Basically, the cam is the central point in a four stroke internal combustion engine which determines an engine's characteristics. Which is why we all worship the one and only single cam :). The cam controls when, how fast and how far the valves are opened, and therefore how fresh air-fuel mixture enters the cylinder and how burnt gas leaves it.
There are three basic factors that determine the power output of a internal combustion engine which are 1) displacement 2) mean effective pressure (MEP) and 3) speed. Displacement and MEP translate directly to torque, and power (HP) is simply the product of displacement x MEP x RPM.
Displacement put aside, the way to more power is raising either MEP or RPM, or, usually, both. (Increasing displacement is described in the big bore section.) More MEP basically means getting more mixture in the cylinder, neglecting other ways to raising MEP as more efficient combustion (by higher compression ratio) or chemical ways (Nitrous oxide injection, nitrous methane etc.) to get more combustible gas into the combustion chamber)
Mixture enters the cylinder through the intake channel. The amount of time and the effective intake cross section (two important factors in determining the amount of mixture that will flow into the cylinder) are controlled by the intake valve and the intake cam which opens and closes the valve. The valve opens a circular gap through which the mixture flows. The cross section of the gap is a function of valve diameter and valve lift. Valve diameter is specified in the physical dimensions of the valve, while the valve lift is laid down in the design of the cam. The area below the intake cam lift curve determines the effective intake section a cam opens over time. The bigger this area is, the more mixture can fill the cylinder. Bigger valves with more lift mean more mixture in the cylinder, more MEP, more power. That easy? Well, before looking deeper into valve dimensions and valve lifts, lets consider another important factor that determines the breathing capacity of an engine.
It was only in the very early days of the development of the internal combustion engine, when 50 RPM were still high speed and earth was still flat that the intake (and exhaust) cycle was perceived just as a static process. The valve opens, the piston goes down, sucking mixture into the cylinder, the valve closes, the piston goes up to compress the mixture etc. Soon it was discovered that such a perception neglects the dynamic aspects of the process. Nowadays the gas flow in an engine is seen as a process of flow dynamics that also accounts for the dynamic properties of the mixture and gas involved.
Soon in the development of engine mechanics it was discovered that the intake valve should open even before the piston reaches top dead center, and that the valve should close later than bottom dead center of the intake cycle.
What happens is this: When the intake valve opens, the outlet valve is still open. The piston has almost finished it's exhaust cycle. There probably is already some vacuum in the cylinder because burnt gas has already left the cylinder in the exhaust cycle, leaving a vacuum which can be used to aspirate new mixture. During the intake cycle, new gas rushes in and develops momentum. Piston reaches bottom dead center, while gas still draws in, driven by the momentum. Gas flow stops when its momentum balances the backpressure building up from the mixture which is compressed in the cylinder. Ideally, the intake valve closes at that precise moment. This point (intake close) is the most important factor for performance issues! (Next important is the opening point of the outlet valve.) If the valve closes too early, the gas flow is interrupted before the cylinder reaches its optimum filling. If the valve is closed too late, the gas flow has reversed already and mixture flows out of the cylinder again.
Unfortunately, there is no singular "optimum closing point", not even for a given engine with given breathing characteristics. Instead, the whole combination of cam and valve timing, gas inertia and momentum that determines the optimum intake closing point depends on the timing not in terms of degrees of camshaft rotation but on timing in terms of absolute time, in other words, engine speed. So when setting the optimum closing point, we can only determine the optimum closing point for a certain engine speed. Above that speed, the valve will close too early, at lower speeds, the closing point will be too late.
Valve Train Dynamics
So why not set the optimum intake close very late, so that best filling and MEP occurs at a very high speed, and have unending power since power follows MEP x RPM? There are other factors which are also influenced by cam design that limit the speed limits of the engine. Under normal operation, the valve train and the valve follows the cam lobe profile when opening and closing. But as it is with the air-fuel mixture that enters the cylinder, the whole valve train also bears mass and hence inertia, factors limiting the amount of acceleration it may experience. The opening of the valve follows the cam profile, and more RPM just mean more stress on the valve train (rocker arms in our examples of Honda SOHC/4s), stress which generates wear. More crucial is the closing cycle, where it is up to the strength of the valve spring to have the valve and valve train follow the cam profile. When an engine is overreved, the closing valve no longer follows the cam profile, a process known as valve floating or valve bouncing. The valve closes too late, which may cause it to collide with either the piston or the other valve, which usually wrecks the engine. Worst case the valve may break, fall down and raise even more havoc on piston, cylinder wall, con rods, crank and crank case.
The factors setting the critical speed up to which the valve train operates normally are:
- mass and hence inertia of the valve and valve train
- spring rate and preload of the valve spring
- cam profile
It is because of valve and valve train inertia that SOHCs can rev higher than OHV engines, and, I hate to admit it here, DOHCs are better performers than SOHCs. They simply have less moving parts that add mass and inertia.
Optimizing The Valve Train
In a given engine there are still enough ways to make the engine cope with higher speeds: Reduce the moving masses of valve and valve train, use valve springs with a higher spring rate or use a cam profile which is 'milder' i.e. puts less acceleration and deceleration on the valve train.
Lets have a closer look at those three:
Reducing the mass: Good, but has limited capabilities. Besides the valves, whose mass cannot be reduced much, it's the rocker arms and the spring retainers which offer room for improvement. As for the rocker arms, bear in mind that the inertia grows with distance to the rotational center, in other words, material removed from the ends of the rocker arms have more effect than at the pivot. Be careful where to grind the rockers and how much - reducing mass should, if possible, not go at the expense of stability.
An easy and proven way to reduce masses at the rocker arm is lightening the valve adjustment mechanism by turning down the adjustment nuts to half height and cutting down the adjuster screws accordingly.
Another good way is using special spring retainers, like the titanium ones available from Megacycle. I would personally stay away from aluminum retainers - the stress resistance of aluminum is lower that steel or titanium so the valve cotters may sink in over time.
Use springs with a higher spring rate or preload existing springs: The easiest, yet most effective way to higher engine speeds. But, as every coin has two sides, harder springs mean more stress on the valve train, hence more wear. Harder springs also mean more power loss in the valve train, power which the engine generates but which will never make it to the rear wheel.
So my advice here is: Don't overdo. There were valve springs available from Yoshimura which, together with other modifications helped push redline to like 11,000 RPM, but unless you have an engine that actually reaches that speed under normal operation, these springs are simply an overkill. I've seen some cams and rockers ruined by overly hard springs.
Preloading the existing springs was pretty common in earlier days, but I wouldn't recommend doing so in a SOHC/4. Although it's fairly simple - just put a plate or big washer under the spring base, it reduces the free spring travel and may cause problems with race cams which often have higher valve lift.
Cam profile: Here we're pretty much left to the wisdom and experience of the cam manufacturer. Softer slopes mean less acceleration but also less effective open area below the cam lobe, therefore less mixture in the cylinder, less MEP and HP. It is an offset that must be carefully balanced.
Important for practical engineering is that a given engine where only the cam is changed will have a different redline because of different cam profiles.
Now, since there are so many different parameters laid down in a cam, it would be hard to compare different cams. Several values have been developed over time to determine a cam as 'mild' to 'hot'. But let's have a look at what values we usually see in a some cam's specifications:
|Cam I||Cam II||Cam III|
|Intake opens||5||19||29||deg. before top dead center|
|Intake closes||30||47||59||deg. after bottom dead center|
|Exhaust opens||35||48||73||deg. before bottom dead center|
|Exhaust closes||5||19||30||deg. after top dead center|
Also note that these values are taken at valve clearances much wider than usual running clearances, usually .04" (1 mm). Each cam profile has a start and end "ramp" whose sole purpose is to slowly eliminate the necessary clearance and to have all parts of the valve train connect to each other before the actual opening of the valve begins. Small variations in valve clearance will change the opening point significantly. If you compare two cams, make sure the measures are taken at the same clearance!
Therefore for determining the timing of a cam, a wider checking clearance is used to make sure the opening/closing point is measured at the beginning/end of the cam lobe, not at the ramp. It is important that the timing marks of each cam are taken at the same checking clearance to make them comparable! The problem of different checking clearances has been overcome by the lobe center method, which is explained below.
Now, the total duration the intake or outlet valve is held open is called, you guessed it, duration. Since intake opens before TDC and closes after BTC, intake duration equals intake opens + 180 + intake closes. Likewise is exhaust duration equal exhaust opens + 180 + exhaust closes:
|Cam I||Cam II||Cam III|
Cam I has the shortest duration, while Cam II has a higher duration and Cam III is even higher. Also the valve lift Cam II and III provide are higher that Cam I. Cam I appears to be a mild cam, while II and III are hotter cams.
At the end of the exhaust / beginning of the intake stroke there is a time at which the exhaust valve is still open while the intake valve is already opening. This is called overlap, and determined as intake opens + exhaust closes:
|Cam I||Cam II||Cam III|
Remember that intake closes and exhaust opens are the two most important timings to determine an engine's characteristics.
Generally speaking, the longer the overlap, the hotter the cam. Since both valves are open during the overlap, a certain amount of fresh, unburnt gas may directly pass through into the exhaust. Which worsens mileage, and emissions. Also, since both valves are open, the practical compression ratio with a cam with long overlap is lower, and often remarkably lower than the theoretical value that is stated with a piston kit. With a long overlap, you may use high c/r pistons without detonation even with standard pump fuel since the practical c/r is lowered by the long overlap. On the other hand, in order to maintain a decent practical c/r it is often advisable to use high c/r pistons with hot cams not to loose power.
Lobe center gives the position of the very middle of the cam lobe in degrees. It is simply determined by (duration / 2) minus overlap:
|Cam I||Cam II||Cam III|
|In lobe center||97.5||85||75|
|Ex lobe center||100||85.5||82.5|
N.B. The lobe center is just a mathematical value, and does not necessarily coincide with the highest point of the cam lobe.
The side effect of unburnt gas flowing through into the exhaust minimizes with rising engine speed, because since the absolute time both valves stay open drops with rising speed. With a hot cam, the engine is capable of reaching such higher speeds, because the cam with the longer duration enhances the engine's breathing capabilities so that it can still maintain a good MEP at higher speeds. At the expense, though, of bottom end torque, where the side effects of long overlap take their toll. So each cam is a balance between top, mid-range and bottom end power, and the right choice of a cam depends of the application and desired characteristics of the engine.
That in mind, let's look at what the manufacturer has to say to the three cams in the above examples.
Cam I is the K0-6 stock cam. (Honda doesn't explicitly comment on it)
Cam II is Megacycle's 125-05: "OK with stock pistons and springs. Best with heavy duty springs. All around power increase with smooth power band. This was originally a production road race profile.
Cam III is Megacycles 125-25: "Use high compression pistons and P.M. or R/D springs [two valve spring sets sold by Megacycle]. Mid-range and top-end power, yet retains good low-end.
Adjustable cam sprockets for the 750 are available from APE, and probably other sources as well. And if you don't want to spend your hard earned money on an adjustable cam sprocket, a round file and some elbow grease will get you there as well.
By adjusting (degreeing) the cam you make sure the cam timing is actually at the marks the cam maker intended it to be. Or, you can change the engine characteristics slightly by changing the degreeing from the stock values.
To degree the cam, you do the following:
Install a degree wheel on the crank. Find TDC, adjust the degree wheel to zero. If you really take this serious, and you do if you want to degree your cam, finding TDC means setting up a dial indicator on the cylinder block with the head removed, and rotate the crank until the piston is 1" (25 mm) from top on it's way up. Note the reading on the degree wheel. Rotate further until the piston is in the same position on the way down. Note the reading as well. TDC - the REAL TDC is just in the middle between these two readings. Now compare that to the "T" marks on the ignition advancer. If its correct, you're lucky, if not, correct the mark.
Assemble the top end but leave off the valve cover. Set up a dial indicator on a intake valve, set it to zero with the valve closed. Rotate the crank until the valve is .040" (1 mm) open. Note the reading from the degree wheel (OPEN). Further rotate the crank until it is 1mm from closing. Note the reading at the degree wheel as well (CLOSE). Lobe center for the intake is:
|OPEN - CLOSE||+90|
Do the same for the exhaust valve. Lobe center for the exhaust is
|CLOSE - OPEN||+90|
If these values differ from what the cam manufacturer says, adjust the cam sprocket until the LCs match the specs. If you do not have specs from the manufacturer, adjust the cam to be symmetric, i.e. identical lobe centers for intake and exhaust.
Checking valve clearance
When fitting a new cam into an engine, it is important to check the clearance between valves and pistons. An easy way to do is to put a small amount of plastilene into the valve pockets of one cylinder. Then assemble the top end (I leave the head gasket out and only lightly tighten the head nuts) including cam and rocker arms and adjust the valve play to almost zero. Then slowly turn the engine over. If it could be turned over a few crank rotations, take off the top end again, and check to how the plastilene in the valve pockets has been squeezed down. There should be at least 1mm (0.025") left - having left out the head gasket during the check adds some more clearance, just to be on the safe side.