This is the guide I used when I wanted to learn more about camshaft design. Freezy74 has been very helpful in explaining how all of this relates to my forced induction engine and the changes we've been making to the setup.
Cam Truth by Buddy Rawls, 8/2000
More articles have probably been written about camshafts than any other engine component. To me, the camshaft, or more specifically the valve timing events are the most important aspect of an engine's ability to perform.
This write-up is not going to describe how an engine works, by using text book explanations for the cycles in the 4-stroke naturally aspirated engine. It is intended to present the relationship of the camshaft to the engine combination and apply that relationship to engine performance characteristics. I will incorporate various lines of reasoning used by the media and enthusiasts, and how they apply to engine modifications. The intent is not to prove right or wrong, but to help provide a clearer understanding. I present this write-up based on my experience of modeling engine parameters to calculate power capability and flow requirements. As always, there are assumptions to simplify the subject. None of the thermodynamic properties, pressure wave phenomena, or combustion aspects will be covered in this write-up. In order to address the various ideas, an engine model will need to be visualized. The engine models are simple, however, I consider them to be a realistic approximation of the engine as a system, focusing on flow and valve timing requirements.
The Understandable Engine
Engine Modifications and Cylinder Pressure
: First, let's look at what an engine does. The first inclination is to say that an engine produces power, typically noted as horsepower. This is not really the case. An engine produces torque (which is measured in terms of work on a dyno) and when applied over a unit of time (as in revs per minute), you get power. The formula that relates horsepower to torque is this: HP = Torque x RPM / 5252. The 5252 is not some sort of experimentally derived constant. It is simply a units conversion. Let me explain in very simple terms: an engine moves or exchanges air; it is basically a pump. It's output directly relates to how much pressure is produced from the trapped charge in the cylinder as the piston travels up and down its bore.
There are many things that go into this: crankshaft geometry, cylinder volume, RPM, inlet and exhaust port characteristics, and sealing capability just to name a few of the more important ones (to me). You can alter the breathing aspects and cylinder volumes and crankshaft geometry, but it all comes down to making cylinder pressure. The sole component of the engine that really controls cylinder pressure is the camshaft and its associated control of the valve events, regardless of inlet/exhaust capabilities and bottom end geometry. Valve timing is made up of timing events measured lift per degree of crank rotation. Most conversations and articles always seem to focus on the valve duration and maximum valve lift. The thing that must be remembered, is that those lifts, durations, and lobe separation angles come from the timing events, and not the other way around.
So when subjects tend to move toward specific durations and lobe separation angles, be aware, that unless you are talking about identical engines (geometry, cylinder volume, inlet and exhaust characteristics, etc.), the specific numbers and comparisons do not mean much. The subject needs to focus on cylinder pressure and not on whether a cam is classified as a racing or a street grind. Almost every performance enthusiast understands that increased flow and/or increased cylinder volume gives increased power. However, not all increases in flow capability or engine size yield positive results. Sometimes a performance decrease occurs. The statement really needs to be reworded to this: increased flow and/or cubic inches gives increased power capability. There is a big difference. The revised statement demonstrates that performance gains are dependent on the set-up in some way.
Simple Cylinder Pressure Model
: For the next portion, consider simple cylinder pressure models. Take a look at what happens as the crankshaft rotates and moves the piston up and down the cylinder bore. Imagine if you could cap off the top of the cylinder bore and install a pressure guage. What happens if you rotate the crank, moving the piston upward, decreasing the overall cylinder volume? Then what happens when the piston moves down, increasing the volume? As the volume changes, the pressure within the enclosed cylinder is increased or decreased. A decrease in volume will generate an increased pressure and a volume increase will generate a decrease in pressure (vacuum).
Now, incorporate a port on top of the cylinder bore. Inside the port, imagine a small piston with a spring that can extend or compress as it is pushed or pulled in the port. Now rotate the crankshaft. The piston moves up and down the cylinder bore, increasing and decreasing the cylinder volume. You can watch the smaller piston in the port move inward or outward, as cylinder pressure increases or decreases. The displacement of the small piston and spring is dictated by the spring constant, which is in units of force per displacement (lbs/in). Lets assume that the pressure variations, as the cylinder volume increases and decreases, stay the same as when the cylinder was simply capped off.
What happens when the spring constant is changed? As the cylinder volume decreases or increases, the small piston and spring move different lengths than before. However, with the assumption that the overall cylinder pressure increase or decrease never changed, only the spring travel changed. This is a simple model of an inlet or exhaust port.
Inlet And Exhaust Port Effectiveness
: Why the analogy of the spring and piston in the inlet and exhaust port? The spring constant can be thought of as the port's effectiveness or capability. It will be a function of the port's shape, as well as it's cross sectional area. An assumption was made that said the increase and decrease of pressure in the cylinder remained the same, regardless of how much the small piston and spring moved. In other words the cylinder could only exert so much pressure and that was a function only of the crankshaft geometry and of the cylinder bore area. Regardless of what the spring rate (or port effectiveness) was, the cylinder volume could only produce the pressures that were obtainable when it was capped.
The next statement should be viewed only in the context and assumptions of the previous paragraphs. The amount of volume displaced in the cylinder was the same regardless of the spring constant and the small piston's position in the port. So, extending the statement a little further for our purposes, the inlet and outlet port's effectiveness has nothing to do with the volume of charge displaced by the cylinder. For the next step, add a second port (with piston and spring) to the capped cylinder assembly. This means a cylinder with two ports, each with a small piston and spring. However, this time two modifications will be made. One port will be fixed to allow the piston to move in one direction and the other port will be fixed oppositely. This means that as the crank rotates and the cylinder volume increases, the one port will allow the piston and spring to extend.
As the cylinder volume decreases the other port will allow compression of the spring. As the crankshaft is rotated, each piston and spring will be extended or compressed. If the spring constants are changed in either of the ports, the extended or compressed dimension will be altered. This is a decent analogy of a cylinder with an inlet and exhaust port, one displaced by pressure and the other by vacuum. Again, the spring rate can be thought of as the port's effectiveness. The mechanical stops that prevent the piston and spring from moving are analogous to the valves closing off the port.
Cylinder Pressure and Torque
: At this point, move from the pistons and springs set-up to a more conventional picture. A cylinder volume with an inlet and exhaust port. Each with a conventional shaped valve to control flow into and out of the cylinder volume. Keep the same assumption about the cylinder pressure increases and decreases remaining the same as when the cylinder volume was capped. Early on, I explained about torque and horsepower. Lets put a torque wrench on the snout of this crankshaft. Now when the crankshaft is rotated, we have capability to see cylinder pressure and torque, working together. Try to follow some typical valve timing rationale.
As the cylinder volume increases, open one of the valves. When the piston starts moving back up the bore and decreasing the cylinder volume close that valve. Continue turning the crankshaft one full rotation until the volume once again starts decreasing. At that point, open the other valve. Continue turning until the volume is the smallest and close the valve. Go through that crankshaft rotation smoothly and watch the pressure gauge on the cylinder change. Note how the torque can change, depending on where in the rotation the valve is open. The goal is to open and close the valves throughout the crank rotations and keep the torque and cylinder pressure maximized. Watch what happens if the crankshaft is rotated slower or faster.
The valve events will need to change for each case of different rotational speed. If you could use some sort of iterative process, for a given rpm, inlet and exhaust capability (effectiveness), and cylinder geometry, it would be possible to arrive at the optimum valve timing setting. This is for illustrative purposes only, because in reality very slow speeds will end up with a very wide range of valve timing that would enable maximum cylinder pressure.
Cylinder Filling, RPM, and Lag
: The write-up has covered cylinder pressure, port effectiveness (using the spring constant idea), and torque (based on crank rotation and valve openings). The obvious next step is to gather these into one model. To do this, let's add realistic inlet and exhaust ports to the set-up, each with their own characteristics and cross-sections. Each port will now have it's own particular effectiveness. The goal is to rotate the crankshaft and optimize the valve events for maximum torque and cylinder pressure while a slow rotational speed is being utilized. If you change the inlet or exhaust port's effectiveness at this point, the torque and cylinder pressures do not deviate much. This is because the port is able to keep up with the displaced volume of the hand-rotated crankshaft. However, as the rpm is increased something different starts happening. At this point, it is important to remember how flow is typically measured.
It is in terms of flow rates. That is a unit of flow volume per a unit of time (cubic feet per minute, CFM). This means that for each volume of air displaced by piston movement for one crankshaft rotation, it becomes a flow rate when the rotations are measured with repect to time (revolutions per minute, rpm). As the rpm is increased, the draw on the port becomes greater. As the charge is drawn or accelerated through the port, the inertial effects create a lag between the movement of the incoming air and the displaced air in the cylinder. Go back to the piston and spring analogy. The piston and spring will be displaced a particular distance, as a function of the spring constant. What happens when a damping action acts in conjunction with the spring constant giving the piston and spring a slight lag as the cylinder volume begins increasing or decreasing with crankshaft rotation?
The piston and spring will not displace the same distance. This means that the lag is delaying the ports' response and will not respond to the complete displaced volume of the cylinder. In short, it moved less volume. Step back and take a look at this. The previous paragraphs describe how the intake valve closing controls the pressure that gets trapped in the cylinder. However, the total cylinder pressure capability, for a given rpm, is not dictated by the intake closing, but rather by the inlet port capability. If the closing of the intake valve is a given for a particular cylinder volume, port configuration, and RPM, how can the cylinder pressure be increased?
The amount of charge actually trapped has to be increased (or a better choice of words, the cylinder volume needs increased filling). For a given configuration, the only way to increase the cylinder filling is through the valve events. If the intake closing is fixed, the only alternative is to open the intake valve earlier. Combine the lag effects and the cylinder filling characteristics and try to optimize the cylinder while the turning at higher, but constant, RPMs. Opening the intake valve earlier will increase the cylinder filling and therefore increase cylinder pressure capability. But there is another player that will make the earlier intake opening cause problems.
Exhaust Outflow Interaction
: Up to this point no time has been devoted to the exhaust. We can work through the crankshaft rotations and see that for a given inlet port effectiveness and exhaust effectiveness, it is possible to arrive at seemingly correct values for the intake valve closing and exhaust opening. The values will control the cylinder pressure characteristics. We can reason that the earlier intake opening will increase cylinder filling and when paired up with good intake closing and exhaust openings will enable the cylinder pressure to increase.
This directly relates to the torque produced by the cylinder volume. I want to pose a question at this time. With the discussion so far, with spring constants and port effectiveness and displaced volumes, what happens to the exhaust outflow as the cylinder volume begins decreasing, due to the crankshaft forcing the piston upwards? Depending on the exhaust effectiveness (or spring constant) and the lagging phenomena, the cylinder volume may not be completely emptied. This means that there is resistance or pressure in the exhaust path. In order to rid the cylinder volume the exhaust has to close later. Remember the exhaust opening has already been optimized.
The time in the crank rotation that is being discussed is Top Dead Center at the intake opening and exhaust closing (this is referred to as overlap). By opening the intake earlier, we increased cylinder filling. Yet, by closing the exhaust later, the intake opening could be contaminated. Residual exhaust pressure could be present, which will lessen the total cylinder filling characteristics. However, we know that because of the lagging effect, the charge has inertia. If the charge has inertia, then it will have a lower pressure behind it (a vacuum). This will all be dependent on the spring constant and damping effects, of the piston and spring model. In the realistic port, it is controlled by the port's effectiveness and the exhaust gas characteristics.
This is a simplified approach made possible after removing the pressure wave effects from the model If the exhaust is effectively designed and timed correctly, it will not have residual pressure at the closing point. This is controlled by exhaust effectiveness, rpm, and cylinder volume. This is a key design point. This is where the argument of sizing the exhaust system to the engine's horsepower may find some valid opposition. Here is why. The inlet port effectiveness and valve timing events control cylinder filling capability. What controls exhaust effectiveness? Valve timing events and the emptying of the cylinder volume. In my line of thinking, exhaust sizing is directly reflective of the inlet characteristics and valve timing events, not the engine's horsepower output. Lets go back to TDC with the intake valve opening and the exhaust closing.
During overlap, there is charge exchange possible between the inlet and exhaust ports (by definition). If the intake and exhaust timing events have been optimized to produce maximum cylinder filling and pressure, that means that the TDC overlap activity has to be a balance of intake opening and exhaust outflow. To increase cylinder filling, the intake was opened earlier; only to the point at which the exhaust closing caused a degradation of this filling capability. For a given inlet and exhaust effectiveness, cylinder volume, and rpm, there is an optimum exhaust closing.
Consider the low-pressure region behind the exhaust outflow. For a low-pressure situation to exist, the exhaust closing has to be timed correctly to sufficiently rid the cylinder volume during the exhaust process. Any earlier and the inlet charge is compromised by exhaust pressure and spent gases still in the cylinder. Any later and the cylinder volume will be contaminated with residual exhaust gases coming back toward the cylinder. This is due in part to the exhaust system seeing atmoshperic pressure, as well as the increasing cylinder volume creating lower pressure situation (vacuum). However, at just the right instant, it is possible to help draw clean intake charge into the cylinder (this is referred to as scavenging).
With the analogy of the inlet and exhaust ports being instantly opened or closed, per the discussion so far, it is possible to reach maximum cylinder filling and emptying for the specific situation. However in the real-life engine, the valves are lifted with respect to crankshaft rotation, or worded differently, a ramp defined by valve lift versus crank angle.
Favorite: Cam Lobe Ramp Rate
: Since the valves cannot be instantly opened or closed (in most engines) it is not possible to completely reach maximum cylinder pressure capability in the previously described engine models. However, it can still be optimized for the specific set-up, a given cylinder geometry, RPM, and port characteristics. It is possible to tailor the intake filling capability with the exhaust outflow capability. Keep in mind, the optimum openings and closings have already been 'roughed in'. This is why the actual openings and closings are important, contrary to many enthusiasts' beliefs.
The media and enthusiasts have been conditioned to think the actual advertised or seat duration is not important. From the previous paragraphs, I hope their importance is realized. The opening and closings control the cylinder pressure capability. The ramp rate is the tuning tool to maintain the inlet and exhaust relationships and maximize the cylinder pressure. Engineering advancements can be seen in action by tracing the power capability of the lobe and follower designs over the past 40 years. For example, 20 years ago, no one would have believed the amount of engine air flow and power output obtainable with a flat tappet lobe design.
If the maximum obtainable cylinder pressure is assumed possible only with an instant opening or closing valve, then fastest lobe ramps are the closest approximation. However, depending on the flow capability of the port, with respect to different valve lifts, RPM, and crankshaft angle, a quick ramp may not be advantageous. It all comes down to filling and emptying the cylinder and maximizing cylinder pressure.
Another favorite: Valve Timing and Flow Relationships
: As stated in the first paragraph, the intent of this write-up is to discuss differing views or reasoning of the media and/or enthusiasts about the camshaft and its role in effecting engine power capability as components are changed. These changes are viewed as any modification that effects the way an engine exchanges air. This could be as seemingly insignificant as better flowing air filters or mufflers, or as major as a complete inlet or exhaust path redesign. Cylinder volume and crankshaft geometry changes, as well as heads, intake, and exhaust changes effect this air exchange.
What happens to the valve timing demands as these components are changed or modified? We have all read countless offerings from the media and engaged in long bench racing discussions on the topic. This write-up is going to present some of these views from a perspective that should address the concepts on their own terms; then their overall effect on cylinder pressure.
Restricted Flow Case
: The subject always comes around to the modification of a stock engine. By stock engine, I am referring to the typical restrictive factory stock (or very mildly modified) intake and exhaust capabilities. There are differing opinions as to what can be done to a restrictive stock type engine to increase performance. These opinions range from 'doing nothing', because it is a waste of time and money to attempt helping such a poor breathing set-up, to the alternative of working within the constraints and finding extremely respectable performance. Though aimed at stock set-ups, this category can very easily encompass many configurations, even all-out race engines. A restricted engine can be thought of as an engine that is limited by its components, manifolds, heads, or carb/EFI configuration to allow the full-displaced cylinder volume to enter. This classification primarily applies to the inlet side of the engine, where cylinder filling occurs. After all the peripheral parts are chosen/assembled, there is only one component that controls how well the air exchange is carried out, the camshaft. How is the typical camshaft swap in this situation viewed? The rationale for 'doing nothing' focuses in three distinct areas: drivability, RPM, and overall power capability. The drivability issues come about because of the idea of the engine loosing all of its low-end power or torque characteristics. The RPM issues come about, because of the need for the engine to turn higher rpm to make power. This is closely related to the low-end power loss associated with the drivability issue. And last, is that the overall power capability of the engine is low. This last area is deeply grounded in the idea of flow equals power, so why bother with the compromised breathing capability of this type of set-up.
High Port Velocity = Excellent Low RPM Performance
: Small restrictive inlet ports provide high port velocity. Small restrictive inlet ports also require lower rpm levels to achieve maximum torque output. So why the drawbacks to adding a camshaft to one of these situations? The name of the game is cylinder pressure. How do you increase cylinder pressure? You have to increase the amount of incoming air. There are arguments about increased camshaft duration and lift can actually out-deliver what the cylinder is capable of moving. The only part of the natural aspirated motor capable of moving air is the displaced volume in the cylinder and the scavenging effects provided by the moving air charge. Then the camshaft merely becomes the driving force to actuate the valves.
Classifying the camshaft as 'too large' needs to be rethought of as the camshaft is 'just not correct for the set-up'. Most enthusiasts would be very surprised at the valve activity that restricted set-ups require for good cylinder filling. The requirement is for the engine to produce ample cylinder pressure for the desired performance level and RPM range. In earlier paragraphs, I discussed intake valve closing as a key ingredient for cylinder pressure. Likewise, at TDC overlap, the inlet and exhaust valve activities have to be timed to promote proper air exchange. With such small volumes of air being moved in the restrictive inlet, in order for a charge exchange to occur ample time is needed. Step away from viewing timing events as an angular dimension. Consider valve timing as a unit of time.
Volumetric flow has the time quantity associated with it. If the engine requires a particular volume of incoming air to adequately fill the cylinder in order to produce high cylinder pressure (equatable to torque), then the model illustrates that increased overlap (as an angular quantity) is a valid solution. However, this only applies to the point of being hampered by the exhaust characteristics. Exceeding the usable lift range of the valves and ports is a question that quickly surrounds the subject of adding increased valve lift. Instant opening and closing valve events enable maximum cylinder filling to occur, when properly phased to the crankshaft activity. Go back to the model that was used to show cylinder pressure. For a given inlet and exhaust port effectiveness, there was an optimum opening and closing for the intake and exhaust valve. To increase the flow capability of the engine through modified valve timing events will take increased lift (over the baseline) at the same crankshaft rotation.
This does not necessarily imply that overall maximum lift has to be increased, but limitations in the cam lobe design will typically force this to happen. There are instances in which the flow can actually decrease with extreme high lifts, but this is tunable with rpm. For instance, if the port shuts off at 6500 RPM with a particular high lift camshaft, what does it do when designed not to exceed 6000 RPM with that same valve lift? It actually becomes more of flow dynamics concern that is seen as port velocities really start climbing. This is analogous to a port's flow capability flattening out (or even falling off) above certain valve lifts at a particular flow bench static pressure reference. If the static pressure is changed the flow characteristics at the same valve lifts can change.
What Can Be Expected From A Restrictive Design?
Adding a performance-oriented camshaft to a restrictive stock-type engine set-up will require timing events that focus on increased cylinder pressure and lower RPMs. Lower RPMs will require earlier intake valve closing values, yet still enable excellent cylinder filling at these lower RPMs. By addressing cylinder filling and pressure as design criteria, low RPM torque in maintained and usually increased.
Drivability is maintained, if not enhanced. In a track situation, the drivability may not be an issue but the cylinder pressure maximization is still the goal. If there is a question about how much power a restrictive engine set-up can produce, look at the sanctioned racing events that require stock breathing characteristics of the cylinder head/s and peripheral equipment, yet allow revised valve timing events. How is it that a 'stock' class drag car can run in the 11's on the 1ŇĹ4 mile, yet the average hopped up street car cannot seem to even break into the 12's. It's about engine requirements. On a side note, it is common for a lot of these combinations to be shifted well below 6500 RPM.
Unrestricted Flow Case
: The usual progression in modifying an engine's breathing characteristics is to add better flowing cylinder heads. Increased breathing enables better high lift flow characteristics, along with increases in inlet and exhaust capability. These are the ingredients needed to increase power over the stock limitations. As I said in the first paragraphs, increased breathing capability does not guarantee increased power. The main design criteria is to optimize and maximize high cylinder pressure.
With high flow capability, the port cross-sections can actually flow more than the cylinder can displace for a given RPM; this is the difference between the restricted and unrestricted cases. Remember displaced cylinder volume becomes a flow rate when combined with time. RPMs allow this transformation to occur. If the engine is not turning high enough RPMs, then there is sort of a reserved volume of air in the inlet not being used. This is a characteristic of low port velocity.
Large Ports = Excellent High RPM Performance
: If two cylinder geometries are each outfitted with different inlet capabilities (cross-section and effectiveness), then for a given RPM, the inlet port velocities will differ. In the earlier paragraphs, there was an assumption made when using the piston and spring model that said, the displaced volume remained equal, regardless of how much the small piston and spring extended or compressed. If the displaced volume in each of these engines is equal, then the inlet port capabilities define the port velocity with respect to the volumetric flow produced by the cylinder. What does this mean?
This means that to be able to reap the benefits of the unrestricted engine: 1) you have to rotate the engine fast enough to use all of its flow capability and/or 2) you have to address the low port velocity at the lower RPMs. In the simple models, we saw how port effectiveness could effect the valve openings and closings and their effect on maximizing cylinder pressure. In each situation, adequate cylinder filling had to occur. At low RPMs, the unrestricted port design sort of has a 'reserve' of usable flow cross-section. This air is not really used at lower RPMs. However it needs to be addressed from standpoint of being clean and able to produce good burn characteristics.
There is a situation, where the incoming charge can be contaminated. This occurs because the inlet port is moving air slowly and the exhaust pressure, during outflow, can slow down the charge even more or at least contaminate the charge to the point that the combustion properties are diminished. This process is called reversion, and when it occurs, adequate cylinder filling has been compromised. Keep in mind an important thought. This set-up has all the inlet capacity the engine could want (and more), yet it is possible to destroy its power making ability before it is ever cranked up if the valve events do not work for the set-up. Flow capability, by itself, does not guarantee a power increase. In the restricted flow case, we looked at overlap as a function of time as opposed to an angular measurement. Now, do the same with the unrestricted flow case.
The engine requires a particular volume of incoming air to adequately fill the cylinder in order to produce high cylinder pressure (equatable to torque). This is the same sentence from the restricted flow case! What happens to the amount of time needed to fill the cylinder? With unrestricted flow, the amount of time needed to fill the cylinder will shorten for a given RPM. In the simple models, it was demonstrated that the total cylinder pressure capability, for a given rpm, is not dictated by the intake closing, but rather by the inlet port capability. So this means that the activity happening at the intake opening is what is effecting cylinder-filling capability. I described the easily contaminated 'reserve' cross-section in the unrestricted flow set-up. With slow port velocity, the cylinder filling is compromised very easily. So for an unrestricted flow situation overlap (as an angular quantity) is not a valid solution.
High Lift Flow Capability
: As stated earlier, the usual progression in modifying an engine's breathing characteristics is to add better flowing cylinder heads. With these cylinder heads come larger ports and increased flow capability at higher valve lifts. For an unrestricted inlet to really maximize its potential, it has to be allowed to turn higher RPM. Here in comes, the reasoning of using larger cams with the larger cross-sectioned and better flowing heads.
We are looking at definite valve lift increases to take advantage of higher lift flow capability and increased valve duration to enable higher RPM potential. This is the typical rationale behind this build-up, but there are some issues that need to be addressed. Although the flow substantially increases at the higher lifts, is the motor actually benefiting from the increased flow capability at lower RPMs? Remember the overlap situation, in which the incoming charge could be contaminated from exhaust to intake charge exchange. For lower RPMs, the unrestricted set-up will respond extremely well to valve timing events that provide very little overlap (as measured in crankshaft degrees). It is not uncommon for a stock specification camshaft to work very well in these situations. It provides the early intake closings for lower RPMs, yet typically has very low overlap angle values.
Another favorite: Mixing and Matching Components
: It is very common to stage modifications based on budget or schedule. Enthusiasts will space out there performance upgrades. Components are installed that will benefit later purchases. This causes problems because of mismatched parts. A camshaft, with valve events designed for restricted flow situation will not work well in an unrestricted set-up.
The slow port velocity will allow so much exhaust to intake charge exchange (reversion), that low RPM performance is severely hampered. The inlet charge is so dirty and slow that it may not ever recover and produce good cylinder filling characteristics until a few hundred RPM at the very top end of the powerband. This is a common reason why lower/narrower lobe separation camshafts get a bad rap. In reality, it's not the camshaft's design that is the problem; it's the pairing up with those valve events and flow capabilities that are incompatible. On the other side of the coin, is the reasoning that says a stock or restrictive combination cannot benefit from improved valve timing. This is partly because the camshafts being tried are geared for better breathing engine set-ups.
Valve events that are designed for an engine with substantial flow capability will not work in a restrictive engine. If the models are correct, then the reason is simply that the valve events for an unrestricted combination do not permit good cylinder filling characteristics in a restricted combination. It is almost as if a restricted flow situation banks on the valve events to open the door for more flow because the heads are so confining. Whereas the unrestricted breathing situation relies on the cylinder head to provide the flow capability and the valve events actually restrict flow in certain situations.
: The main idea is to optimize cylinder pressure. Every combination will have different requirements. The cylinder geometry in conjunction with the entire inlet and exhaust paths determines the valve timing needs. An engine that has valve events reflective of the requirements can really provide good power and drivability as well. There are all sorts of formulas out there that equate cylinder head flow to horsepower. However, these formulas only apply if the flow is being adequately used by the cylinder volume and geometry.