How to choose a camshaft

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://www.carcraft.com/techarticles/ccrp_9812_secrets_of_camshaft_power/index.html Secrets of Camshaft Power] by Marlan Davis (Car Craft, December, 1998).  

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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.

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.

Lift is a number that is best matched to your cylinder heads. Head flow for common domestic head castings can be found [http://users.erols.com/srweiss/tablehdc.htm '''here''']. Heads are flow tested at different lift levels, and at different amounts of "depression" (usually measured in in/Hg). When comparing heads, be sure the depression is similar, or be prepared to convert the results from one depression to another depression, using a calculator. Another difference that can be found when comparing head flow, is the size of the cylinder the head is sitting over, and yet another thing is whether an intake manifold is in place (rarely done), or if an exhaust tube is in place (more common), or is clay is used to radius the openings (fairly common).

More lift is generally better provided two things are addressed: the valves, retainers, and springs are capable of the lift you plan without binding, and the heads flow more as the valve is lifted higher.  If a head port starts decreasing flow above 0.500" lift, there is no reason to try to use a 0.700" lift cam, but in most cases more lift is better up to the point where the heads start losing flow.  Since the aftermarket has different rocker ratios available for most engines, the lobe lift of the cam can result in more lift than the stock ratio rockers would give.

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.

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 less 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 efficiency suffers because some of the air/fuel mixture gets pushed out with the exhaust, along with some a/f charge getting pushed back into the intake manifold (called "reversion").  

Another thing to remember is that larger engines tend to lessen the effect of having a longer duration. The same duration cam in a small displacement engine will have a higher peak RPM than if it was installed in a larger displacement engine. For example, if a cam provides a 6500 RPM peak hp in a 305 SBC, the same cam might peak at 5500 in a 400 SBC.   

This number (usually not found on the cam card) represents the amount of duration in camshaft degrees when both the exhaust and intake valves are open at the same time. This factor is ground into the cam and can't be changed without physically altering the camshaft lobe profiles. Increasing duration at the same LSA will increase overlap. Decreasing LSA at the same duration will also increase overlap.

Overlap and LSA are closely tied together. Increasing overlap contributes to a race cam's choppy idle, along with the intake valve closing point and the exhaust valve opening points. The extra time the valves are open at the same time 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 the engine reaches higher RPM, the 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.   

More overlap can fool electronics and cause tuning headaches with EFI. It can also make tuning a carburetor a bit more difficult. More overlap makes a choppy idle and tends to make peakier power for the same reason as a narrow LSA does. More overlap and the subsequent lower intake manifold vacuum might mean giving up vacuum-driven accessories like power brakes. Some cars even use vacuum to operate the climate control, headlight covers, door locks, and windshield wipers.

===Ramp speed/lobe intensity===

One more point about the cam profile is ramp speed. For a given duration, more lift means the lobe ramps (the opening and closing faces on the sides of the lobe) are steeper (more aggressive). That is to say, the cam lobe has has to accelerate the lifter faster to get to the peak lift within the available amount of duration duration. Faster ramp speed can give more "area under the curve", which usually equates to a broader, less peaky powerband. The downside for flat tappet cams is that the steeper ramps mean they contact the lifter at a greater angle, so the potential for wiping out a cam lobe or lifter is greater. Manufacturers are well aware of this, so they try to design the lobe profiles to optimize power, yet maintain good durability. Cam profiles like the Comp Cams XE-series and Lunati's Voodoo line are both at the edge of how fast the valve can be safely opened and closed. That's why they caution against using a higher ratio rocker arm when using these grinds.  

This number represents where the intake lobe's peak lift occurs in relation to crankshaft rotation. It is the point of maximum lift of the intake lobe and is measured in ''crankshaft'' degrees. A cam ground "straight up" will mean that the exhaust lobe's peak lift will happen at the same amount of degrees before top dead center, as the intake valve will peak after top dead center. ICL is machined into the cam. When cam manufacturers machine the snout of the cam for the cam sprocket, they will drill the holes with the cam slightly advanced, retarded, or straight up.  When installed with stock components, the ICL can't be altered. Aftermarket timing chains and sprockets often have provisions for altering how the sprocket attaches to the cam and therefore you can counteract the ICL ground into the cam. If the LSA value is the same as ICL, the cam is ground "straight up." If the ICL is less than the LSA, it is ground advanced by the difference. If ICL is more than the LSA, the cam is ground retarded. For instance, if the cam has a 110-degree LSA with a 106 ICL, the cam is advanced by 4 degrees.

While is is true that you cannot change the lobes of a camshaft after it is ground (unless you re-grind the lobes), you can alter the characteristics of the camshaft in your motor by installing it in either a retarded or advanced position relative to the crankshaft rotation. For instance, the manufacturer recommends the camshaft to be installed straight up, neither advanced or retarded from his design. However, you have determined that you are making too much horsepower down low and can't hook the tires up. You want to trade off a little of the lower end power for some higher end power. You might, in this case, install the camshaft slightly retarded. Although all four events (intake open, intake close, exhaust open, exhaust close) will be affected by changing the camshaft timing, the most important one will be the intake closing point. If you retard the camshaft, you will be closing the intake later, thus bleeding off some of the cylinder pressure and resulting in less low end power. Vice versa if you advance the camshaft. More bottom end, less top end.

Intake centerline can be altered either by the crankshaft grind or the use of a camshaft sprocket that can alter if the cam is installed advanced or retarded. Later ICL (retarded cam timing) will shift the power curve up just a bit due to closing the intake valve later. With the faster engine speeds, the intake valve can stay open later without the risk of pushing intake gasses back into the intake runners.  Earlier ICLs (advanced cam timing) will foster low end torque for the opposite reason. At low speeds, closing the intake valve sooner will trap more intake air at low RPM.

==How the cam specs affect an engine's output==

Choosing a cam is often something that seems shrouded in mystery. The manufacturers have a hundred years of technology to draw from and millions of dollars expended on the research, development and testing of camshafts. They have used that experience to come up with thousands of lobe profiles and grinds that attempt to cover the whole broad spectrum of engines and applications. It's possible that an off-the-shelf grind might be perfectly fine, but it can't hurt for you to know the finer points. Most companies have tech lines to help you pick the right grind, but they are also in the business of selling products. Use their expertise, but knowing more about it can help you understand how your choices will affect how your engine runs.  That way, you can take the manufacturer's generic recommendation and fine-tune it to how you intend to use the vehicle.

Now that you know some definitions and general trends, you should take a look at downloading [http://www.compcams.com/camquest/default.asp Comp Cams' CamQuest]. It is free software that lets you compare cams and how they affect power output. For more in-depth research, purchase some dyno simulation software like Desktop Dyno 2000 or DynoSim. They allow you to alter the cam specs and the results are displayed graphically on a simulated dyno chart.