32-39 Choosing Camshafts 10/23/13 9:32 AM Page 36
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MANY CAMS TODAY HAVE “ASYMMETRICAL” GRINDS THAT USE DIFFERENT PROFILES FOR THE UPSIDE AND DOWNSIDE RAMPS ON THE CAM LOBES, AS WELL AS DIFFERENT LOBES FOR THE INTAKE AND EXHAUST VALVES. lobe, the curvature can’t be too steep, otherwise the lifter may not follow the lobe and bounce on its way back down. Roller cams are much better in this respect because a roller on the bottom of a lifter can follow a more radical lobe profile. A roller cam can open and close the valves faster for more total airflow with the same lift and duration.
Valve Timing Issues Valve timing includes the points at which the intake valves open and when they close, and when the exhaust valves open and when they close. Of these four timing events, intake valve closing actually has the greatest impact on now much power a particular camshaft will make. If the intake valve closes too soon, the cylinder may not fill completely during its intake stroke.
Circle 36 for more information
Circle 35 for more information 36 October 2013 | EngineBuilder
Holding the intake vale open longer allows more airflow into the cylinder – up to a point. If the intake valve remains open too long past the point where the piston has reached and passed bottom dead center, the upward motion of the piston can start to reverse airflow and push air back out of the intake port. The point at which the exhaust valve opens has the second greatest impact on performance. If the exhaust valve opens too soon, cylinder pressure may be lost before it can complete its work. If the exhaust valve is opened too late during the exhaust stroke, it increases the pumping effort required to push the exhaust out of the cylinder. The closing of the exhaust valve and the opening of the intake valve, by comparison, have the least impact on performance. If the exhaust valve closes too quickly, some exhaust may remain in the cylinder and dilute the incoming air/fuel mixture during the following intake stroke. Holding the valve open longer (even as the intake valve starts to open) creates a scavenging effect that helps pull air through the cylinder into the exhaust, but you don’t want too much valve overlap as this can rob some of the air/fuel mixture that would otherwise remain in the cylinder (it also increases exhaust emissions). The opening of the intake valve has to occur early enough so the cylinder has time to fill with air/fuel mixture, but if it starts to open too soon (before top dead center), you can get reversionary airflow back into the intake manifold. The point at which the intake and exhaust valves open and close is determined by the ramps on the cam lobes. As soon as the lifter or follower reaches the point where the ramp starts from the lobe base circle, the valvetrain begins to move and starts to open the valve. The opening point may be measured at a specified amount of lift (such as .004˝ or .050˝) and listed as lasting so many degrees of crankshaft rotation. For example, a typical street performance cam might have a listed duration of 224 degrees for the intake and exhaust valves measured at .050˝ of lift, with a maximum lift of .470˝ (with stock 1.5 ratio rocker arms). The lobe separation between the