a preferable combination in 2 bbl classes). Shorter duration cams with wider lobe separations usually yield much flatter torque curves. Longer rod engines seem to prefer shorter durations and wider lobe separation.
• Stock exhaust manifolds or a highly restricted exhausts usually respond well to shorter exhaust duration and wider lobe separation. Power increases are most evident at higher rpm where exhaust back-pressure is greatest and reversion is most prevalent.
• Most unported heads approach 85 or 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.
• 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.
• 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 26 or 27 degrees may be very hard on the valve train. CamCraft’s 24 degree XTLZ profiles are a notable exception to this.
• Camshaft intensity is a measurement term coined by Harvey Crane to compare ramp characteristics of camshafts.
- Hydraulic Intensity is the difference between .004? duration and .050? duration.
- Minor intensity is the difference between .010? duration and .050? duration.
- Major intensity is the difference between .020? duration and .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.
–Tech Tip courtesy of Camcraft Performance Cams