Valve Timing & Performance
If racing is your thing, or you are an automotive hobbyist,
then you are most likely not afraid of swapping cams in your engine. However,
the trick to making your car faster is not just replacing the cam, but
replacing it with the "right" camshaft. This is where your
understanding of valve timing becomes crucial in selecting the proper cam. To
help you better understand valve timing, this page will cover blow down overlap
and the ram effect.
BLOW DOWN
On the power stroke, the combustion pushes the piston down in the cylinder.
During this stroke, it is necessary to open the exhaust valve before the piston
gets to the bottom of the cylinder. This will allow the excess pressure in the
cylinder to "vent out" just before the piston reaches the bottom of
the stroke. The term "Blow Down" is used to describe this event.
Timing the exhaust valve in this manner assures no pressure
is left in the cylinder to push against the piston on the exhaust stroke.
Otherwise, there could be 20 PSI (or so) pushing against the piston as it
starts up the cylinder. This would require some of your engine's power just to
push the exhaust out of the cylinder!
High RPM engines need to have the exhaust valve open sooner
so the pressure has a better chance to exit the cylinder. However, at lower RPMs, opening the exhaust valve too soon means you didn't
take full advantage of the power stroke.
OVERLAP
As the engine cycles, there is a period when both the intake and exhaust valves
are open at the same time. This valve timing is known as "overlap."
Think of this as the exhaust and intake cycles overlapping each other.
The valves are timed so the intake valve opens slightly
before the piston reaches top dead center (TDC) on the exhaust stroke.
Likewise, the exhaust valve is timed to close just after the piston starts down
on the intake stroke.
The objective of overlap is for the exhaust gas which is
already running down the exhaust pipe, to create an effect like a siphon and
pull a fresh mixture into the combustion chamber. Otherwise, a small amount of
burned gasses would remain in the combustion chamber and dilute the incoming
mixture on the intake stroke. This valve timing is a product of the cam's
duration and separation specs.
The science involved with overlap is quite complex.
Pressures, runner lengths, temperature, and many other aspects influence how
well the overlap effect works.
RAM EFFECT
When the piston reaches the bottom of the cylinder on the intake stroke, the
intake valve doesn't immediately close at this point. The intake valve remains
open even though the piston is starting up the cylinder on the compression
stroke. The expression "ram effect" is used to describe this event.
Timing the intake valve in this manner allows an additional
amount of fresh mixture to be rammed into the cylinder. The effect is very
similar to water hammer in plumbing. What happens is that during the intake
stroke the fresh mixture is running fast enough down the intake manifold and
into the cylinder that it can not instantly stop when the piston stops at the
bottom of the intake stroke. Just like the water hammer effect, the incoming
mixture is rammed into the cylinder even though the piston may be starting up
on the compression stroke.
High RPM engines can have the intake valve remain open
longer to take advantage of this ram effect. However, at low RPMs, the ram effect is not strong enough and the piston
will start to push the fresh mixture back out of the cylinder. Of all the
different valve timing effects, this one can have the greatest impact on your
engine's performance.
Copyright AutoWare 1998