The camshaft is not a silver bullet. In fact, if anything in the engine is really that special, it’s the cylinder head, which when removed from the engine most race mechanics keep wrapped in a towel to ward off prying eyes. That’s where the power is, and not all of it centers around the ports.

A lotta cam
But the cam does play an important role. Someone has said the camshaft is the "equal sign" of the engine. It's the catalyst that unites all the other parts in a way that makes them their most effective. Better cylinder filling is of course the goal of opening the valves for a longer period via an aftermarket cam’s extended valve duration. But there is a drawback. It can be counterproductive. Exposing the cylinder to the outside air for longer periods slows the intake airstream. The result is less optimum mixture distribution in the combustion chamber at lower engine speeds—the fuel separates from the charge. In addition, extended valve duration cuts into the time the cylinder has to compress its mixture—the engine has less compression. So while added valve duration often yields a high-rpm power increase, a side-effect is often lost low-rpm power.

What powerband?
Extended valve duration also increases valve overlap, that period during which both the intake and exhaust valves are open. As valve overlap increases, the wave (lengths and angles based) tuning of the intake and exhaust systems becomes demonstrably more effective. These latent effects are “wakened up” and utilized. However, again there is a tradeoff because the engine thus modified depends less on its mechanical parts and more on its wave pulses, which are somewhat tenuous, to run well at all. Rather like how a highly-developed two-stroke engine behaves. Therefore, though power-adding, wave tuning is effective for only a very narrow rpm range—the engine runs better than stock, but only above a certain rpm, below which it runs noticeably worse than stock. Many have experienced this.

How this relates to the exhaust
A lot can be said about tuned exhausts: diameters, angles, lengths, the speed of the pulsed waves, and all the rest. It makes for interesting reading; there really is some physics going on there that excites the mathematically-minded. But one important thing should be considered at the outset: it does no good to put on a tuned exhaust if the camshaft isn’t there to support it. The camshafts in Honda street bikes made until well into the 1990s are all designed with almost no valve overlap. Thus exhaust tuning can’t work—the valves aren’t open to receive the waves resulting from tricky exhaust design with an eye on tuning. A "tuned exhaust" on a vintage Honda cannot improve engine performance unless the camshaft is also changed. Of course, the weight reduction and reduced back pressure in very loud exhausts having large baffles can have a small performance effect no matter the cam used. For that matter, not all “performance” exhausts are tuned. Exhaust pervayers Dennis Manning and Terry Vance went on record a lifetime ago as saying their products owe more to serendipity and style than to engineering. And despite Internet conversations to the contrary, there is no appreciable tuning in stock exhausts.

Lift or duration?
There is another, less often considered, more innovative way the camshaft can be used to increase cylinder filling, by opening the valve farther instead of holding it open longer. More valve lift, in other words. Like the valve that is held open longer, the valve that is opened farther allows more air into the cylinder, potentially increasing power. However, unlike longer-duration cams, since cylinder exposure to atmosphere is not increased, the gases moving in the tract are not slowed, and the charge enters the cylinder still well mixed and combusts nicely. And the engine isn't made more dependent on wave tuning by the usual expedient--and almost inevitable outcome of--extended valve overlap. Nor is there any loss of cylinder compression. There is plenty of low-rpm power. That sounds like a win/win. Why aren't all camshafts made this way then, with an emphasis on lift instead of on duration, especially performance cams?

Valve acceleration
There are three reasons. First, there is valve acceleration. The valve that is opened farther in the same amount of time is one opened more quickly, i.e. accelerated harder. The valve is then closer to the point of "float," and valve float is a serious threat. Thus high-lift cams are potentially problematic. It takes some careful reengineering of the rest of the engine to make them work, making such cams a greater liability for the manufacturer in the hands of home-brew engine builders.

The light finally dawned
Second, because of the motoring world's historic fixation on valve duration, a certain politic of never opening a valve farther than 25 percent of its diameter has long prevailed. Until the mid 1980s, few if any production Honda valves did. This 25 percent figure came from an interesting geometrical fact: when the valve is opened to a distance equal to one-fourth its diameter, the flow curtain around the valve's periphery is as effective as it will ever be. However, consider for how long that valve is fully open. An almost infintessimal period. Openings beyond 0.25d effectively extend the amount of time the valve is near fully open, without actually increasing overall valve open duration and thus without adversly affecting tract speed and ultimately combustion. More open time without more duration. In other words, when the valve is opened to say 0.35d-0.40d—as most production engine's valves are today—the valve, instead of “seeing” 0.25d for a thousandth of a second, actually dwells a bit at 0.25d, increasing flow potential without the drawback of extended duration. Engine builders discovered this principle and engineers have confirmed it—it is the new tradition. And it works. Many engine builders go out to 0.50d or more.

Consumerism
But the third and the real reason not many aftermarket camshafts are designed for lift is that such cams are very difficult to market. Their numbers aren't as impressive. Not to mention they require knowing what you're doing, and as we've revealed already, that is not a given.

Degreeing the stock cam
Speaking of unimpressive, 60s and 70s Honda SOHC four cams are pretty tame, typically producing only 5-6mm lift and 10-40/40-10 timing yielding a scant 230 degrees duration of the valve. Efforts at special degreeing of them, in the absence of other engine changes, has not proven very effective. But there is one exception: worn cams and cam chains. A worn cam opens its valves later than normal, as does also a worn chain. Thus an intake open timing that when new was 10 degrees BTDC can only a few thousand miles later profile at 5 degrees or even 0. I have observed this firsthand countless times in Hondas, whose cams and cam chains wear quickly. A stock engine therefore can regain some of its lost power by either replacing the cam and cam chain—which of course is best—or by re-degreeing the worn cam to the original 10 degrees through the expedient of slotting its sprocket. Camshaft degreeing of a stock engine is in this instance fruitful, and it is something I have done many times when the customer has not wanted to replace the cam.