Basic Camshaft Knowledge
A camshaft actuates (opens and closes) the engine valves. Rotating at one-half the crankshaft or engine speed, the camshaft is responsible for the correct timing (with respect to piston location) of air/fuel mixture and exhaust gas flow in and out of each cylinder.
In today’s cars, the camshaft is located in either the engine block – an overhead valve design (OHV), or in the cylinder head in the overhead cam layout (OHC). Some engines even employ a double overhead cam (DOHC) design in which there are two camshafts – one for the intake valves and the other for the exhaust.
Although there are differences in physical construction, the operating principle is the same in either case. As the camshaft rotates, the lobe pushes up on a lifter or down on the lash or valve cap. In an OHV engine, a pushrod is seated in the lifter and acts as an intermediate link between the lifter and rocker arm.
In both engine designs, the rocker arm or lash cap compresses the valve spring to open the valve. As the cam continues to rotate, the force is relieved and the valve closes.Usually, the contour on both sides of the lobe is the same (the contour, or profile, controls the acceleration of the valve). However, if it differs from side to side, the camshaft is said to be asymmetric. The Ford 2300cc engine has asymmetric lobes. If the intake and exhaust lobes have different profiles, the camshaft is referred to as dual pattern design. Some of the so-called “mileage” camshafts are dual pattern.
The measurement that indicates the amount, in inches, that the cam opens a valve is the lift. There are two types of lift – cam lift and valve lift. Cam lift is the distance the lifter is raised by the cam lobe. Valve lift is the amount of valve spring compression (how much the valve opens) due on the mechanical advantage of the rocker arm. (OHC engines that do not use cam followers or rocker arms measure valve lift directly by cam lift. In these engines, cam lift and valve lift are identical.)
- Always use new lifters when installing a new camshaft. The bottoms of the lifters are spherically convex ground, and the cam lobes are ground with taper. The mating of these two surfaces ensures that the lifter will rotate, thus reducing the chance of premature wear. If you install used lifters on a new or reground camshaft, early failure is almost certain.
- Check piston to valve clearance. Insufficient clearance will cause valve to piston contact. This is a very expensive mistake. A minimum clearance of .100″ on the intake valve and .125″ on the exhaust must be maintained. Keep in mind that when you advance or retard the cam; it dramatically changes the clearance. Always check the clearance with the cam degreed in the correct position.
- Be sure to use the recommended valve springs for the camshaft being installed. Either incorrect pressure or incorrect spring application will almost certainly lead to premature camshaft failure. Most aftermarket cams have higher lift than a stock profile. Therefore, stock valve springs will “coil bind” or “stack” before the cam reaches its full lift, causing the cam to fail immediately. This happens because there is not enough installed height with the stock springs. When a new cam is installed, always check the springs for “coil bind.” With the valve at full lift, check the clearance between the coils. You need to maintain a minimum of .060″ between the coils at this point. Even if the stock springs are not “binding,” they are rated only for stock lift, when above stock lift is present, the “open” spring pressure is extremely increased. When excessive spring pressure causes cam failure, it can be the result of incorrect springs, as well as short valves, improper retainers, and many other factors unrelated to the camshaft or valve springs. The only way to ensure the correct pressure is to actually check the installed height .
- Be sure to lubricate the camshaft prior to break-in. The first few minutes are the most critical period in the life of a camshaft. Apply a generous amount of cam lube on the lobes and fuel pump lobe. Apply lube also to the bottom of each lifter. To ensure that the engine is primed with oil correctly, use a priming tool and a drill before initial start-up.
CAMSHAFT TUNING TIPS:
Advancing and Retarding:
By advancing the cam, the valves open and close earlier. Duration and overlap remain unchanged. Advancing raises the cylinder pressure (due to earlier valve closing) This improves low end and mid-range torque at the expense of some top-end power. The result is similar to using a shorter-duration cam since the intake valve closing point is more critical than its opening point.
Retarding the cam so valves open and close later has the opposite effect. This should increase top-end power, at the expense of low end, and mid-range torque. Thus: 1) Advance Cam: More low and mid-range torque, 2) Retard Cam: More top-end power.
Advancing and retarding are easily accomplished with offset bushings or keys for the cam or crankshaft, depending on the engine. The bushings and keys are usually supplied in increments of 2, 4, 6, and 8 crankshaft degrees. Remember that one crankshaft degree equals two camshaft degrees.
How Much is Enough?
Trial and error is usually the best method when advancing or retarding a cam to alter performance. Our experience indicates that cam advance of 2′ to 6′ should give the best overall performance. These settings have helped top-end power in many engines.
Before attempting to advance or retard a cam, you must know the actual valve timing, not the manufacturer’s specifications.
Advancing and retarding a camshaft will move valves closer to the piston. Valve-to-piston clearance must be checked after advancing or retarding alterations to prevent possible engine damage. Also, changing the cam timing will also change ignition timing, which must then be reset.