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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. Maintaining this momentum is especially important in 2-bbl classes as these engines do not have enough power to accelerate powerfully out of the turns like 4-bbl engines. A tight car has no place in 2-bbl racing.

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 very few racers who can’t hook up 600 lb.ft. of torque on 8-inch tires if you 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 has wheel spin, it apparently isn’t that easy. Many experienced 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 (or use a data acquisition system in a simulated test).

Failure to accurately degree in the camshaft is another culprit. There can easily be four or even six degree variations in different brands of timing sets. Proper valve timing quickly becomes a crapshoot 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 customer’s valve timing is then you cannot recommend changes to help your racer get the most out of his engine.

Even if class rules prohibit advancing or retarding the cam, you still need to know where it is installed. In the few instances where moving the cam is prohibited, you may get the cam manufacturer to actually grind the proper advance into the cam AND verify it. This means buying that timing set from the cam supplier. It may not be perfect but it should be very close. Of course this will add a bit to the cost, but how many top 3 finishes will it take to recoup that cost.

Along with supplying your racer customer with a cam appropriate for their 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. You need to know about your customer’s driving style and rpm range in order to base your 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 manufacturer as to the maximum lash you can use before running off the ramp and damaging the valves. Increases at one end mean decreases at the other end when making these changes. Usually 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 the following 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 expense.

One other important thing to know is the minimum rpm your customer has encountered. Few drivers can accurately tell you what their lowest rpm really is. It is hard to be fast if their cam has a power range of 4,500 to 7,000 rpm and their restarts are 3,500 rpm. The lowest rpm encountered is as important if not more important than the maximum when selecting the proper cam. It is usually lower than most people think.

Note: If the customer sees little or no change in performance after all of these changes the problem is most likely NOT the camshaft. It may be time to look elsewhere.

–Tech Tip courtesy of Camcraft Performance Cams low lift no dwell at max lift: these lobe designs have large nose radius and rounded nose. they are more stable at higher rpm and usually have good wear characteristics.low lift with dwell at max lift: these cams have an almost flat nose. the minimum nose radius is at each end of the flat area and is necessarily smaller than the non dwell type. this cam will have more area under the curve and make more torque. it will not be as stable at high rpm and it will be a bit harder on valve springs. wear characteristics will not be quite as good.This is a cam with high lift. By necessity the nose radius is much smaller than the low lift cam. If too much lift for the duration is designed into the lobe the nose radius will be too small to support the load and it will have poor wear characteristics. Often when using high ratio rockers it is advisable to back off on the lift a bit to increase the nose radius.

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Charles Reichard

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