32-39 Choosing Camshafts 10/23/13 9:32 AM Page 34
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tively small intake runner volumes for good air velocity and throttle response at low- to mid-range rpm, a splitplenum 180-degree hi-rise intake manifold and properly sized carburetor.
Don’t Overcam Your Work The biggest mistake many engine builders make is to overcam an engine. Using a cam that has too much valve lift, too much duration and/or too much valve overlap for the application can have negative consequences. Everybody likes big numbers, but if the cam specs don’t really match the engine you’re building or the application the engine is going into, you’ll end up with an engine that underperforms and fails to meet your customer’s expectations.
Keep Your Eye on Valve Lift Increasing valve lift opens the valve further so more air/fuel mixture or exhaust can flow past the valve. Valve lift is increased by using taller lobes on the camshaft and/or higher ratio rocker arms. Increasing valve lift improves airflow up to a point, so you want more lift in a performance cam. But airflow eventually peaks out because of restrictions in the cylinder head, intake or exhaust system. Increasing valve lift beyond this point is pointless because there’s nothing more to be gained. There are also physical limits as to how far the valves can be opened before it creates interference problems between the valves and pistons, between the valve spring retainers and the tops of the valve guides, and between the
coils of the valve springs. Modifications can be made to increase clearances (such as cutting larger valve recesses into the tops of the pistons, reducing the height of the valve guides and/or lowering the spring seats), but eventually the limit is reached beyond which no further increases in valve lift are physically possible. So the only way to increase airflow further is to hold the valves open longer (increase duration) by opening the valves sooner and closing them later, and/or by improving the scavenging effect of the exhaust to pull air/fuel mixture through the combustion chamber by increasing valve overlap. There’s a lot of science that goes into determining optimum valve lift, and how quickly the valves open and close. Ideally, you want a fast acting cam that
Circle 34 for more information 34 October 2013 | EngineBuilder
A lot of development work has gone into revamping lobe profiles in recent years to optimize performance, and many cam suppliers have introduced new product lines that reflect these improvements.
opens and closes the valves quickly to maximize airflow. You also want the cam to reach peak lift as quickly as possible, although airflow at mid-lift actually has more of an impact on total airflow because it happens twice during each valve cycle (once as the valve opens and again as it closes). Consequently, you want cam lobes that open the valves quickly, hold the valves open when airflow is greatest, and then close the valves quickly to minimize compression losses. With flat tappet cams, the curvature of the ramp on the side (flank) of the lobe that opens the valve can’t be too steep, otherwise the lifter may dig into the ramp. Likewise, on the closing side of the