How to choose a camshaft
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===Phasing the camshaft=== | ===Phasing the camshaft=== | ||
− | While is is true that you cannot change the lobes of a camshaft after it is ground (unless you weld and re-grind the lobes), you can alter the characteristics of the camshaft in your motor by installing it in either a retarded or advanced position relative to the crankshaft rotation. For instance, | + | While is is true that you cannot change the lobes of a camshaft after it is ground (unless you weld and re-grind the lobes), you can alter the characteristics of the camshaft in your motor by installing it in either a retarded or advanced position relative to the crankshaft rotation. For instance, if you have determined that you are making too much horsepower down low and can't hook the tires up, you ''might'' want to trade off a little of the lower end power for some higher end power. In this case, you would install the camshaft slightly retarded (although harnessing the power would be the preferred thing to do- a 0.10 second better 60 foot time equates to about a 0.15 second reduction in 1/4 mile ET). |
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+ | Although all four events (intake valve opening, intake valve closing, exhaust valve opening, exhaust valve closing) will be affected by changing the camshaft phasing, the most important one will be the intake closing point. If you retard the camshaft, you will be closing the intake later, thus bleeding off some of the cylinder pressure and resulting in less low end power. Vice versa if you advance the camshaft. More bottom end, less top end. A rough estimate is a 4 degree change in cam phasing will change the cranking pressure by 5 psi (advancing increases pressure, retarding decreases pressure). To put that into perspective, a rough estimate says a one point change in static compression ratio (as in going from 9:1 to 10:1, or vice versa) changes the cranking pressure by 20-25 psi. | ||
Intake centerline can be altered either by the crankshaft grind or the use of a camshaft sprocket that can alter if the cam is installed advanced or retarded. A later ICL (retarded cam timing) will tend to move the power curve upwards, due to closing the intake valve later. With the faster engine speeds, the intake valve can stay open later without the risk of pushing intake gasses back into the intake runners. An earlier ICL (advanced cam timing) will tend to increase low end torque because at low speeds, closing the intake valve sooner will trap more intake air at lower RPM. | Intake centerline can be altered either by the crankshaft grind or the use of a camshaft sprocket that can alter if the cam is installed advanced or retarded. A later ICL (retarded cam timing) will tend to move the power curve upwards, due to closing the intake valve later. With the faster engine speeds, the intake valve can stay open later without the risk of pushing intake gasses back into the intake runners. An earlier ICL (advanced cam timing) will tend to increase low end torque because at low speeds, closing the intake valve sooner will trap more intake air at lower RPM. |