How to choose a camshaft
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[[File:Cam dimensions1.jpg|thumb|350px|]] | [[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. | ||
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| + | ===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. | ||
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| + | 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). | ||
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| + | 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. | ||
===Duration=== | ===Duration=== | ||
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#subtract the lobe separation angle, then | #subtract the lobe separation angle, then | ||
#multiply the results by 2 | #multiply the results by 2 | ||
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===LSA=== | ===LSA=== | ||
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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. | 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. | ||
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| + | ===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. | ||
===Intake centerline (ICL)=== | ===Intake centerline (ICL)=== | ||
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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. | 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== | + | ====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. | 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. | ||
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*Racy street/strip cam: 0.333" and higher lobe lift (0.500” and higher lift w/1.5 rockers); duration @ 0.050” lift around 232° and higher | *Racy street/strip cam: 0.333" and higher lobe lift (0.500” and higher lift w/1.5 rockers); duration @ 0.050” lift around 232° and higher | ||
| − | == | + | ==Things that can "frag" a camshaft and lifters== |
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'''See:''' [[Camshaft install tips and tricks]] | '''See:''' [[Camshaft install tips and tricks]] | ||
==Summary== | ==Summary== | ||
| − | Choosing a cam is often something that seems shrouded in mystery. | + | 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, | + | 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. |
| − | 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. | + | 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 engine builder 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 combination and a clumsy, finicky engine that won't put a smile on your face. |
==Resources== | ==Resources== | ||
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