How to choose a camshaft
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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. | 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. | ||
| − | === | + | ===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 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. | |
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| + | ===Intake lobe centerline (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. | ||
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| + | ==General Trends== | ||
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| + | ===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. | ||
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| + | ===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. | ||
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| + | ===LSA=== | ||
| + | Lobe separation for a given duration primarily changes the amount of overlap. | ||
====Narrower LSA:==== | ====Narrower LSA:==== | ||
| + | A narrower LSA will ''increase'' overlap. This has a tendency to reduce engine output at lower RPM and increase engine output at higher RPM. A narrower LSA tend to make more peak power but a little less average power. | ||
*Moves torque to lower RPM | *Moves torque to lower RPM | ||
*Increases maximum torque | *Increases maximum torque | ||
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====Wider LSA:==== | ====Wider LSA:==== | ||
| + | A wider LSA 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. | ||
*Raise torque to higher RPM | *Raise torque to higher RPM | ||
*Reduces maximum torque | *Reduces maximum torque | ||
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*Increases piston-to-valve clearance | *Increases piston-to-valve clearance | ||
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===Overlap=== | ===Overlap=== | ||
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