I have heard it said before and it is worth repeating for this article that if a Briggs motor has a heart it is the camshaft. More questions are raised over
this one reciprocating piece than any other part of the motor. There are many areas to consider in examining cams. They are lift. duration, timing, profile, center lines and the every popular Briggs easy spin. We
will examine all of these aspects of camshaft function and design in this article and I will leave you with my specific thoughts on the best cams for Briggs stock classes as the rules currently permit. We will first try and discuss
cam dynamics in a very general form and then move on to the unique aspects of the Briggs camshaft under WKA stock class rules.
A general description of valve train function in any race engine is relatively
straight forward. The valves must be lifted as high as possible to give maximum air flow both in and out of the cylinder. The rate of valve lift(relative speed of opening) must be as fast as possible to give the maximum amount of
air flow for the maximum amount of time.
The opening of the intake valve must start upward at a slow rate in order to take up valve clearance as well as reducing shock to the valve train. Once this has been
accomplished, the lob can push as hard as possible to raise the valve as fast as is possible, but as the lifter nears the nose of the cam the rate of valve lift must slow down so that the valve spring is not overcome and allow the
components to loose contact. On the backside of the intake, the lob can let the valve drop as rapidly as possible. Since the intake valve is being cooled by the rush of cold methanol, the lobe doesn't need a long closing time
frame. Because of the relative cool temperature of the intake valve it can slam home against the seat without fear of warpage unlike we have with the exhaust.
On the exhaust side we have a different situation.
The lobe must again start slowly taking up valve slack. Once this is done, we still want the valve opened as rapidly as possible in the shortest amount of valve timing. It is the backside of the exhaust lobe that is different.
Because of the red hot temperatures experienced by the exhaust valve the cam must let the valve down on the exhaust seat in a rather soft manner. This will prevent warping or possible fracturing of the valve and seat. Due to this
problem the closing of the exhaust must be fairly long in order to gradually slow down the valve closure and set the valve down gently on it's seat.
VALVE TIMING
More than any other thing,
overall performance from a camshaft is determined by the timing events of the cam. These are the points when the intake and exhaust valves open and close. This timing is described by measuring the relative point in crankshaft
rotation. The points that these vents occur as well as the overall duration(relative time the valve is open) is a standard method for comparing camshafts.
Many factors play in determining the best camshaft for a
given motor. These include the efficiency of the intake or exhaust port, the carburetor, the exhaust header and on and on. More than anything else the valve timing is affected by engine speed. At slow cycling speeds the relative
elapsed time between the valve events is long. However as the cycle rate is increased the timing for each event gets smaller and smaller. As engine speed increases, the time between the events will decrease to such an amount that
efficiency is impacted. The only way to counteract this is to increase the duration between the valve events. It is very common in high speed racing engines to extend the valve open duration.
What is the effect
of extending the duration? First, the longer duration raises the effective engine speed (rpm) range. If there is a larger duration between valve events there is more relative time between events at higher engine speeds. This allows
the rpm to go higher before we reach restrictive event timing. This will only give more horse power if the intake or exhaust system can give increased flow or fuel mixture. As we increase the duration, low speed power will be
DECREASED. This is caused by both the increase of the duration and the phasing of the intake and exhaust cycles.
There is a small period of valve timing where the exhaust valve is closing and the intake valve
starts to open. Both valves are actually open at the same time. This event is called OVERLAP.
Interesting things happen during overlap. Since the intake valve is being opened while hot exhaust gases are rushing
out of the exhaust track, an effect known as intake draw-though can occur. This effect can actually 'pull' the intake mixture into the cylinder. This effect actually increases with rpm. The faster velocity in the exhaust during
high rpm will create a lower pressure in the exhaust track(trust my physics prof). If the intake is opened as the last of the exhaust gases are being pulled from the cylinder the intake charge is sucked into the chamber. This
effect can be enhanced by opening the intake valve earlier in the cycle. This is great until the pressure in the exhaust gets lower and draw though actual begin to pull some of the intake mixture out of the exhaust pipe! This is an
important consideration in a restricted motor.
Exhaust opening happens at the changeover from the power cycle to the exhaust phase. Generally it is to your advantage to open the exhaust valve sooner. If the valve
is opened during the period of still high combustion pressure, the pressure will zip past the valve creating an initial surge of flow. This event is sometimes called exhaust blow down. Open of the exhaust is a compromise . Open it
too soon and combustion pressure(power) will be lost out the exhaust port. In general very little effect will be seen from changes in the opening in a race engine.
