Camshaft. Few words stir the soul of the gearhead as much as this one. And few engine parts grace a catalog with more honor, more mystery, more significance. Yet, little real understanding of cams exists, and with very few exceptions (and those in even more unique contexts), aftermarket high performance camshafts yield less than hoped-for outcomes. Let's explore a little bit about camshafts. This article has the potential for digging relatively deeply into this subject, so prepare for as many as five installments. We'll see.
Strokes versus phases
The first four-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 worked best when their intake valves opened *before* the piston's intake stroke actually began and their exhaust valves closed some time *after* the exhaust stroke ended. Thus the four-stroke engine has phases that are only loosely connected to their pistons' strokes. Strokes are mechanical, phases are pneumatic, thus they are linked, but are not the same.
Intake air inertia
The engine's phases depend on 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 instead of a gain. 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 the mixture loses its momentum and stops, but before it reverses direction. This is the whole point of making sure the cam, and in turn the intake valve, is in the correct position to make this happen. Whether manufacturer or modifier, this is what you strive for.
However, the manufacturer plays to a more careful set of rules. The result is the stock cam times its valves conservatively, providing the cylinder very limited exposure to the atmosphere, unlike the performance camshaft's extended timing which exposes the cylinder longer. This is because etended timing harms air velocity and density. Therefore, though the engine's power can be increased through inertia tuning, that increase is focused more around a particular rpm range, and not spread as broadly over the rpm as before. In fact, the "cammier" the engine, the less power it might have even than stock at most engine speeds, making added power only at the one that coincides with the ideal inertia rpm. Moreover, that rpm will be higher than normal due to the resulting slowed intake air speed, which reduces combustion efficiency due to reduced fuel and air homogenation. Combustion, and thus torque, suffer. No free lunch, as they say.
This well-known drawback can be partly overcome by nudging cam timing to put the intake valve's closing at the point that captures that last little bit of the intake's inertia, just before it falls, closing the door on it, so it can't back up. As described earlier. Trial and error finds this ideal point that maximizes cylinder filling and pressure and is the whole reason behind "degreeing" camshafts. Degreeing a cam is not simply mistiming it. The nudge is less than one-third of one sprocket tooth's distance. More on this in the next post.
The cam fallacy
Cams remanufactured to stock specs, in which there is a growing popularity, often outperform bigger, "high performance" cams. Many times my customers have asked for bigger cams. I generally discourage them. When they insist they always find they have made a mistake. "I've lost my low end", one says. "Your engine is over-cammed," I reply. No one wants to believe it. They have to experience it for themselves. Something few seem to appreciate is how well the stock cam works in most Japanese multicylinder engines. Short of riding at full throttle, i.e. racing, aftermarket cams usually fail to satisfy. For the kind of riding most of us do, stock cams really are best.
Next post, Camshaft anatomy.