Calculating corrected compression is helpful when installing a long duration cam with a late-closing intake valve because knowing corrected compression will allow the engine builder to optimize the mechanical compression ratio for the best low-rpm performance.
Camshaft Selection Factors
* Application and Riding Style
* Engine Displacement
* Mechanical Compression Ratio
* Important RPM Range
* Total Bike Weight
* Drivetrain Gearing
* Induction System Design
* Exhaust System Design
* Stroke Length
* Rod Length
* Rod/Stroke RatioTable 4
Camshaft SelectionAlthough many factors must be considered for optimized cam selection, some of the most important are engine displacement, compression ratio, gasoline octane, gear ratios, total vehicle weight, and most importantly rpm range. Selecting too large or small of cam will maximize horsepower and torque at the wrong rpm. Maximizing horsepower and torque in the correct rpm range is more important than high peak-power numbers.
When choosing a cam, start by matching duration, lift, overlap, and the IVC timing to the engine's application and parts combination. However, keep in mind that cams with similar catalog specifications may perform differently due to slight differences in lobe profile and opening and closing ramps. The following are additional cam selection tips for normally aspirated engines. Power-adder applications such as nitrous oxide, turbocharging, and supercharging require additional considerations that should be discussed with your cam manufacturer.
Duration Considerations: An engine with large displacement will usually run well with more duration than a smaller engine. A heavy bike and/or one with low numerical gearing requires a cam that will provide strong low and midrange power to help the bike accelerate. In contrast, since a lighter bike requires less low and midrange power to accelerate, it can tolerate a longer duration cam and later closing intake valve while still having acceptable acceleration. Keep in mind that for a given engine displacement, increasing duration moves the power band to a higher rpm, improving top-end horsepower at the expense of low-end torque. And the same holds true for closing the intake valve later.
Compression Considerations: For pump-gas engines, which are octane limited, remember that the intake valve closing and mechanical compression ratio must be coordinated to eliminate detonation. The engine's mechanical compression ratio also has a relationship with the exhaust valve opening. Since a higher compression engine has increased cylinder pressure and faster combustion speed, it can benefit from an earlier opening exhaust valve. In the case of a lower compression engine, the exhaust valve opening should be delayed to extend the time available for cylinder pressure to react on the piston. Increasing the mechanical compression ratio when increasing duration or closing the intake valve later will minimize the loss of low-speed torque.