Airflow Considerations: The cylinder head's exhaust-to-intake (E/I) airflow ratio can provide clues to matching the cam to the head. For example, a head with a good intake port and excellent E/I ratio (high 70-percent range or greater) would probably do well with a single-pattern cam profile (assuming valve lift is optimized to port flow) unless the exhaust system is very restrictive. A single-pattern cam has the same amount of duration for the intake and exhaust. Many cams ground for Harley V-Twin engines are a dual-pattern design, having more exhaust duration than intake duration. Greater exhaust duration helps compensate for a weak exhaust port or restrictive exhaust system by providing additional time for scavenging the cylinder. If an engine has roughly a 70 percent or less E/I ratio, start with a dual-pattern cam having more exhaust than intake duration. Power-adder applications typically benefit from a dual-pattern cam because they generate a large exhaust volume. Engines with an E/I ratio between 70 and 80 percent should be tested with both single- and dual-pattern cams to determine best power.
Make sure the piston domes match the combustion chamber, provide the desired compression r
Valve-lash Testing: If you are running a solid-lifter cam, experimenting with valve lash can give an indication of whether the installed cam is optimized to the engine combination. Increasing lash opens the valve later, closes the valve earlier, shortens duration and decreases net lift. If power increases with more lash, the cam may have too much duration for the combination. However, if power goes down, more duration may improve power. Valve-lash testing works on both the intake and exhaust sides. Since large lash variations are hard on the valvetrain, keep test time to a minimum. Experimenting with different rocker ratios is another method for determining whether an engine would have better performance with more or less cam. And unlike valve-lash testing, rocker ratio experimentation works with both solid and hydraulic lifters.
Changing Cam Timing: Retarding or advancing a cam 4-degrees will move the engine's power band either up (retarded) or down (advanced) approximately 300 to 400 rpm. Closing the intake valve later and/or opening the exhaust valve earlier tend to move the power band higher. Basically, retarding a cam increases top-end horsepower, while advancing a cam improves low-end torque. If power improves when the cam is retarded more than 4-degrees, the cam is too small and requires more duration. On the other hand, if power increases when the cam is advanced more than 4-degrees, the cam is too large and less duration would probably improve performance. All else being equal, a cam with a later closing intake valve requires an increased mechanical compression ratio to maintain a given low speed performance.
Cranking Compression: An octane-limited engine with excessive cranking compression is prone to detonation and can result in disappointing top-end power. Engine cranking compression can be an indicator of how optimized a cam is to an engine. Excessive cranking compression can indicate too small of cam-one with too little duration and an intake valve that closes too early. An engine that runs best with the ignition advance set much less than its normal timing range is another indicator of excessive cylinder pressure and possibly too small of cam. Retarding the cam is one way to reduce cranking compression. Installing a longer duration cam with a later closing intake valve is another.