A PRIMER ON CAMSHAFTS
by Henry D. Manley III
The camshaft has been called the heart of the internal combustion engine, and there is
a great deal of truth in this statement. The camshaft controls the valve train making
possible the entire four cycle event. In stock American passenger car engines the camshaft
is designed to deliver smooth quiet idling and good low speed performance. To achieve
these ends as much as one fifth of the potential horsepower is sacrificed for two basic
reasons. First, the valves are not moved off their seats radically, and second, the valves
are not left open for long durations. Of course, there is some overlapping of the valves,
and this is not particularly condusive to good performance, but at high engine speeds this
overlap actually becomes beneficial as will be shown later.
A racing camshaft basically opens the valves higher and leaves them open longer than a
stock camshaft. However, the exact profile of the cam, that is, how the cam actually
accomplishes this event, is of primary importance. The ideal situation would have the
intake valve snap open instantly at approximate top dead center ( TDC ) and remain open
until 40° degrees after top dead center ( ATDC ). Unfortunately, such instantaneous
action would pound the valve train to pieces and prove completely impractical. Therefore,
even racing camshafts are a compromise between stock cams which activate the valve train
only mildly and some yet-to-be invented instant valve opener.
Racing camshafts are designed to open the valves sooner and leave them open longer than
stock cams, as well as lift the valves higher. The exact number of degrees that the intake
valve opens ( BTDC ) and closes ( ABDC ), and the exhaust valve opens ( BBDC ) and closes
( ATDC ), depends of course on the grind. Each shaft is ground for a primary purpose, as
explained later, but regardless of the grind there is more overlapping of the valves than
with a stock cam. This overlap is inevitable because of the increased duration, but at
high engine speeds some benefit is derived from the overlap as there is a mild
supercharging effect of the exhaust gases which tends to suck in additional fuel and air.
The basic reason for overlap can best be explained by an examination of the four cycle
event.
The intake valve starts to open before the piston reaches TDC on the exhaust stroke of
the previous cycle. In this way the valve will be fully open by the time the piston moves
over TDC and down on the intake stroke. The incoming charge of fuel and air builds up a
form of kinetic energy which causes the mixture to continue flowing into the cylinder
after the piston has reached BDC and starts up on the compression stroke. The intake valve
finally closes at some point 60° degrees or more ABDC so that there is no throttling
effect on the incoming fuel which would cause a limitation on the engine's top speed
performance. When the piston is at TDC on the power stroke both valves are closed to seal
off the combustion chamber for the explosion of the fuel mixture. As the piston travels
down on the power stroke the exhaust valve begins to open 60° degrees or more BBDC. A
slight loss of power results from this early opening of the exhaust valve, but the overall
effect is positive because the exhaust gases begin to escape the combustion chamber under
their own power thus avoiding a throttling effect on the piston as it moves up on the
exhaust stroke. The exhaust valve remains open after the piston has reached TDC because
the kinetic energy built up by the escaping gases actually causes the gases to continue to
flow even when the piston has started down on the intake stroke. The piston's transition
at TDC from exhaust stroke to intake stroke is precisely the area where overlap occurs. As
illustrated by this review of the four cycle event, the exhaust valve is open ATDC to
allow more complete scavenging of the chamber, while the intake valve opens BTDC to
facilitate the flow of the incoming charge. The actual amount of overlap can be quite
large, especially in camshafts designed for top end performance. As previously explained,
this large overlap will effect low speed performance to such an extent that the car may
even run slower at low r.p.m.'s with the racing cam than it did with the stock cam. For
this reason great care should be exercised when deciding which cam should be used in an
engine. |
