How to choose a camshaft
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The camshaft is the brain of your engine, mechanically opening and closing the valves. It dictates when the valves open and close, how long they are open and closed and when they are open and closed in relation to crankshaft position. The camshaft has a very large effect on the type of power your engine makes. | The camshaft is the brain of your engine, mechanically opening and closing the valves. It dictates when the valves open and close, how long they are open and closed and when they are open and closed in relation to crankshaft position. The camshaft has a very large effect on the type of power your engine makes. | ||
| − | This article assumes that you already know the most basic fundamentals of camshaft operation and starts with describing camshaft parameters. It is designed to help you select the right cam and decipher the numbers so you know WHY its a good cam. For more basic information on camshaft operation and the definition of its components, see [http:// | + | This article assumes that you already know the most basic fundamentals of camshaft operation and starts with describing camshaft parameters. It is designed to help you select the right cam and decipher the numbers so you know WHY its a good cam. For more basic information on camshaft operation and the definition of its components, see [http://www.carcraft.com/techarticles/ccrp_9812_secrets_of_camshaft_power/index.html Secrets of Camshaft Power] by Marlan Davis (Car Craft, December, 1998). |
==Camshaft specifications explained== | ==Camshaft specifications explained== | ||
| + | [[File:Cam dimensions1.jpg|thumb|350px|]] | ||
When you look at cam specifications (typically referred to as a cam card), they will list several numbers that are very important to how this particular cam will operate in your motor. The photo above outlines a pushrod engine which is what you'll encounter most of the time in the hotrodding world. The cam is located in the block. The lobes push against lifters which push on pushrods, and the pushrods transfer their motion to the rockers. This in turn operates the valves. | When you look at cam specifications (typically referred to as a cam card), they will list several numbers that are very important to how this particular cam will operate in your motor. The photo above outlines a pushrod engine which is what you'll encounter most of the time in the hotrodding world. The cam is located in the block. The lobes push against lifters which push on pushrods, and the pushrods transfer their motion to the rockers. This in turn operates the valves. | ||
===Duration=== | ===Duration=== | ||
| − | This is the amount of time (stated in crankshaft degrees) that the cam will hold the valve off the seat. Some cams have the same duration for the intake and exhaust valves. They are typically called single pattern cams. Those with different numbers for intake and exhaust are often called split pattern or dual pattern. | + | This is the amount of time (stated in crankshaft degrees) that the cam will hold the valve off the seat. Some cams have the same duration for the intake and exhaust valves. They are typically called single pattern cams. Those with different numbers for intake and exhaust are often called split pattern or dual pattern. The design is ground into the cam and can't be altered without physically changing the camshaft lobe profiles. |
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| + | If the duration is not stated on the cam card, it can be easily calculated by adding the intake opening point in degrees to the exhaust closing point. | ||
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| + | If those points are not known isn't known, you can estimate the duration by using the advertised duration and the lobe separation angle: | ||
| + | #Add the intake and exhaust advertised durations, then | ||
| + | #divide the results by 4, then | ||
| + | #subtract the lobe separation angle, then | ||
| + | #multiply the results by 2 | ||
===Lift=== | ===Lift=== | ||
| − | This is how far the lobe of the cam will push the lifter in a linear distance. | + | This is how far the lobe of the cam will push the lifter in a linear distance. It is measured by subtracting the base circle radius (or diameter) from the radius (or diameter) at the tallest point. This number is also ground into the cam, however the actual lift seen at the valve will change with rocker arm ratio. |
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===LSA=== | ===LSA=== | ||
| − | Lobe Separation Angle, sometimes called | + | Lobe Separation Angle, sometimes called lobe ''displacement'' angle. The LSA is a measurement in ''camshaft'' degrees that states how far apart the maximum lift points of the exhaust and intake lobes are. This number is ground into the cam and can't be altered without physically changing the camshaft lobe profiles. |
[[File:LobeSeparationSmall.gif]] | [[File:LobeSeparationSmall.gif]] | ||
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==How these things affect an engine's output== | ==How these things affect an engine's output== | ||
| + | The following is a rough guide to how the cam specs relates to how the cam is used: | ||
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| + | *Stock/near stock cam: 0.260" to 0.273” lobe lift (0.390” to 0.410” lift w/1.5 rockers); duration @ 0.050” lift around 200° | ||
| + | *RV/mild performance cam: 0.300 to 0.307" (0.450” to 0.460” lift w/1.5 rockers); duration @ 0.050” lift around 212° to 222° | ||
| + | *Hot street performance cam: 0.320" or so lobe lift (around 0.480” lift w/1.5 rockers); duration @ 0.050 lift around 230° | ||
| + | *Racy street/strip: 0.333" or higher lobe lift (0.500”-plus lift w/1.5 rockers), duration @ 0.050” lift around 232°-up | ||
===Duration=== | ===Duration=== | ||
| − | Increasing duration will tend to shift the power and torque curves | + | Increasing duration will tend to shift the power and torque curves upward. Longer durations lend themselves to higher RPM operation, because at higher RPM the amount of time the valve spends open is smaller than at lower RPM. Keeping the valves open longer allows the cylinders to fill with more air and fuel. Since the valve may be open considerably longer than the intake stroke, it does tend to reduce power and torque at lower RPM. At lower RPM the intake valve is open too long for maximum efficiency, so the suffers. and some of the good stuff you just sucked in there gets pushed back out because the valve is open longer than optimal for low-RPM operation. Another important factor to remember is that larger engines tend to "tame down" a cam's duration. The same duration cam in a small displacement engine will have a higher peak RPM than if you installed it in a larger displacement engine. For example, if a cam provides a 6500 RPM peak hp in a 305 Chevy, the same cam might peak its HP at 5500 in a 400 Chevy. Here is a comparison between two engines. The only thing I changed about these two simulations is the duration of the camshaft. Notice that the engine with the larger cam makes more power, but you would have to rev it 1000 rpms faster to get it. Notice also the huge loss of torque down low. This is an extreme example just for comparison. |
===Lift=== | ===Lift=== | ||
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===Overlap=== | ===Overlap=== | ||
| − | Overlap and LSA are closely tied together. Increasing overlap is what gives engines a choppy idle. The extra time the valves are open together causes what is called ''reversion'' which is a situation in which the exiting exhaust gasses are partially pushed back up into the intake runner at low speeds. This causes big fluctuations in vacuum and uneven fuel metering. Once at higher | + | Overlap and LSA are closely tied together. Increasing overlap is what gives engines a choppy idle. The extra time the valves are open together causes what is called ''reversion'' which is a situation in which the exiting exhaust gasses are partially pushed back up into the intake runner at low speeds. This causes big fluctuations in vacuum and uneven fuel metering. Once at higher RPM, that overlap is helpful since the fast-moving exhaust gasses make a slight vacuum and help to pull in new intake charge which is called ''scavenging''. Overlap is also very important to intake manifold vacuum. Less overlap will improve idle vacuum. This has benefits to be discussed later. |
===Intake lobe centerline (ICL)=== | ===Intake lobe centerline (ICL)=== | ||
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