(Written for Oval Track Racers)
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.
Driver style plays a big part in the picture. A driver who can keep the rpm up in the turns will be able to use a bigger cam than one who drops a lot of rpm. The same cam in the car with lower turn speeds will not have enough torque at the turn exit because it will be below the maximum power band.
Car setup and the ability of the driver to accurately feel what the car is doing is more important than the last 20 horsepower. This is especially true with dirt cars. I have talked to few racers that can’t hook up 600 LB/ft of torque on 8 inch tires if you can give it to them. However the more power out of the turns you give them the slower they go. This is frequently due to driver interpretation of poor acceleration as lack of power rather than wheel spin. While it seems that any driver with some experience can tell when he is spinning wheels, it apparently isn’t that easy. Many experienced top drivers admit it is hard to differentiate between wheel spin and lack of power except in extreme cases. It is often helpful to paint a small section of a rear tire and have an observer watch the car in the turns for signs of excessive wheel spin.
Failure to accurately degree in the camshaft is another culprit. There can easily be 4 or even 6 degree variations in different brands of timing sets. Proper valve timing quickly becomes a crap shoot without a degree wheel. Published valve timing is only a suggested starting point based on dyno testing and track experience. There are variables that may call for different valve timing in your specific combination. If you don’t know where your valve timing is then the cam grinder or engine builder cannot recommend changes to help you get the most out of your engine. Even if class rules prohibit advancing or retarding the can, you still need to know where it is installed.
Now is a good time to mention that while your cam grinder or engine builder should be able to supply you with a cam appropriate for your application, it will be necessary in most cases to change jetting and fine tune the cam timing and lash settings to the driver’s preference and ability. The cam grinder or engine builder cannot know about driving styles and rpm ranges in your application without good accurate information on which to base his recommendations.
There are two easy ways to change the characteristics of your cam. Advancing and retarding the cam can move the power band up or down a few hundred rpm. Advance for more bottom end and retard for more top end. It usually takes about a 4 degree change for a driver to feel it. The second way is to change the valve lash. Tightening the lash will increase the top end power while loosening it will increase bottom end power. You won’t hurt anything by tightening the lash but check with your cam grinder as to the maximum lash you can use before running off the ramp and damaging the valves. When making these changes, increases at one end mean decreases at the other end. Often it is the decrease that makes the car faster.
When checking valve to piston clearance be sure to check it with the cam advanced and retarded about 6 degrees from the recommended point to allow for any timing changes you may wish to make in the future. Making these changes to cam timing is important to not only optimize your present combination but to indicate a direction for future cam changes.
Let’s examine two scenarios. Car A is slow off the turns (no wheel spin) but has great power the last half of the straights. Loosening the valve lash (or advancing the cam) will increase power out of the turns with a slight power loss at top rpm.
Car B is slow off the turns but suffers from excessive wheel spin. The cure here is to kill a little power off the turns by tightening the lash (or retarding the cam). This will enable the car to hook up and get more power to the ground. Increased top end power will be an added bonus but the real need was to get horsepower off the corners down to a level that the car and driver can handle.
Changing to a shorter duration cam for car A or a longer duration cam in car B will accomplish the same thing but at a much higher cost in time as well as dollars.
One other important thing is knowing the minimum rpm encountered. Few drivers can accurately tell you what their lowest rpm really is. It is hard to be fast if your cam has a power range of 4500 to 7000 and your restarts are 3500. The lowest rpm encountered is as important if not more important than the maximum when selecting the proper cam.
If you see little or no change in performance after all of these changes (you did do all of them didn’t you?) the problem is most certainly NOT the camshaft. It’s time to look elsewhere.
54 Atrium Trail
Arden, NC 28704
“Home of the Torque Monster Cams”
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Some thoughts on choosing the proper cam.
Longer duration cams require tighter lobe separation to have any power off the corner. (not usually a preferable combination in 2 bbl classes.)
Short 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 exhaust 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. Our 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 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.