Know-How Notes: How A Camshaft Works
Other than carburetors, there is nothing more mystical than how a camshaft works. Often dismissed as voodoo or magic, the brains of your engine is only mysterious to the uninitiated. Once you understand the basics, it is really isn’t rocket science. Ok, so maybe it is rocket science, but unless you are out to learn how to design a new profile, this article on the basics should shed some light on how a camshaft works.
The basic function of a camshaft (or just cam for short) is to turn rotating motion into linear motion. Driven by the crankshaft, the cam spins half as fast. So when the crank is spinning at 3,000 RPM, the cam is spinning at 1,500 RPM. This allows the 4-stroke action to take place inside the engine. Each rotation of the cam provides the opening and closing of the valves. There is one full rotation of the camshaft for every two rotations of the crank.
The key to understanding how a camshaft works is through the basic terminology: base circle, lift, lobe separation angle, and duration.
The base circle is the round portion of the cam lobe where the valve lash adjustments are made. A slight high spot on the base circle is called base circle runout.
This is the maximum distance that the cam lobe pushes the lifter above the base circle. This is not valve lift.
This is the total lift for the cam, calculated by multiplying the cam lobe lift by the stock rocker arm ratio, such as 1.5:1 or 1.6:1. For example, if your cam lobe lift is .325 and you’re using a 1.6:1 rocker arm ratio, you can take .325 x 1.6, which is .520 inches of overall valve lift. Increasing the rocker ratio is a quick way to up the lift and gain a little duration with the same cam.
The higher the lift, the larger the space between the valve seat and the valve allows more air and fuel to enter/exit the combustion chamber. Higher lift applications often require different valve springs and components. There is also such a thing as too much lift, and can lead to the valve hitting the piston, which ends in catastrophic failure.
The length of time the valve is held off the valve seat from the cam lobe, which is measured in degrees of crankshaft rotation. This is also called “advertised duration”. The other specification, “duration at .050-inch” is the distance measured in degrees of crankshaft rotation from when the valve is open at .050-inches until it is .050-inches from closing. Duration at .050” is considered the more important spec.
Increased duration adds power to the engine, particularly in high-RPM applications. This is due to the fact that the added open time allows more air/fuel to enter and combustion gases to exit the cylinder. In lower RPM applications, however, the longer open times can lead to a loss in cylinder pressure.
Lobe Separation Angle
This is the angle in degrees between the centerlines of the intake and exhaust lobes. A 114-degree lobe separation angle means that the peak opening points of the intake and exhaust lobes are 114 degrees apart. The LSA shows the valve overlap, which is the amount of time that both valves are open on the same cylinder. If the lobe separation angle was 0 degrees, both the intake and exhaust valve would open and close at the same time. Measurements typically range between 104 and 115 degrees. A 114-degree cam is considered mild, where a 111-degree cam is fairly rough for a street engine. The term “lopey” comes from this spec, as a large overlap causes the engine to have a rough idle. You can hear the overlap as rump, rump, rump sound. That lope sound starts showing up around the 112-ish LSA, but there are other factors at play. The wider the LSA, the smoother the engine runs, the more narrow the LSA, the rougher the idle.
Narrow LSA angles yield a lower RPM torque range with higher maximum torque, higher cylinder pressures, rough idle, and lower idle vacuum. Wide LSA angles move the RPM range up with lower maximum torque, lower cylinder pressures, smooth idle, and higher idle vacuum.
This is the position of the center of the cam profile based on the engine at top dead center. The centerline is the center of the #1 cylinder intake lobe relative to TDC for that cylinder. This is how you check and change the initial timing of the camshaft through a process called “degreeing”. Advancing the cam pushes the valve timing events sooner, which yields increases in bottom-end torque. In this state, the piston to valve clearance decreases on the intake side and increases on the exhaust side, while retarding the cam sets the valve timing events later, has the opposite effect on the valve to piston clearances and increases the higher torque range.
For performance minded people there is a ton of power potential with just a simple cam change. Selecting the right cam for your application requires a bit more information, but now you are armed with the basics. For your average daily driver, a cam with short duration and nominal lift will give you the best economy, but if you want a street demon, big duration, big lift and a narrow LSA is your ticket to ride.
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