 Techline-August08-PtIV Cam Specs and Choices
Pt One of this story:
As promised, we’ll continue with cam characteristics and how they affect engine performance. We’ll then get into choosing a cam for your particular application.
Lobe Separation Angle
The acronym LSA refers to Lobe Separation Angle also known as the Lobe Displacement Angle or the Lobe Spread. I try to include the various vernaculars that refer to the same principle or components because nothing is more confusing than encountering different terms and not realizing they actually mean the same thing.
For starters, here are some basic facts about LSAs. First, a tight LSA has the respective intake and exhaust cam lobes (centerlines) closer together. Second, a wide LSA has a wider angle between the high points (LCs) of the respective cam lobes. Third, other factors being equal: tightening the LSA produces more valve overlap around TDC, while widening the LSA results in less overlap. Fourth, at Bottom Dead Center (BDC) of the intake cycle, a wide LSA produces a longer delay to valve closure after the piston has passed BDC.
The calculation of LSA gets more involved since we need to add the intake and exhaust Lobe Centerline figures; then divide their sum by two to determine the LSA. We learned a little earlier on that to calculate the Lobe Centerline Angle for the intake, we divide the intake duration by two and then subtract the intake valve opening timing to determine intake lobe centerline, which is the maximum lift-point of the intake cam lobe. To determine the exhaust LCA, divide the exhaust duration by two and subtract the closing timing to get the exhaust Lobe Centerline Angle (LCA), which is the maximum lift-point of the exhaust cam lobe. To show you how to calculate the LSA, let’s use Bob Wood’s BW-408B cam specifications:
Intake LCA plus Exhaust LCA, divided by two equals LSA.
This comes out to be
100 + 102/2 = 101 degrees LSA.
The strength of the low-pressure area (vacuum) developed in the cylinder during the intake stroke, which is a necessary function for cylinder fill, is directly controlled by the amount of valve overlap. However, the LSA is not the same as overlap. Ably assisting the engine’s ability to draw in a fresh fuel/air charge is the scavenging effect, which is a colloquial mechanical term more properly known as the Principle of Momentum. Scavenging is when the high velocity of the exhaust gases rushing out of the cylinder through the exhaust port creates a strong low pressure area in the cylinder, which pulls the fresh fuel/air charge from the intake tract into the cylinder at high engine rpm.
Scavenging directly relates to the cylinder heads, exhaust system, and other factors that affect an engine’s breathing capabilities. For example, weak cylinder heads (in terms of flow), like those found on the TC 88s and, to a lesser extent, on the TC 96 reduce the scavenging effect.
Another would be a poor scavenging exhaust system; the most obvious example being drag pipes on a street bike. Here, the wrong LSA might not put enough fresh fuel/air charge into the cylinders for the engine to make the power it should.
Typically, a wider LSA of 112 to 116 degrees will mean less overlap and higher engine vacuum at idle while a narrower lobe separation (101 to 108 degrees) provides just the opposite. Wild full-race cams may be the exception to this rule. They can have a wide LSA, like 118 degrees, and still have lots of overlap and make little vacuum at idle. This is because the intake valve has so much duration, opening very early in the cycle and not closing until long past when the exhaust valve begins to open. These engines have that particular rumbling sound at idle that we all love because of the intake and exhaust tracts working together. In old Harley vernacular, we refer to this as a lumpy cam.
LCAs are the maximum-lift points of the intake and exhaust cam lobes. With street cams, the wider you move the lobe centers apart, increasing the LSA (to something like 116 degrees), the less overlap you typically have. Although both wide (112 to 116 degrees) and narrow (101 to 108 degrees) LSAs do have an effect on overlap, it is still possible to get overlap with very wide LSA race cams. (These LSA figures for wide and narrow angles are approximate and used for comparative purposes only.) Some LSAs are narrower than 101 degrees of crankshaft rotation or wider than 116 degrees.
The overlap phase begins with the opening of the intake valve. The timing of this is critical to engine vacuum, throttle response, exhaust gas emissions, and, particularly, gas mileage. The area between the intake valve opening and the exhaust valve closing, and where it occurs, which is the amount of overlap, is one of the most significant markers in the engine’s combustion cycle. If the intake valve opens too early, exiting exhaust gases will push the incoming intake fuel/air charge back into the intake manifold. If the intake valve opens too late, the engine will lean out, thus hampering engine performance. If the exhaust valve closes too early, it will trap some of the burnt gases in the combustion chamber, bastardizing the new intake charge and reducing engine power. If the exhaust valve closes too late, it will over-scavenge the chamber, taking out some of the fresh intake charge, again creating an artificially lean condition and reducing power output. If the overlap phase occurs too early, an overly rich condition will develop in the exhaust port, negatively affecting gas mileage and exhaust emissions, which is a big EPA no-no. Therefore, it’s easy to see that everything about overlap is critical to the performance of the engine. High-torque cams never have more than 2 degrees of separation between the exhaust cam LCA and the intake cam LCA.
Cam Selection
We measure LSA in camshaft gear rotational degrees of 360 degrees for a complete cycle, not to be confused with other engine cycle measurements that measure in crank rotational degrees of 720 degrees to complete a full cycle. The crankshaft or flywheel assembly moves at twice the rotational speed of the camshaft, or looking at it the opposite way, the camshaft rotates at half the time of the crankshaft rotation. The experienced mechanic will know this attribute of rotational speed and can listen to the cadence of running engine components to easily isolate symptomatic noise for a correct diagnosis. To the uninitiated, it is just all noise. While lift, duration, and overlap are very important in the calculation of which cam profile to use in a particular situation, they do not go far enough for the thinking mechanic. Therefore, the manner in which the intake and exhaust lobes time to each other will give more of the needed information.
One should consider many factors before choosing a cam. You do not have to be a rocket scientist, and you also don’t have to understand fully all that we have gone through. However, you need to understand some cam information to be able to select a knowledgeable adviser and separate marketing trivia from fact. You also should be able to apply the available factual cam information to your bike model, total weight, and riding style. For example, the optimal cams for a Dyna will be different than those
suitable for a bagger.
Rule I: bigger isn’t always better. One problem most riders encounter when trying to select a cam is that a radical cam costs the same as a mild one. This being the case, many will feel the need to purchase a more radical cam because it sounds like they’re getting more lift, duration, and overlap for the same amount of money. The truth is, you may be purchasing less for the same money because that wilder cam will bog down your ride. The overriding rule is not to purchase a cam too radical for your bike or your needs. This is the number one mistake enthusiasts and mechanics alike make. The bigger-is-always-better syndrome does not apply to performance mechanics. This condition seems endemic to the Harley rider. Bigger is better with big-cubic-inch engines that require increased duration and lift because of the engine’s demands for much more air.
Modern, stock, EPA-compliant engines need an increase in the length of time the valves are open, which means using a longer duration cam. Now, a longer duration cam allows you to also increase the compression ratio, since cylinder pressure will begin to build later in the compression cycle since the intake valve is held open longer. Larger bore and longer stroke engines can handle even longer duration cams. I only make this point for informational reasons, since I’ll always increase compression whether the engine uses a longer duration cam or not. However, increasing compression without increasing duration will cause more heat generation, invariably leading to detonation. Therefore, an input fuel modifier is necessary to bring the fuel/air ratio to manageable levels. The ECM fuel tables link to the ignition tables, so both are modified with an input system like Terminal Velocity. Power Commander is an output modifier that adjusts ECM signals after they leave the computer. However, the Power Commander has an ignition component that other output modifiers do not. Larger-cubic inches will also lower the powerband created by the longer duration cam whereas the same cam in a smaller engine will raise the powerband.
Story continues in the next thread. Check back issue for pix and extra items. |
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