Editing 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, I suggest www.howstuffworks.com 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. === What do all the numbers mean? === ---- '''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. The design is ground into the cam and can't be altered without physically changing the camshaft lobe profiles. '''Lift:'' 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. [[http://www.tpub.com/content/constructiongrader/TM-5-3805-263-14P-2/img/TM-5-3805-263-14P-2_71_1.jpg]] '''LSA:''' Lobe Separation Angle, sometimes called Lobe Center Angle or Lobe Displacement Angle. This is a measurement in ''camshaft'' degrees that tells you how far apart the centerlines, or 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. [[http://www.lunaticamshafts.com/Images/Tech/Cams/LobeSeparationSmall.gif]] '''Overlap:''' 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. '''Intake and Exhaust Centerlines:''' Intake Centerline: *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. Exhaust Centerline: *This number represents where the exhaust lobe's peak lift occurs in relation to crankshaft rotation. It is the point of maximum lift of the exhaust lobe and is measure in crankshaft degrees. '''Phasing the camshaft:''' 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. == How does all of that affect my engine's power? == ---- '''Duration''' Increasing duration will tend to shift the power and torque curves up in RPMs. Longer durations lend themselves to higher RPM operation. Why? At high RPMs, the amount of time the valve spends open is smaller than at lower RPMs. Keeping the valves open longer allows the cylinders to fill with more air and fuel. Since the valve is open for considerably longer than the intake stroke, it does tend to reduce power and torque in the lower RPMs. At lower RPMs its open too long 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. [[http://i162.photobucket.com/albums/t264/curtis73/wikiarticle.jpg]] '''Lift''' Lift is a number that is best matched to your heads. Head flows for common American head castings can be found on the internet. They are flow tested and the numbers published at different lift levels. 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 you lift more. If your heads start decreasing flow above .500" lift, there is no reason for .700" lift, but in most cases more is better up to the point where the heads start losing flow. Since the aftermarket has plenty of rocker ratios available for most engines, the lift that is ground into the cam is usually sufficient, but shopping around between cam brands and product lines might yield slightly different lifts that you can use to fine tune. One more finer point about lift that I like to mention 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 more aggressive. That is to say, they have to accelerate the lifter faster to get to the peak lift in the given duration. Faster ramp speeds usually pay off big time because they get the valve lifted higher, faster. The sooner you can get the heads flowing their peak air, the more air can get sucked in the cylinder. The downside for flat-tappet cams is that the steeper ramps mean they contact the lifter at a stronger angle. The potential for wiping out a cam lobe or lifter is greater, but manufacturers know that and design ramps to be as fast as they can be without destroying components. '''LSA''' Lobe separation for a given duration will alter a few different things. Primarily it changes the amount of overlap. Narrower LSAs will increase overlap. This has a tendency to reduce engine output at lower RPMs and increase engine output at higher RPMs. Narrower LSAs tend to make more peak power but a little less average power. Wider LSAs tend to make less peak power, but a broader powerband. Changing the LSA also changes the valve timing events; opening the exhaust valve sooner and closing the intake valve later, both of which affect how the engine ingests air. '''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 RPMs, 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 '''ICL''' 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 ICLs (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 RPMs. == General Trends == ---- '''Duration''' - More duration means power in the higher RPMs. Best for: lighter cars, lower rear axle gearing (higher numerically), higher stall converters, bigger head ports and flow, higher compression (to compensate for the low cylinder pressures at lower RPMs), and lower transmission gearing. Less duration makes power in the lower RPMs. Best for: heavier cars, tow vehicles, higher rear axle gearing (lower numerically), lower stall converters, smaller head ports and flow, lower compression (to prevent too much cylinder pressures during cranking) and higher transmission gearing. '''Lift''' - as stated before, lift should be matched to the head flow. Lift doesn't change the RPM range of the engine, it just alters how much area is available for flow. Maximizing how much air can be sucked in is a benefit on any engine. '''LSA''' - Wider LSAs broaden the torque curve and consequently the HP curve. Best for: street cars, computerized EFI cars, fuel efficiency. Narrower LSAs make more peak HP in a more narrow RPM range. Best for: race cars, carburetors, power (at the risk of losing some MPG). '''Overlap''' - 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. '''ICL''' - altering the cam timing with ICL, advance, or retard, can fine tune where the power comes on in the RPM band. A few degrees in either direction can be a way to fine tune things, but as a wise racer once told me, "if you have to change the ICL, you've chosen the wrong cam." Altering ICL should be left to those in the know, and most off-the-shelf cams have been designed by cam companies who know what they're doing. == Other cam design factors == ---- '''Roller vs. Flat tappet''' (to be ammended later) '''Cam Engineering and materials''' (to ba ammended later) == Things that will "frag" a camshaft and lifters == ---- *1. Failure to remove all [[rust]]-preventative from cam and lifters with solvent once you get them home. *2. Failure to wash the cam and lifters with hot soapy water to remove the remainder of rust-preventative not removed with solvent. *3. Failure to properly massage an extreme pressure lubricant such as Molybdenum Disulfide into the pores of the metal on all lobes and lifter faces. *4. Failure to use an extreme pressure lubricant additive to the engine oil for camshaft break-in. *5. Failure to use the proper valve springs for cam break-in. You can't use the 350 lb over-the-nose springs that you'll eventually use in the motor and expect the cam to live. *6. Failure to check for valve spring coil bind. *7. Failure to check for retainer to valve guide clearance. *8. Failure to check for binding at the rocker/stud interface. *9. Failure to check for piston/valve clearance. *10. Failure to run the motor at high rpms (2500 or higher, alternating 1000 rpm's up and/or down) for the first 35-40 minutes of its life. NO IDLING. NO IDLING. NO IDLING. *11. Failure to clearance lifters in their bores so that they spin freely. *12. Failure to initially adjust the valves properly. *13. Failure to inspect the distributor drive gear for wear. Too much wear can allow the cam to walk in its cam bore and contact an adjacent lifter. == Summary == ---- The manufacturers have spent a hundred years of technology and millions of dollars in research and testing. They know what they're doing, but they also 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. In short, choosing a cam is often something that seems shrouded in mystery. Now that you know some definitions and general trends, I would like to suggest you download CamQuest here: [http://www.compcams.com/camquest/default.asp]. It is a free software that lets you compare cams and how they affect power output. For more in-depth discovery, purchase some dyno simulation software like Desktop Dyno 2000 or DynoSim. They allow you to alter any cam timing event and the results are displayed graphically for you on a simulated dyno chart. To summarize, the whole system has to match; carb, intake, head flow, exhaust, cam, torque converter stall speed, rear axle ratio, tire size, transmission ratios, and vehicle weight. Some of those things are already decided for you within a small range, like vehicle weight and transmission ratios, while others are easily altered like rear axle ratios and tire size. Choosing a cam with this knowledge might make it a bit easier to understand the reasons why a professional might recommend a certain cam and it might help you make wiser decisions about your cams in the end. Either way, the right cam choice can make the difference between a well-sorted drivetrain and a clumsy, finicky engine that won't put a smile on your face. ==References== ----- For help with camshaft choice : http://www.camquest.com/ [[Category:Engine]] [[Category:Camshaft]]

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