Lobe Center; What is it?
What is "lobe center"? What does it do? Why is it important?
Lobe center is a routine; a method, of fine-tuning the timing of a camshaft so that the intake valve closes at a particular point. The intake valve closing point is the most important valve event in the engine; just how important was discovered by accident.
The first 4-stroke engines didn't even have camshafts. Their valves opened and closed atmospherically, that is, at the prompting of these steel-pistoned, slow-revving engine's positive and negative pressures. Naturally, these movements didn't coincide precisely with the top and bottom of the piston's stroke, though it was at first assumed that they should. Later, when lightweight aluminum pistons permitted higher rpm, camshafts were added to better control valve action, and engineers were surprised to discover that those earliest engines "knew" what they were doing. Their cammed successors responded best when their intake valves opened before the intake stroke actually began and their exhaust valves closed some time after the exhaust stroke ended.
The reason, it turned out, is inertia -- the intake charge's momentum keeps it flowing into the cylinder despite the piston's upward movement after completing the intake stroke. Delaying the intake valve's closing to take advantage of this phenomenom increases cylinder filling and power. That's good. But it's kind of tricky. How long is long enough? If the valve is held open too long, the mixture's momentum dies and the gases back up in the port, sucking mixture out of the cylinder and causing a loss of power. On the other hand, if we close the valve too early, we've limited how full the cylinder can get. Either way, power is lost. The ideal thing is to close the intake valve at precisely the millisecond that the mixture loses its momentum and stops, but before it reverses direction. This is the whole point of "degreeing" cams, and when a certain lobe center is recommended, it is supposed that this will put the cam, and in turn the intake valve, in the correct position to make this happen.
So what's the deal with the cam lobe's center? Why not just track that critical intake valve closing point? The answer, in a word, is ramps. All cams have them. Acceleration ramps which gather up the clearances in the valve train before the valve is shot off its seat, and deceleration ramps which cushion the shock of the valve's closing. The problem is that these ramps move the valve so slowly that determining when it has opened or closed is very difficult. Because of this, engine builders have developed a method of ignoring the ramp portions of a cam when measuring valve movement. The ramp isn't there to move the valves anyway, its sole purpose is to babysit the valve train. These tuners disregard the valve's movement until it has moved a certain amount, which ensures that the valve is well clear of the ramp. That amount is represented below by the dotted line. It is called the "checking height."
The problem is that ramps aren't all the same size. The ramps on a pushrod engine's cam for example are huge, because that engine's spindly valve train demands very gentle opening and closing curves. The ramps on overhead cam engines on the other hand are much smaller, because there are fewer valve-related parts flailing about needing cushioning. Each engine design has its own size ramp and its own checking height, some 0.020", others 0.040", and still others 0.050". Comparison of these different cams is impossible, as two different checking heights give two very different timing conditions. What's needed is a system of measuring cam timing which is independent of engine design. That's the lobe center method. Although it still requires the use of a checking height, the lobe center approach indexes the cam based on an average of its opening and closing points -- its "center" -- and not one of those points. This means that any checking height can be used and the outcome will be the same every time, because no matter what the starting and stopping points are, the center of a thing is always the center.
Why isn't the cam lobe's center measured directly? Because it isn't the lobe's physical center that is being sought, it's its mathematical center -- the average of its opening and closing points. In some cams, the physical and mathematical centers are the same, but not in all cams. The cam lobes in overhead cam rocker arm engines for example are asymmetrical -- their opening and closing flanks are shaped differently to compensate for the change in geometry the system undergoes as the rocker arm slides over the lobe.
To sum up, the lobe center cam timing method serves three purposes. First, it is an attempt to time the intake valve at just that critical closing point that will maximize power. Second, it is a sort of arbitrator in the world of high performance engine tuning -- it gets everyone speaking the same language, so that meaningful comparisons can be made when comparing different cams. And third, it gives us gearheads something to do to bikes that are otherwise seldom in need of service... And all it takes is a simple equation:
© 1996-2015 Mike Nixon