 | Three Secrets of High Performance Engine Building
by Mike Nixon |
Here are three significant concepts that can add to our understanding of the high performance engine. Each of these three topics is broad enough to involve a number of related issues, as we'll see. And as topics, together they represent some of the most searching analysis of what constitutes engine performance as you are likely to have recently considered. I hope you find them interesting. Enjoy.
Secret #1: Engine Efficiency
We begin our study with efficiency. The word means simply "what you get compared with what you wanted." Mathematically, efficiency is expressed as output divided by input. For example, 1000 BTU of energy in the form of fuel might be fed into an engine, and the result might well be 200 BTU of power output. That's 1000 in and 200 out, or 200 divided by 1000. This would indicate an efficiency of 20 percent. Let's look at engine efficiency in three steps, in the same order in which the engine deals with them.
Volumetric Efficiency
Efficiency begins in an engine with Volumetric Efficiency. Volumetric efficiency refers to how well the cylinder is filled. Most production powersports engines fill their cylinders less than 100 percent full. After air enters the cylinder and the intake valve closes, the cylinder typically contains less air volume than if you took the cylinder off the engine and set it out on your driveway! In other words, instead of having 14.7 psi of sea level air pressure in it, the cylinder has 13 psi or so. That is, a slight vacuum. The cylinder has been "charged" with less than its full capacity. And the engine must work with this amount, never getting any more than that due to design limitations. However, the engine can be altered to fill its cylinder more, and even over-fill it. If through one of several means we manage to get slightly more than normal capacity into the cylinder, ending up with say 16 psi instead of 14.7, we call the cylinder "supercharged." That's where the legendary supercharger gets its name -- it's simply a device that overfills the cylinder. The supercharger is mechanically driven from the engine and results in positive displacement of air -- the faster the engine turns, the more extra air that is pumped in. Another mechanical supercharging method is the use of the turbocharger, a supercharger that is driven by exhaust gases and therefore does not result in positive displacement. That is, the turbocharger's movement of air is not linearly proportional to engine revolution. However, supercharging does not have to be done mechanically. There are also non-mechanical means of supercharging the cylinder. A common one is through chemical supercharging. The use of alcohol, nitrous oxide and other oxygen bearing substances force added oxygen into the cylinder, with the result cylinder over-filling and over-pressurization. Finally, methods that are neither mechanical nor chemical are also used to coax more air into the cylinder. Pulse and inertia tuning are good examples of this kind of supercharging. Pulse tuning, for its part, capitalizes on the lengths and angles of the intake and exhaust tracts to harness the inevitable resonances within these tracts to performance advantage. Carefully calculated lengths and angles control pulses in the intake and exhaust tracts to persuade the charge to move faster or slower as needed to increase cylinder filling. Inertia tuning is very similar, but a little different. Inertia tuning juggles tract gas speed and density with valve timing for increased cylinder filling. Both pulse and inertia tuning are used to significant effect in today's high performance engines. Without them, the engine's cylinder would not be as completely filled, and power would be wasted. Combustion Efficiency
But cylinder filling is just the first area of potential waste in the engine. Whether normally charged ("aspirated") or supercharged, once the charge is in the cylinder, it must be burned to unlock the fuel's BTU energy. And of course there is loss here too. Combustion Efficiency simply considers how much of this charge gets completely burned. The piston is moving all the time combustion takes place. It is still coming up as combustion starts, which actually promotes combustion but may lead to detonation, the instantaneous and massive explosion of the whole charge due to excessive heat and pressure, with the result a waste of BTU energy as the piston is hammered instead of pushed. Combustion is also ongoing as the piston strokes downward again. This downward movement rapidly changes the cylinder's pressure, discouraging steady flame movement across the cylinder. Other things that affect combustion efficiency are combustion chamber shape, intake tract air speed, and spark plug location. Flatter chambers burn more of their contents than do arched ones, which is why modern engines have such chambers. High intake charge speed promotes good combustion because the air and fuel stay more thoroughly mixed as they enter the cylinder. A centrally-located spark plug causes combustion's flame to reach deeper into the remote edges of the cylinder, turning more BTU into useful work, and eliminating the end-gases that often initiate detonation. During combustion then, some of the power we're trying to produce is lost, and we haven't even got to the crankshaft yet!
Thermal Efficiency
And the game still isn't over. The charge has entered the cylinder, and the combustion chamber has burned it, and each of these two has resulted in some inevitable waste in the power-producing process. However, there is more waste yet. There is still the conversion of that thermal energy into pressure, that is, "piston push". And there is loss here also. Thermal Efficiency therefore concerns itself with how much of the heat of combustion ends up actually pushing the piston downward. Usually, as much as 60 percent of combustion's heat is wasted heating up the engine or going out the exhaust pipe, leaving very little to drive the piston. Maximizing thermal efficiency is difficult, but not impossible. Well-shaped combustion chambers that finish their job quickly waste less of combustion's heat because there is less time for that heat to radiate away from the combustion process into the engine castings. Smaller chambers offer less surface area that will soak up combustion's heat, improving thermal efficiency even more. Coating the piston top, which is really the combustion chamber's floor, with a ceramic layer, also prevents heat absorption, this time into the piston, with the result greater piston pressure. Even higher cylinder compression can help, because it makes the charge more ready to burn, speeding up flame travel and resulting in more charge consumption. Combustion chamber design is so critical that engineers often use the engine's torque per cylinder to rate a combustion chamber's effectiveness. How much pressure the engine produces in a given size cylinder directly reflects the combustion chamber's efficiency. Even before the crankshaft turns, the first three steps in the power-producing transaction each waste some of the precious power that will ultimately be outputted.
Secret #2: The Role of Rpm
There are other engine efficiencies as well, such as Mechanical Efficiency and Power Stroke Efficiency, and these are very important also. But you get the idea. There are a lot of leaks in the engine's processes, and each one affects the one following it because the inefficient outcome of each step ends up being the next step's input, and already there is a loss. Plugging all these leaks (as well as possible) is the art of the high performance engine builder, and helps explain why performance engine building is so much work. The next concept to be considered, and the second "secret" of the high performance engine, is the interesting role and effects of rpm.
MEP vs. Rpm
There are just two major ingredients in horsepower. These are combustion's push (we have just looked at this in our consideration of thermal efficiency), and revolutions per minute (rpm). Combustion's push has a fancy name -- Mean Effective Pressure (MEP). MEP is simply the single pressure which, acting on the piston, could theoretically do the same work as all the sucking and blowing that goes on inside the cylinder. Engineers use MEP as an indicator of the stress on the engine from the inside-out -- i.e. the constant strain inside the pressurized container. The beauty of MEP is that it is platform-independent. It doesn't matter whether the engine is a four-stroke or a two-stroke, single cylinder or a V-12. It is concerned only with cylinder stress. However, cylinder pressure isn't alone. Before you can have power, you must also have time, and in an engine, time is measured in rpm. MEP and time go together. They are the twin giants holding up the horsepower universe -- the bookends bracketing all that is important about how an engine makes power. They are that significant. And, there is an interesting relationship between them -- they are essentially opposites. Find for example a high performance engine that makes its power through a lot of rpm, and you will discover (through calculation) that it produces a relatively low MEP. Conversely, the engine that produces power mostly through MEP will be one that doesn't need much rpm. Of course, many engines benefit from a careful combination of both, but even then, every engine works on an emphasis on one or the other, due to its internal design. This fact, the relationship between MEP and rpm, quickly evaporates all the barstool arguments. It also produces some fascinating conclusions. For example, to make a high-rpm engine make more power, do you perform modifications that make even more use of rpm, or do you go after MEP? Oddly, such an engine will usually respond best to modifications that increase MEP, not those which make use of rpm. Clearly then, contrary to popular wisdom, rpm is not always the major player in the game of increasing engine performance, even in high rpm engines. Piston Speed
Some engines of course will need to be modified for higher rpm to get more power out of them. There are a number of problems that arise with high rpm however, and the first to be considered is the valve train. The four-stroke engine's valve train must be rigid enough and accurate enough to avoid "floating" its parts. Valve float is when the valve (and/or its operating parts) has accumulated so much inertia (through high speed) that the valve spring doesn't immediately return the valve after the cam lobe has passed. It's a dangerous thing because other parts intermittently share the same space, and interference is therefore inevitable. Along with detonation, valve float is every four-stroke engine builder's nightmare. But the valve train can be made to cope. Stronger springs, lighter valves, and less flexible (and even fewer) valve train components all result in more faithful adherence to the dictates of the camshaft and are therefore more high rpm compatible. The minimalist shim and bucket valve system for example evolved as a result of this quest for the ultimate valve train integrity. However, even with the valve train taken care of, rpm can still be a problem. The other reciprocating parts in the engine are also susceptible to being moved too quickly for their good. These are of course the piston and connecting rod. There is a limit to how fast these two relatively heavy parts can be yanked around. At some point, their inertia will just be too great, and the failure of the rod and even the crankshaft bearings (which absorb much of the rod's loading) will result. For this reason, an engine's Piston Speed has traditionally been a measure of its maximum safe rpm. Piston speed as a measurement was conceived back in the days of steel pistons that couldn't be thrown around too quickly without "fragging". Today however, though still referring to the piston by name, piston speed more than anything indicates the reciprocal loading on the connecting rod, not the piston. The piston's (and rod's) speed increases with rpm because the distance of its stroke never changes, though the crankshaft turns faster and faster and there is less time for the piston to traverse its stroke. The piston's speed is also anything but constant, since it stops at the top and bottom of its stroke. The piston's maximum speed then is actually near the middle of its stroke, and so calculated piston speed is really an average. The piston's speed also increases with stroke. That is, for a given rpm, an engine having a longer stroke pushes and pulls its piston faster. This is one big reason modern high rpm engines have relatively short strokes. What all this means is that when modifying an engine, piston speed is often the first consideration, even if it has little to do with the durability of the piston itself. It is rather a general rule of thumb for rpm.
Ring Flutter
Piston speed is an archaic design limit that has been violently stretched in recent years thanks mostly to advances in metallurgy. A modern sportbike has a piston speed in the 4,000 feet-per-minute range. Twenty years ago this was a racing-only level, yet now it's covered by a three-year factory warranty! Therefore, piston speed is no longer the rpm gauge that it once was, but more of a general limit depending on crankshaft and connecting rod technology. Everything else being equal (we have already dealt with the valve train), today's performance engine's rpm limit is indicated by something else entirely, and that is Ring Flutter. Ring flutter is a piston ring problem. The piston ring of course seals compression and combustion pressure inside the cylinder. But it doesn't do this job by itself, at least not during combustion. During combustion, high pressure gases get behind the ring and push it downward in its groove and outward against the cylinder wall. This is how a piston ring with only a few pounds of radial tension can resist over 1000 psi of peak combustion pressure. It borrows from that pressure. But this works only if the ring stays on the bottom of its groove. As rpm increase, the tendancy is for the ring to float upward, especially on the piston's downstroke. That's when it reaches maximum acceleration (near the middle of the piston's stroke) and at the same time almost completely loses effective combustion pressure. If the engine rpm is also very high, these three occurances (maximum acceleration point, loss of cylinder pressure, and very high rpm) combine to cause the piston to literally out-accelerate its own piston ring, leaving the ring momentarily "floating" in its groove. The ring drifts upward, off the bottom of the groove, and touches the top of the groove. This cuts off the path of whatever gases are present, and the ring then collapses inward on itself against combustion's forces, failing to seal the cylinder. The result is blowby of the residual gases past the piston, which reduces lubrication and overheats the piston and cylinder. Also, since the piston gets up to 80 percent of its cooling through its rings, and the ring isn't against the cylinder wall any more, the piston heats up even further, often to the point of seizure. There are a number of ways to prevent ring flutter. Obviously, keeping rpm down is one way. However, for a high rpm high performance engine, this is not an option. Another is for the engine to be built with a very short stroke. Even at high rpm, the short stroke moves its piston more slowly. This has commonly been the tactic used by modern engine designers. However, along with this is simply the use of thinner piston rings. Thinner rings are less massive. With their reduced weight, they build inertia more slowly and can withstand high acceleration without floating. All modern high performance engines use very thin piston rings. One of the advantages of using today's forged high performance pistons is that most of them are designed for very thin, high performance rings. Rolling Element Bearing Failure
Older engine designs that still use rolling element bearings at their crankshafts and connecting rods can also develop rpm-related problems. These bearings aren't as high rpm compatible as their plain shell counterparts. Especially when they are undersized, as they are on Harley Evolution engines, a design rooted in the 1930s, yet one that favors high-rpm tuning. This engine's rolling element main and rod bearings can be spun so fast that their rollers skip across their races, ultimately damaging them. Such is the problem the many 150 hp big inch Harleys and clones are experiencing. These are not long-distance machines, with their connecting rod bearings becoming trash in just 5,000 miles.
Secret #3: Cylinder Head Basics
So we see that high rpm not only isn't always the route to added performance, but when it is the indicated path, it has its hurdles that must be dealt with. Being familiar with these issues is important. Finally, we come to high performance engine "secret" number 3 -- the four-stroke cylinder head. There is a lot of mystique surrounding the high performance four-stroke head. At the races, when a competitor is tearing down his engine, he will often wrap his cylinder head in a towel, away from prying eyes. Many have said that the four-stroke engine's camshaft is the heart of the engine. However that really isn't so. The camshaft is merely the "equal sign," the part that brings all the rest of the engine together. The real heart of the engine is the combustion chamber. Engine performance truly begins there. Combustion Chamber Shape
The shape of the combustion chamber is more important than many realize. It is far more important than what is done to the ports, or how big your carburetors are. Current combustion chamber technology has resulted in very flat combustion chambers. The reduced volume of this shape results in a shorter flame period, which means ignition timing doesn't have to be advanced as far. (The ignition advance required to get maximum power is in fact an indicator of how well the combustion chamber does its job.) In fact, modern engines run much less ignition timing than older ones do. The result is free power because for whatever number of crankshaft degrees that are saved, the engine is no longer fighting itself, with the piston compressing an already-burning mixture. Remember also that the piston is part of the combustion chamber. With their flat-topped pistons, high compression is possible in modern combustion chambers without detonation. There are no nooks and crannies in which harmful, detonation-producing end-gases can hide, enabling sportbike manufacturers to give these 86-octane-fed engines amazing 12:1 compression ratios. This new age engine's flatter pistons also present to its chambers less surface area and therefore they absorb less of combustion's heat. One of the advantages of the many multi-valve chambers we have today is that with so many valves in the chamber, these valves are more vertical, further allowing very efficient, shallow combustion chamber shapes. Older engine designs with more horizontal valves and necessarily deeper chambers are at a serious handicap, comparitively, and typically cannot run as high a compression and need considerably more ignition advance. One of the best aftermarket cylinder heads for the Harley Evo engine has the valve stems pulled toward each other, relative to stock, for more vertical valves and a shallower, better burning chamber. (Actually, the valve heads are moved apart, not the stems together. Consequently, this head works only on a very large big-bore cylinder.) Aftermarket heads like this one are the most effective items in the Harley performance market. Naturally, stock cylinder heads can't be improved by moving their valves around, but they can be reshaped to give at least some of the same benefit. Then there's squish. Squish, that area of the combustion chamber that serves to push end-gases toward the spark plug, to both eliminate end-gases and generate healthy charge agitation just before the spark, is a very important part of the high performance combustion chamber. So effective is every aspect of squish -- from its total surface area to its angle to its relation to the piston crown -- that racing teams usually guard each season's combustion chamber specs very carefully. Though the combustion chamber is often ignored by many engine builders, the professional has long known its the secret of its power potential. Camshaft Technology
No part of the four-stroke high performance engine embodies more mystery to the average enthusiast than the camshaft. There is a lot that is taken for granted about this usually very hyped-up component, but it is an interesting and quite simple fact that, just as the engine wants either MEP or rpm to make more power, likewise its valves want to be opened either farther or for a longer period to increase power. However, unlike MEP vs. rpm, both of which make more power, the power resulting from holding the valve open longer or opening it farther is very different in each case. Let's examine this. The idea of fooling with valve timing remember is increased volumetric efficiency. One of the ways the camshaft can contribute to this is by holding the engine's valve open longer. That is, by increasing the valve's open duration. This will potentially give more opportunity to the engine for cylinder filling. But there is a drawback. Exposing the cylinder to the outside for longer periods affects the intake and exhaust tracts by slowing the movement of their contents. The result is less optimum mixture distribution at lower engine speeds, because the fuel separates from the airstream when it is moving too slowly. 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 usually offers a high rpm power increase, a side-effect is a loss of low rpm power. Another potential benefit of extended valve duration is that it usually increases valve overlap, that period during which both valves are open. As the valve overlap increases, the aforementioned pulse and inertia tuning of the intake and exhaust systems become dramatically more effective. However, again there is a drawback, and that is that the extended valve duration engine depends more heavily on pulse and inertia tuning to run well at all. Though powerful, pulse and inertia tuning is effective for only a very narrow rpm range. An engine cammed this way therefore runs better than stock, but only above s certain rpm, below which it runs much worse. The other way the camshaft can be used to increase cylinder filling, by opening the valve farther instead of holding it open longer, brings a very different effect. Like the valve that is held open longer, the valve that is opened farther allows more air into the cylinder, increasing power. However, since the time that the cylinder is exposed to the outside is not increased, the gases moving in the tract are not slowed, and the charge enters the cylinder still well mixed and combusts nicely. The result is no loss of low rpm power. Furthermore, since valve timing isn't changed, there is no loss of cylinder compression, and the engine isn't made more dependent on pulse and inertia tuning. Why then aren't all camshafts made this way, with an emphasis on lift instead of on duration, especially performance cams? There are three reasons. First, there is valve acceleration. The valve that is opened farther yet taking no more time to do so is one that is opened more quickly, i.e. accelerated harder. Remember that as the valve is accelerated, it tends to "float," and that float is the greatest danger in engines having old-fashioned spindly and flexible valve trains, which until a few years ago meant most of them. Second, because of the engineering world's historic fixation on valve duration, a certain politic of camshaft design has prevailed. It is still a cherished belief in many engineering circles that a valve never needs to be opened farther than 25 percent of its diameter. (Until the late 1980s, few if any production motorcycles deviated from this principle.) This 25 percent figure comes from an interesting geometrical fact. It happens that when the valve is open to a distance equal to 0.25 its diameter, the flow curtain around the valve's periphery is maximized. That is, it's a big as it is ever going to get. This is true as far as it goes, but like the old saying, it doesn't go far enough. There is more to this issue than flow curtain. The interesting thing is, opening a valve to 0.30d and more (racing engines open them to almost 0.50d) results in an interesting bit of trickery as far as time is concerned. For, larger openings actually extend the amount of time the valve is opened, without actually increasing valve open duration and thus adversly affecting tract speed and ultimately combustion. No, that is not a typo. More open time without more duration. Here's how it works. A valve that is open to 0.25d is at that magical full flow point only an infinitessimal period at operating speed. Almost negligibly, really. So the 0.25d flow curtain benefit is largely theoretical. However, when the valve is opened to say 0.35d (as most production sportbikes' valves are today), the valve then, by virtue of being opened past the 0.25d point, has some time to dwell at 0.25d, even though the total open duration is unchanged. Neat, huh? Finally, the third reason not many aftermarket camshafts are designed with lift in mind instead of duration is that such cams are very difficult to market. Its numbers aren't as impressive, for one thing. Plus, this kind of cam takes a lot more care to install properly, care that most engine builders unfortunately just aren't going to take. In fact, this is our next topic of discussion.
Cylinder Head Set-Up
One of the most "secret" of all engine building techniques is proper cylinder head set-up. Many people assume that an aftermarket cam can be purchased from a catalog and put in the engine and away you go. Not so. But this is the perception, and this is why camshaft manufacturers can't sell really well-designed camshafts, but instead offer ones heavy in duration that any fool can install without trouble and get at least dubious results from. Most serious engine builders design their own cams. Check around and you will find that this is so. However, whether the camshaft you use is a custom or one off the shelf, it absolutely must be installed correctly. At the very least, that camshaft must be degreed, but that's another discussion altogether. There are an even dozen valve-related checks that must be made on the engine when installing the aftermarket cam. Following is an outline of those checks. Let's begin by defining some terms that may not be familiar to everyone, but all of which refer to the valve spring. - Spring Free-Length:
Spring free-length means the length of the valve spring uninstalled. This is usually the only specification given by the service manual. Its purpose in the stock engine is merely to identify the correct spring, and more importantly, to enable you to easily spot fatigued springs. However, high performance engine builders use spring free-length to calculate other important spring specifications. So we must start here.
- Spring Installed Height:
Installed height means the height of the valve spring once it is installed. In other words, the spring is already partly compressed even before the valve has moved. The purpose of installed height is simply to gauge valve seat pressure. Valve seat pressure is important in any engine, stock or modified.
- Spring Full-Open Length:
A spring's full open length is its length when the valve is fully open. This is the spring's shortest working length, and therefore it reflects the hardest that the spring will work. The primary purpose of spring full open length is to help the engine builder determine if there is sufficient spring pressure to control valve float, about which more will be said later.
- Spring Coilbind:
When a valve spring is coilbound, it is completely compressed, its coils touching one another -- metal to metal. This should never occur in operation, but we need to know when it could theoretically occur, so as to know how much room we have to stay away from it.
Now let's look at setting up an engine's valves and valve springs for performance cams and pistons. The following twelve valve related checks are performed on most four-stroke engines, though as noted, a few are specific to rocker arm engines, and a few to shim and bucket engines.
- Installed Height:
As mentioned earlier, the spring's intalled height is the spring manufacturer's way of ensuring that you get the correct valve seat pressure. The modified engine's increased cylinder pressure requires better sealing. Also, at high rpm there is less time for valve cooling, making good seating even more crucial. Some spring manufacturers use installed height in place of full-open pressure (our next step) to identify their springs and match them to the camshaft, though that is not the correct way. Checking installed height is easy. When the manufacturer's spec is available, simply assemble the valve components without the spring and measure how much room there is for the spring. In most cases, you will have to adjust the components to get the recommended installed height. If there is not enough room for the spring, this may mean sinking the valve deeper into the combustion chamber (the least desirable solution, as it will affect combustion), or machining the cylinder head spring seat area. If there is more than enough room, the spring can simply be shimmed to the correct installed height. If the spring manufacturer's spec is not available, as happens often with high performance parts for Asian engines, the process is a little more complicated. You must add together the spring's coilbind length, the maximum valve lift, and a 0.060" safety margin. The result will be an installed height that assumes you want the maximum seat pressure possible given the cam and springs you have.
- Full-Open Pressure:
All performance camshafts, whether duration or lift emphasizing, increase how quickly the valve opens, adding to valve acceleration and making controlling valve float a real problem. Increases in engine rpm that often accompany performance modifications further add to the issue. The spring's full-open pressure is your first line of defense against valve float, and it will often need to be considerably more than stock. There are two ways to determine full-open pressure, and both require a spring tester. You don't really need one of those $1000 units. There are several makes of small hydraulicly-operated spring testers available that you can use in a bench vise. In method A, the length method, you must know the spring's installed height. Simply subtract from this the maximum valve lift, and compress the spring(s) in the spring tester to this length. If using method B, the pressure method, simply compress the spring(s) the amount of the maximum valve lift and note the pressure. Then mathematically add the known seat pressure.
- Valve Free Travel:
The valve's free travel, the amount that the valve can travel from closed to when there is guide interference, is very important. It determines retainer-to-guide clearance. Assemble the components without the springs, and measure from the bottom of the retainer to the valve guide seal. For safety, the amount must be 0.060" more than the maximum valve lift.
- Coilbind Clearance:
The purpose of checking for coilbind clearance, how close to coilbind the spring comes when operated, is to validate your spring choice. That is, it will reveal either over-shimmed or incorrect rate valve springs. Just repeat the full open test (length method) and then see if you can compress the spring 0.060" more without it coilbinding. If not, it's the wrong spring.
- Valve-to-Piston Clearance:
The relationship between the valve and the piston necessarily changes in an engine modified with a different piston or camshaft, or when the cylinder deck clearance or cylinder head surface are modified. In most cases, this relationship must change for maximum performance. That is, to make full use of an engine design, the valve must usually be brought much closer to the piston. This is by the way why so many Asian cars tangle their valves when their cam belt breaks, whereas American cars with similar designs do not. The Asian cars (and virtually all motorcycles) are built with much closer valve-to-piston tolerances, because maximum performance design usually makes better use of this clearance, resulting in less of it remaining. The two traditional methods for checking valve-to-piston clearance are the clay method and the indicator method. Method A (clay) -- If a hydraulic lifter engine, temporarily substitute solid lifters. Then lightly oil the combustion chamber and piston, cover the entire piston crown with a thick layer of clay, and reassemble the engine. Turn the crankshaft through at least two revolutions, and remove the cylinder head. After carefully removing the clay, bisect it and measure the valve-to-piston clearance with a caliper. Method B (indicator) -- Install soft (dirt bike carburetor) springs in place of the valve springs. If a hydraulic lifter engine, temporarily substitute solid lifters. Put a degree wheel on the crankshaft and find true TDC. After rotating the crankshaft to TDC overlap, place a dial indicator on the valve retainer and "zero" the dial. Then push down on the valve and note on the indicator how much the valve moves. Repeat with the crankshaft at 30o either side of TDC, in 10o steps. The valve needs to move at least 0.060".
- Valve-to-Valve Clearance:
Another potential problem area is valve-to-valve interference. This normally happens only when the camshaft is changed or larger valves are fitted. Two methods again. Method A is similar to Method B above for the valve-to-piston check, except insert a 0.060" piece of solder through the spark plug hole and determine whether the solder can pass between the valves. Method B is a Harley-only method, and requires that the manufacturer supply you with the "TDC lift spec," which is simply the amount the valves are open at TDC overlap. You simply put the valves in the head without springs or other parts, on the bench. Stop collar (using drill stops) the valves to the TDC lift spec, then see if you have at least 0.060" between the valves.
- Retainer-to-Rocker Cover:
These next three checks are rocker related checks that are quite common on Harley-Davidsons, though they may be appropriate for other rocker arm engines as well. On Harleys especially, the retainer-to-rocker-cover check is important. When the valve protrusion is increased on Harley Evos, as it often is, the valve spring retainer moves closer to the rocker cover, potentially interfering with it. Or, the cover may be aftermarket, or the heads, as is the case with most clone bikes, giving you the same potential problem. You want to avoid the valve spring retainer touching the rocker cover or any other casting. Assemble the head and rocker cover, and mark with a grease pencil where interference looks likely. Grind some clearance in there if possible. Sounds crude, but it's often necessary and you don't want to overlook it.
- Rocker Arm-to-Cover:
On Harleys again, changes to valve protrusion or any of the top end castings may cause the rocker arm to come closer to the rocker cover. Note the potential clearance issues on assembled parts and clearance as necessary. Clay can be used for this as in the valve-to-piston check. Put the clay on the inside of the rocker cover.
- Retainer-to-Rocker Arm:
This one is very common on modified Harleys and clones. The rocker arm contacts the valve spring retainer's edge due to larger than stock retainers or high valve protrusion. It can also result from incorrectly adjusted adjustable pushrods. The best Hot-Rod retainers are curved for this reason. Assemble and check.
- Rocker Arm Geometry:
Another frequent Harley issue, this is where the rocker arm's movement isn't evenly divided between opening and closing. The result will be excessive valve guide wear. Check to see that the imaginary center of the rocker arm shaft intersects the valve stem at a 90o angle when the valve is open to half its lift. This will divide the rocker's motion evenly, lengthening the life of the valve and valve guide. This problem is in fact why Harley-Davidson has a valve protrusion spec in the first place, and why shorter than normal replacement valves are widely available.
- Valve Follower Travel:
Leaving the Harley stuff now, and back to the import machine. Whenever camming a shim and bucket engine, you must check that the bucket (technically the follower) will in fact move the required distance. In some Asian cylinder heads, the bucket bottoms out in the casting a mere 0.100" or less after the stock valve lift. Failure to check this will result in a considerable amount of damage, including a broken camshaft. To check it, simply assemble the finished head with the bucket, and either dial-indicate or caliper-depth the fully closed bucket position. Then remove the valve and spring, put the bucket back in the head, and after letting it drop to the bottom of its bore, measure its position again. The difference between the two measurements should be 0.060" more than the maximum valve lift.
- Shim and Bucket Valve Protrusion:
Another common shim and bucket problem is that of assembling the cylinder head and then finding that even the smallest of the factory's shims won't get the valve clearances to spec. This can even happen on an un-modified head that has had a thorough valve job done on it. Usually, the problem is that the valve protrudes so much that the smallest shim will barely fit, if at all. Even if the smallest shim fits, you will then have no room left for future valve adjustments, which is not acceptable. On the old Z1, whose valves could be ground, the answer was simply to "tip" the valves 0.020~0.030", bringing the valve protrusion back down to factory spec. This enabled the middle size shims to be used for valve adjustment. However, on virtually every other engine, the valves are plated at their tips, so tipping is not possible. In this case, you must use aftermarket non-plated valves (such as stainless steel) and then tip them.
Summary
So now you know three of the best-kept secrets of high performance engine building. To recap, the first is that efficiency is important. Efficiency in at least three forms (volumetric, combustion, and thermal) is what the engine builder is really improving when he or she improves engine performance. The second "secret" says that rpm is half of horsepower. It is not all there is to power, as many assume. Rpm plays an interesting role, to be sure, but not the one most people assume. The third "secret" is that the cylinder head is the heart of the engine, where most of the real work of the engine takes place. It is also the hardest part of the engine to build properly, because it requires the most forethought, and nothing less than fanatical attention to detail. I trust you have found this article interesting and stimulating. If so, I would appreciate some feedback. Want more? Go here for a little more theoretical treatment of this subject.
Article taken from : www.motorcycleproject.com
