Cam Shaft Information
Mar 20, 2018 9:13 PM - 466 Views
UNDERSTANDING CAMSHAFT SPECIFICATIONS
There are many numbers and terms used when describing a camshaft’s design that must be understood when choosing a cam. It’s good to know exactly how each of these specs affects the engine’s performance, but one of the most important is duration, so pay special attention to that one. Also, look at the recommendations given by the cam companies for our six sample vehicles at the end of this article. Compare how the specs change from one example to the other, then refer back to the paragraphs just below to see why one has more duration or lift than the other.
LIFT: The cam’s basic function is to open the valves. Lift refers to how far the valve is opened (or lifted) off its seat. A street performance cam will usually have between .450- and .550-inch lift. More lift can increase power, and increased lift without changing duration increases power without affecting the point of peak power on the rpm band. The rocker arms have a direct effect on lift because they don’t have a 1:1 lever ratio. A cam that has .318 inch of lobe lift (that’s how far it lifts the lifter) will open the valve .477 inch with 1.5:1 rocker arms (.318 x 1.5 = .477) and .508 inch with 1.6:1 rockers.
Generally, a stock engine will tolerate .500-inch lift before the valves hit the pistons or the valvesprings hit coil bind, but any time lift is increased, these clearances should be checked.
DURATION: Duration is how long the cam holds the valves open. It’s expressed in degrees of crankshaft rotation (remember, the cam rotates at half the speed of the crank). A 280-degree-duration cam holds the valves open longer than a 260-degree-duration cam. Holding the valves open longer allows more air and fuel into the engine and also allows more to get out through the exhaust. Longer duration (higher number) improves top-end power but almost always sacrifices low-end torque. Lower duration improves low-end torque and makes the car idle better, but it limits top-end power, and you can get only so much valve lift with a short duration cam due to the rate-of-lift limitations of the lifter. Roller cams, which we’ll discuss below, have the advantage of allowing high rates of lift with relatively short duration.
The confusing thing about duration is the difference between “advertised” and “at .050-lift” duration. At .050-lift duration is measured from the point where the cam moves the lifter up .050 inch until .050 inch before the lifter is all the way back down. Most cam manufacturers differ in where they start and finish measuring for advertised duration. Some start at .004-inch lift, some at .008-inch and some measure it somewhere in between. That’s why the .050-lift numbers are the best to go by. A 280 cam (advertised duration) from one manufacturer could actually have less at-.050 duration than a 278 cam from another, due to the different points at which the companies measure duration
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