This illustration shows the basic parts of a cam lobe. The nose height compared to the bas
Special thanks to Billy Godbold at Competition Cams and Mike Golding at Crane Cams for their assistance in the preparation of this article.
Camshaft issues always sell magazines, and readers always ask for more. Just when we thought you were sick of it, our most recent readers' poll demanded even more info on how cams work. That's why this story will offer more on the hows and whys of camshaft specifications than any other in recent memory. This time we'll count on you to know the basic definitions of terms such as intake opening, duration, and lift as we go into the theory of how each aspect of cam design tends to affect engine power.
Car magazines have published rain-forest-loads of issues dealing with camshafts. Why? Because the cam is one of hot rodding's most common, most visceral, and most baffling upgrades.
Looking at the intake valve first, its opening point is critical to vacuum, throttle response, emissions, and gas mileage. At low speeds and high vacuum conditions, premature intake opening during the exhaust stroke can allow exhaust gas reversion back into the intake manifold, hurting the intake pulse velocity, and contaminating the fresh intake charge. A late-opening intake gives smooth engine operation at idle and low rpm, plus it ensures adequate manifold vacuum for proper accessory operation (assuming the other three valve opening and closing points remain reasonable).
As rpm increase, air demand is greater. To supply the additional air and fuel, designers open the intake valve sooner, which allows more time for the intake charge to fill the cylinder. With an early-opening intake valve, at high rpm the exiting exhaust gas also helps draw the intake charge through the combustion chamber and out the exhaust-that's good for purging the cylinder of residual gas, but it also increases fuel consumption by allowing part of the intake charge to escape before combustion and can make for a rough idle.
The valves' opening and closing points are tied to the four-stroke engine's combustion cyc
The intake closing point has more effect on engine-operating characteristics than any of the other three opening and closing points. The earlier it occurs the greater the cranking pressure. Early intake closing is critical for low-end torque and responsiveness and provides a broad power curve. It also reduces exhaust emissions while enhancing fuel economy.
As rpm increase, intake charge momentum increases. This results in the intake charge continuing to flow into the combustion chamber against the rising piston far past BDC. The higher the engine's operating rpm, the later the intake closing should be to ensure all the charge possible makes it into the combustion chamber. Of course, closing the valve too late will create significant reversion. It's a fine balancing act.
In a perfect world, the optimum intake closing point would occur just as the air stops flowing into the chamber; would get the valve seated quickly and not waste time in the low lift regions where airflow is minimal and there is no compression building in the cylinder; wouldn't be so fast that the valve bounces as it closes, allowing the charge to escape back into the intake port and disturb the next charge; and, in hydraulic street cam applications, would ensure the closing ramps are not so fast that they result in noisy operation.
Shown are timing figures for a theoretical cam installed advanced, at split overlap ("stra
Overall, the exhaust valve opening point has the least effect on engine performance of any of the four opening and closing points. Opening the exhaust valve too early decreases torque by bleeding off cylinder pressure from combustion that pushes the piston down. Yet the exhaust has to open early enough to provide enough time to properly scavenge the cylinder. An early-opening exhaust valve may benefit scavenging on high-rpm engines because most useful cylinder pressure is used up anyway by the time the piston hits 90-degrees before BDC on the power stroke. Later exhaust valve opening helps low rpm performance by keeping pressure on the piston longer, plus it reduces emissions.
Excessively late exhaust valve closing is similar to opening the intake too soon-it leads to increased overlap, allowing either reversion back up the intake, or the intake mixture to keep right on going out the exhaust. On the other hand, late closing events can help purge spent gasses from the combustion chamber and provide more vacuum signal to the intake at high rpm. Early exhaust closing yields a smoother operating engine. It does not necessarily hurt the top-end, particularly if it's combined with a later intake valve opening.
As engine operating range increases, designers must move all the opening and closing points out to achieve earlier openings and later closings, or design a more aggressive profile to provide increased area under the curve without seat timing increases.