Camshaft Theory

Bob Evans

Grumpy Old Admin
Staff member
I have heard it said before and it is well worth repeating for this chapter, 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. Seems there is always the hot camshaft of the month, so we need to understand the inner workings of a stock camshaft in order to make intelligent buying decisions.

There are many areas to consider in examining cams. They are lift, duration, timing events, profile, center lines and the ever popular Briggs’ easy spin. We will attempt to examine all of these aspects of camshaft function and design in this chapter 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.

The 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 or how fast we get to max. lift) must be as fast as possible to give the maximum amount of air flow for the maximum amount of cycle 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 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. There is an exception to this today known in most circle as “Slapper” cams but we’ll discuss these in the next chapter. 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 warping unlike we have with the exhaust valve.

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 points in crankshaft rotation. The points that these events occur, as well as, the overall duration(relative time the valve is open) is a standard method for comparing camshafts. The method for expressing these events is measured in degrees relative to the full 360 degree involved in one complete revolution of the crankshaft. Two points in the cycle serve as base points and are 180 degrees apart. Top Dead Center (TDC) and Bottom Dead Center(BDC). A camshaft that opens the intake valve at 30 degrees Before Top Dead Center(BTDC) will have longer valve open time than one than opens the valve around 20 degrees BTDC, if they both have the same closing points.
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(RPM) the timing for each event gets smaller and smaller as everything is working faster. As engine speed increases, the relative time between the events will decrease to such an extent 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 as well as lift.
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 that is needed. 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 help 'pull' the intake mixture into the cylinder and actually increases with rpm. A good thing. 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 pushed 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 actually begins to pull some of the intake mixture out of the exhaust pipe! Not good. Overlap is a very important consideration in a stock or restricted class 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. Opening of the exhaust is a compromise . Open it too soon and combustion pressure(power) will be lost out the exhaust port. Open it too late and you will not get all of the spent gases from the cylinder thus mixing with the new fuel mixture and diluting it. In general, very little effect will be seen from changes in the opening in a race engine. Of the four major timing events this one seems to be the least important.
Exhaust closing will have a much larger effect on engine performance. If the valve is closed later in the cycle, the engine will generally want to rev up better than a motor whose exhaust closing is earlier (22 degrees ATDC verse 15 degrees ATDC). Early exhaust closing numbers also seem to help low RPM torque. Later exhaust closings will also help to cool the motor in comparison to early closings, as it allows more of the hot gases to escape. The only disadvantage is that we are increasing overlap by closing the valve later. The will again hurt a restricted motor and low end torque.
Intake closing is possibly the most important timing event. Close the valve early(82- 78 degrees BTDC) and torque will be helped in the lower rpm ranges. For higher rpm motors a later closing(75-70 degrees BTDC) can be used, as the engine will be operating in a higher rpm range and the overall duration will need to be increased. This logic also applies to the easy spin opening if you believe what Briggs says about the easy spin basically being the closing point for the intake valve. Later easy spin starting numbers such as 51-55 degrees will help a higher rpm motor. A discussion on easy spin will follow. With the higher RPM levels we are seeing these days, many of the slapper style cams are closing the intake valve around 72-75 degrees BTDC verses the cams made prior to 1995.
 
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