Camshaft Basics

Most people when deciding to upgrade their engines look to change the camshaft. The cam is probably the least understood component in the engine. Hydraulic, mechanical, roller and hydraulic roller are the 4 basic types of cams in use today.

The terms associated with camshafts are not always understood, or more often are misunderstood. Practically everyone interested in changing a camshaft will understand what "Valve Lift" is, but the terms "Cam Centerline" and "Lobe Separation" are terms which will get different explanations from different sources.

Here are some basic terms

Valve Lift - To figure valve lift take the distance that the cam lobe lifts the tappet and multiply it by the rocker arm ratio, this will give you the distance that the valve is opened or "valve lift".

Duration - This is the time that the valve is off the seat during tappet lift, measured in CRANKSHAFT degrees. As there has to be some point in which you begin to measure the lift of a cam there are usually two figures given on a spec card. The Advertised Duration and the Duration at some arbitrarily chosen point (Usually .050" lift) Some manufacturers measure duration at a different amount of lift and this can cause confusion. Most Cam manufacturers use the .050" figure, but it is wise to be sure when comparing different grinds. When checking a cam you should always check it at the tappet rather than the valve because of minor variations due to lash, and rocker arm ratio.

Centerline - The Centerline of a Cam is the actual position or phasing of the cam in relation to the Crankshaft. Meaning the position of the center line of the #1 INTAKE lobe of the cam in relation to the position of the #1 piston measured in crankshaft degrees of rotation AFTER TDC.

Lobe Separation - This is the PHYSICAL configuration of the cam in relation to the actual spacing of the intake and exhaust lobes from each other. Lobe separation is ground into the camshaft. You CANNOT change it (Unless you regrind the cam). You CAN change the Centerline by degreeing. These two terms are often confused with each other.

Symmetrical- A cam that is Symmetrical has both sides of the cam lobe exactly the same. In other words, the intake ramp of the cam lobe that accelerates the lifter to actuate the valve has the same shape as the portion of the ramp on the downside of the lobe that lowers the lifter. These designs are very easy on the valve train as it is a smooth transition from open to closed.

Asymmetrical - An Asymmetrical cam has opening and closing ramps that are different. These profiles are usually found on high performance cams and offer a high velocity opening and a lower velocity closing ramp in order to snap the valve open quickly and then set it back down more gently.

Dual Pattern - Again, a grind that is usually found in a high performance cam. The Intake lobe configuration is different from the exhaust lobe. Usually the exhaust lobe is ground with slightly more duration that the intake lobe.

Cam Walk - A phenomenon that occurs with Roller cams due to slight inaccuracies in the lifter bore spacing. Most Roller cams use a cam button to control the tendency of the cam to unscrew itself from the block. Bushing the lifter bores can control this problem but is very expensive. A cam button will work quite well in 99% of cases.

The Proper Way to Set Your Valves

When you are dealing with mechanical cams, setting the valve lash is a very important aspect of your engine.

The lash must be adjusted when the tappet is on the heel of the cam lobe. Remember that this is the lowest part of the lobe, So the tappet is as far down in the bore as it will go. Since you can't see this when the heads are on, the way that you determine when the tappet is in the proper spot to adjust is as follows;

Adjust the Intake valve lash when the EXHAUST valve begins to open, this will assure that the intake valve is on the heel of the intake lobe.

Adjust the Exhaust valve when the INTAKE valve has just closed .

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