Some thoughts on choosing a proper camshaft for your dirt tracker
1- 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.
2- Short duration cams with wider lobe separations usually yield much flatter
3- Longer rod motors prefer a shorter duration cam with wider lobe separation
4- Longer duration cams require tighter lobe separation to have any power off
the corner. (not usually a preferable combination in 2 bbl classes.)
5- 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
6- 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
7- Dyno testing doesn’t test drivability or throttle response of the engine.
8- 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.
9- 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.
10- 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.
11- 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.
12- Glowing exhaust pipes may be an indication of over scavenging by the
exhaust. A 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.
Camshaft intensity is a measurement term coined by Harvey Crane to compare ramp
characteristics of camshafts.
Hydraulic Intensity is the difference between the .004 duration and the .050
Minor intensity is the difference between the .010 duration and the .050
Major intensity is the difference between the .020 duration and the .050
Lower numbers indicate more radical profiles but too low can be too radical and
lead to noisy valve train and even to broken parts.
Back to the toolbox!