Valve TimingChanging the timing of a cam's opening and closing events can significantly change the engine's power curve. Of the four opening and closing valve events, the intake valve closing (IVC) has the greatest effect on engine-operating characteristics. The IVC begins the point where all valves are closed and starts the change from the engine's intake cycle to the compression cycle. All engine power is made when the valves are closed and cylinder pressure can build on the piston. All other things being the same, the earlier IVC occurs, the greater the cranking compression and low-rpm torque. As rpm increases, intake charge inertia increases and generates a "ram effect." Closing the intake valve later takes advantage of the ram effect at high rpm and increases cylinder filling for greater power. The higher the engine's operating rpm, the later the intake valve should be closed to maximize cylinder filling at high speed. However, the tradeoff is that low rpm cylinder filling will be reduced, thereby reducing low speed performance. Note the IVC when comparing cams with similar specifications. All else being equal, the later the closing, the higher the rpm peak torque and horsepower will occur. Performance benefits are sometimes realized by designing a cam with a different amount of duration and valve timing for the front and rear cylinders.
A "rough" estimate of valve lift can be made by measuring the diameter of the camshaft bas
A cam can be advanced or retarded by removing the cam drive gear and pressing it back on at a slightly different position. This procedure advances or retards all intake and exhaust events on the V-Twin engine. To eliminate the hassle of removing the cam drive gear, some manufactures offer a multi-index cam shaft with multiple keyways machined into the cam, which allows advancing or retarding the cam about 4-degrees. Although advancing a cam advances all valve-timing events, it most importantly closes the intake valve earlier, which increases low rpm cylinder pressure and usually improves low speed torque at the expense of high-rpm horsepower. As can be expected, retarding a cam closes the intake valve later and improves top-end horsepower at the expense of low-speed torque. It is best to make sure the carburetion, ignition timing, gearing, and exhaust systems are already tuned before experimenting with cam timing.
IVC and CompressionAn engine's mechanical compression ratio has an important relationship to intake valve closing. Mechanical compression ratio is a mathematically calculated number based on cylinder and chamber volumes. However, IVC occurs when the piston is past BDC and moving up the cylinder on the power stroke. Therefore, cylinder volume is less at IVC, which means the engine's compression is less than the mechanical compression ratio. Calculating compression and taking into account the reduced cylinder volume at IVC is called the corrected compression ratio.
In general, an optimized Big Twin engine will run on 91-octane pump gas without encountering detonation with a 9:1 to 9.2:1 corrected compression ratio. In some cases, you can go slightly higher, maybe up to a 9.5:1 corrected compression ratio, but the combustion chamber design, engine assembly and tune-up must be spot-on. Moreover, keep in mind that an engine with a 10:1 mechanical compression, long-duration cam, and late IVC could potentially have a lower octane requirement than a 9.5:1 compression engine with a short-duration cam and early IVC.
Considering the IVC of most street cams, achieving a corrected compression around 9.2:1 generally requires a mechanical compression ratio in the range of 10:1 to 10.8:1. By comparison, a race engine with a 16:1 mechanical compression ratio will have about a 13:1 corrected compression ratio and will run without detonation if designed correctly and fed race gas.