Musings on Camshafts
Camshaft. Few words stir the soul of the motorcycle gearhead as much as this one. It's a traditional, long-revered part of the perfomance mystique. No part of the four-stroke high performance engine embodies more mystery to the average enthusiast, and no engine part graces a parts catalog with more honor, more place, more significance.
The four stroke engine relies on its valves to get air into and out of its cylinder. You don't need to look all that closely at an engine's valve timing to discover that valve opening and closing cooincide only roughly with the piston's strokes. This designed-in valve timing "cheating" acknowldeges the fact that air streaming into and out of the engine has inertia, so opening valves early and closing them late, slightly out of step with BDC and TDC, is useful to accomodate air movement that persists despite piston position. Since it actually overrides valve timing to a degree, it's this air movement that literally controls the engine, not the valves. But what in turn controls the air movement? Three things:
- The aforementioned inertia of the moving air
- Shock waves (pulses) created in the air by valve opening and closing
- The effect on the air of intake and exhaust tract geometry, i.e. shapes
In an engine with a limited warranty and the widest possible user target, generally only the first of these three, air's inertia, is even partly at work. The rest, no. The stock camshaft is made so mild that it necessarily resticts the effect of the other two principles. The shock waves, though there, bounce ineffectively off of closed valves. Similarly, the intake and exhauts' cross sections, lengths and angles create pressures and suctions that are unrealized and ineffecitve, not taken advantage of, for the same reason, because the valves are closed. (By the same token, there is no such thing as a tuned exhaust for 60s through 80s standard Japanese road bikes). A performance camshaft then is one whose valve timing is carefully opened up to expose the cylinder to wave phenomenon in the intake and exhaust tracts.
It is very much like the two-stroke engine. In the two stroke, ports in the cylinder take the place of the cam and valves. And like the four-stroke, air movement in the two-stroke is contolled by more than just the mere port opening and closing, but also by waves and pulses, particularly those in the exhaust tract. In fact the two-stroke engine is famous for how its exhaust system "leads it by the nose", so to speak, so sensitive is it to exhaust tuning, including port timing. When camming a four-stroke engine then, extending its valve timing, what you are actually doing is making it work more like a two-stroke. That is, to the degree it is made more dependant on waves and pulses in both intake and exhausy tracts, it functions like a two-stroke, because it is more attuned to these phenomenon than to strict valve movement.
Two things result from making the four stroke more wave oriented. One, it gets fussy and twitchy, just like the two stroke. That is, a little less consistent, less civilized, the idle rougher, starting more difficult, throttle transitions less smooth. The very thing manufacturets try to avoid when they make cams as mildly tuned as they do. The second thing that happens is power production, while increased, gets more focused, narrowed around a particular rpm range, and not spread as broadly over the rpm as before, the other thing manufacturers do not like to have. These two things happen because the extra power effects of wave and pulse action are highly dependant on, and exhibited mainly at, specific engine speeds, i,e. rpm. In fact, an engine given extended valve duration depends more heavily on pulse and inertia tuning to run even as well as stock. So if timed correctly, the engine makes more power than stock. But if timed disadvantageously, the engine will actually make less power than stock, despite having a high performance cam.
Further, this rpm tend to be higher than normal because exposing the cylinder to wave phenomone for long periods in order to take advantage of its effects unfortunately also slows the airstream's speed, which has the negative effect of lowering combustion efficiency due to the resultingly reduced charge homogenation. The fuel droplets are not as thoroughly mixed in the air, thus the air burns less smoothly and combustion, and thus torque, suffer. Fuel separates from the airstream when it is moving too slowly.
Duration vs. lift
For a very long time, this was a problem for cam designers, this desire to open valves longer to take advantage of wave action but at the same time avoid slowing air speed. But eventually tuners began to understand that there was another way to hold valves open longer that did not slow air speed. This is how the high lift cams common today emerged, out of this discovery. Cams built to the high lift ethic provide improved access to the cylinder without the penalty of slowing the air speed and ruining the engine's low speed manners. You don't unfortunately find many camshaft manufacturers who want to make cams to this new ethic however, mostly because such cams are not amateur-friendly. Added valve lift decreases critical piston and valve clearances, making the careful, skillful checking of these clearances critical and putting most installers of the cam at risk of almost sure failure. High lift cams are too difficult to install by folks who think installing a performance cam is the same as installing a tire.
Degreeing the cam
Camshaft micro timing, nudging the cam within its factory-provided slots that one to three degrees that will optimize the aftermarket camshaft's power potential, is important. During the 1970s I was absorbing a lot of material on engine building by race tuners such as Yoshimura. One day my service manager at the dealership had me install a big bore and an aftermarket camshaft in a bike. Took it out for a test blast, and man, what a disappointment! He took it out also and was no more impressed than I was. Gonna be hard to sell this one to the customer, he had to be thinking. Well, some of the things I had been reading wbout kind of came together all of a sudden and I whipped that cam out, slotted its sprocket, put the cam back in at the factor marks but nudged forward from stock a couple degrees, and took the bike for another ride. Amazing! You would have thought I had added another cylinder. My boss was impressed too. And puzzled, he asked me what I had done to make the difference. Though I did not know it then, what I had done intuitively was to time the cam to coincide with the engine's intake air current. Specifically, the timing change put the intake valve's closing at the ideal point to capture that last little bit of the intake airstream's inertia, just before it fell, and then closed the door on it, to maximize cylinder filling and pressure. The important fact is, it is the intake valve's closing point that did the trick that day. That is the whole reason behind timing cards, cam numbers, and most of all, lobe centers. The lobe center timing method is the most important aftermarket camshaft installation tool, because it takes a lot of the myth, hype, and personality out of camshafts. Instead of each cam manufacturer having its own proprietary and unique (and importantly, very unlike the factory method) timing routine, lobe centers clears the playing field by offering a universal timing method that works on all cams and gives the same result no matter what goofy method the cam maker originally specified. We owe a huge debt of gratitude to whomever thought of it. Lobe centers is not about what cam timg is best for each model bike, though folks try to make it that. It's simply a way to position the cam for the best benefit, and of course this means intake closing.
Cam and follower wear
Production Japanese bike camshafts are made of cast steel or iron. They are actually pretty soft and wear surprisingly quickly, typically 0.001" for every two to three thousand miles. The exception is one or two hypersports models (eg ZX-10) whose cams are chrome-moly. But most are cast steel or iron. They can be drilled with a standatd HSS drill bit effortlessly. Instead of the cam being hard, the rocker or other type of follower is the hard part, and this is how the two parts survive in harmony. Unfortunately, the Japanese go the cheap route and make their followers hard not by the choice of material but by hard chrome plating. Thus the follower has only a very thin hard layer against the camshaft, a layer that is very much at risk. Any dirt or other impurity that finds its way between the cam and follower can result in the chrome plating being compromised, with the plating starting to wear, and it then quickly takes out the cam, due to the chrome being so much harder. The follower turns into a rasp. Certain models of Japanese motorcycles are actually well known for this happening, and aftermarket cam manufacturers have in recent years added follow rehabilitation to their skillsets. They remove the funky chrome and in its place put a hardened and polished steel layer. Some camshaft manufacturers actually require the followers be remade this way whenever one of their cams is sold. Good for them. It really is necessary.
Cams are bolted into Japanese overhead cam engines in ways that are surprisingly low tech. First, there are no bearings other than the cylinder head surface combined with the hold-down clamps, or holders the OEMs call them. While the alloy used in the head is specially designed to be friction resistant and often will fail after (yes, after) the cam does, the steel/aluminum interface remains a potential point of failure, especially when high pressure, high flow oiling to the cylinder is for whatever reason compromised. Over the decades, as valve lift has steadily gone up in production models, cam holders have morphed from the individual holders of the 70s and 80s into the full size hold-down plates today's sport bikes have. This full width plate design resists the flex that would otherwise lead to failure as a result of the high valve lifts. It is largely the individual cam holders that limit the race development of 80s engines, even those that already have shim and bucket valve trains. The clearance in these cam holders is surprisingly large also, some 0.005" on average.
Cam holder bolt torque specs in many OEM manuals are incorrect, and using them results in over-torquing leading to bolt failure, and worse, distortion of the cam holder, which can result in cam problems. One such problem is mentioned below. The factories often specify 12 or more foot-pounds for the 6mm holder bolts. This error is of course repeated all over the Internet. Every mechanics school student knows very well that A) a foot-pound torque wrench is the incorrect tool for a 6mm bolt; that B) a 6mm bolt is never tightened over 100 inch-pounds (which is less than 8 1/2 foot pounds); and that C) a best practice spec in a 6mm bolt threaded into aluminum is 85-90 in-lbs, which is the spec I have used for decades. Kawasaki published a warning about this also, in reference to the KX250F.
Cam lobe anatomy
Cam lobes are utterly fascinating! A cam is not just an ellipse machined onto a shaft. Its lobe is actually a complex shape with several distinct radii, or zones, each with its own job to do. The base circle is the baseline, the reference point for all the other curves, the point of departure from which the are all measured, the valve fully closed point, and thus the area that provides the valve cooling time. The nose is obviously the point at which the valve is fully open. In some engines the valve's full open distance can be determined from the cam nose, in others it can't because rocker arms necessarily alter it. The sides of the lobe are called its flanks. The flanks accelerate and decelerate the valve. On some cams the acceleration and deceleration flanks are identically shaped. Once again, rocker arm engines are an exception. As the cam lobe rolls beneath the rocker arm's pad, the leverage ratio of the rocker arm changes slightly, resulting in a difference between the valve's opening and closing speeds. To compensate, cams for rocker arm engines have lobes whose opening and closing flanks are very differently shaped. Engines with roller rockers have uniquely shaped cam lobes also; much less pointy, more like a circle and very obvious in appearance. So, not all camshaft lobes have symmetrical flanks, and not all lobes are shaped like the the pointy end of an egg. There are ramps on the cam lobe too. Detectable only with a dial indicator, the ramps are very small transition zones between the base circle and each flank. A camshaft lobe's ramps are some of the most important parts of the lobe. They're there for the valve. For 15 or so crankshaft degrees, the ramp, or clearance ramp as it's also called, cushions the valve to prevent its opening and closing from being as harsh as it would otherwise be. Many are not aware that these ramps determine valve clearance. Yes, valve clearance is a product of a camshaft's clearance ramp. Or more accurately, an inverse product. That is, the more ramp a cam has, the less valve clearance is specified. And the less ramp, the more clearance.
A camshaft lobe looks shiny and very smooth. Cam makers will tell you that smoothness is is important. Even barely visible ripples or undulations in a cam lobe's surface are bad. At high engine speeds they produce oscillations in the valve train, that is, the rocker arm or other follower type, and these quickly result in wear and part failure. Imperfections in the cam lobe also accelerate rocker arm wear, potentially very dramatically.
Cams fail, of course. The most common failure is ground up lobes due to lubrication issues. A mechanic knows to look for attendant signs when finding this. Blue discoloaration, for example, tells the story of gradual lubrication loss, typical when the engine is allowed to get low on oil. The same bluing on the piston pin, and sometimes also darkening at the small end of the connecting rod, will accompany this evidence, confirming that there was reduced lubrication over a period of time. Sometimes the cam has a ground up appearance on its journal and not its lobe. The typical scenario is oil pressure loss at the journal due to a plugged oil passage. There will little or no bluing because the increased friction is between aluminum and steel instead if steel and steel. As mentioned above, in these cases the aluminum is not the first to tear but the steel cam journal is.
Some camshafts have their sprockets pressed on at the factory instead of bolted on. These sprockets can slip, and since they are not marked, that is, indexed, a slipped sprocket is easy to overlook, and for the average person impossible to correct. For some time, Yamaha and Kawasaki four-stroke motocrossers have unfortunately been rather famous for this. However, to give them their due, in many cases the cause has been found to be cam holder over-torquing, as mentioned above.
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