Selecting a cam or, more accurately, deciding the cam events that the engine needs has traditionally been regarded as a black art. Most publications feed you just enough info for you to get into trouble (in most instances) or for you to decide you should let a tech guy at a cam company make the decision for you. Either way, this can be bad.
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Writers don’t, in 99 percent of the cases, have the knowledge to educate you to a point where you have a working idea of what’s needed and why. In the case of the cam company’s tech guy, you are at least dealing with a person who has, in most cases, a lot of experience with what works, but not necessarily why it works. If you want proof of this, you can rerun a little experiment that I did with some friends.
We took an engine that had already been built and dyno’d to determine the optimum cam. At this point, four major cam companies were anonymously called for a cam spec for this engine. There were two stipulations: The cam had to be a flat hydraulic, and the intake duration (at 0.006 follower lift) had to be 280 degrees. We received four very different cam recommendations. Among these was an overlap variation ranging from 52 to 64 degrees, and lobe centerline angles (LCA) ranging from 110 to 114. Also, single- and dual-pattern cams were both put forth.
The cam and valvetrain components chosen are critical to engine’s success as a power producer. The subject of optimal valve events has been poorly covered in countless books and magazine articles. In this chapter I will start you down the road toward what will actually work first time around in your engine.
This prompts the question: Can they all be right? Certainly not, but the situation actually took a turn for the worse when we called three months later for a cam spec for the same engine. In three out of four cases, we got a different tech guy but in all cases we were given a different cam spec than the original. So we now had eight different cams, all supposedly near optimal for the job. The real kicker here was that dyno testing had shown that, for our engine (as opposed to a generically similar one), a single-pattern cam on a 108-degree LCA produced the best power curve (highest average output over the used-RPM range), yet not one of the cam companies had recommended a 108 LCA cam!
To be sure, all the suggested cams would have produced far more output than the stock cam each would have replaced. However, none would have produced the near-optimal results of the cam finalized by the dyno testing. The point I am coming to here is that in spite of the fact the four-cycle internal combustion engine has been with us for more than 100 years, there is still a great shortage, even within the cam industry, of knowledge about what valve events work for a given engine spec. (In the next three chapters, which all pertain to the valvetrain, I will supply the information you need to know to make a top professional judgment when it comes to spec’ing-out a cam for your engine.)
So what qualifies me to be this critical of previously printed matter and the cam industry in general? Simple: It’s better than half a lifetime’s worth of dyno testing cams (more than 45 years) to draw on and a pile of race and championship wins, pole positions, fastest laps, etc. And, on top of that, achieving sufficient expertise on the subject to teach it at university to some of the top winning-race/engine-building pros in the business. With all that said, let us now get down to the promised “real” camshaft science.
So what qualifies me to be this critical of previously printed matter and the cam industry in general? Simple: It’s better than half a lifetime’s worth of dyno testing cams (more than 45 years) to draw on and a pile of race and championship wins, pole positions, fastest laps, etc. And, on top of that, achieving sufficient expertise on the subject to teach it at university to some of the top winning-race/engine-building pros in the business. With all that said, let us now get down to the promised “real” camshaft science.
Mechanical Attributes Simplified
Let’s be clear about things here. Although I can simplify the basics of cam/valvetrain dynamics, what you learn here won’t come close to getting your university degree in cam dynamics. The subject is unbelievably complex. What I do is give you the basics, which will open the door to a far greater understanding than you probably have now.
First, let’s look at the camshaft’s basic descriptive attributes. These are shown in Illustration 9-1 and represent the starting point of understanding how to time a cam in, why certain factors change its characteristics, and how optimally it may work. The most important factor to understand here is that the lobe centerline angle (LCA) and the duration of both the intake and exhaust directly affect the overlap produced.
9-1. Here is what is seen looking at a pushrod V-8 cam end on: intake lobe lift (1), exhaust lobe lift (2), intake duration (3), exhaust duration (4), overlap (5), lobe centerline angle (LCA) (6), cam advance (A) and retard (R) (7). For a multi-cam engine, the advance/retard and LCA is phased by the valvetrain gears.
9-2. Shown here is an intake cycle from start to finish. The duration (in this example, 270 degrees) is indicated by arrow No. 3. The degrees opening before TDC is indicated by arrow No. 1 and the degrees after BDC to the closing point by arrow No. 2. The No. 4 line indicates the intake centerline angle, which is exactly halfway between the opening and closing points.
Illustration 9-2 shows the points where the intake valve opens and the position of the piston in the bore with a cam of 270-degrees duration at the lash point. The blue partial circle indicates this duration. Illustration 9-3 shows just how you see it on a cam spec card.
The next move is to take a look at the valve lift (or, as cam designers call it, the lifter displacement curve), produced by the cams form. This is shown in Chart 9-4 (note the caption comments on the duration at various tappet lifts).
If you have absorbed all that, what we are going to do now is take a lift curve and dissect its dynamics. In designing a lift curve, a cam designer has to consider not just the lift imparted to the tappet (or lifter or follower) but also the velocity, acceleration, jerk, and jerk II. If you are Harvey Crane, Jr., then this list goes on to include snap, crackle, and pop!
All tappet motion is quoted in terms of crank degrees, so lift is “so many thousandths of an inch” at “so many degrees” from the starting point (i.e., 0.045 lift at 30 degrees from the starting point). Velocity is not quoted as a particular speed in inches per second (because engine speed changes) but rather in thousandths of an inch per degree of crank rotation. The velocity is the “rate of change” of position or displacement of the tappet in relation to the motion of the crank in degrees.
9-3. By melding the intake- and exhaust-duration arcs (at the top) together, we form the valve-opening event (at the bottom). In this example, the inlet opens (IO) 25 degrees before TDC and closes 55 degrees after BDC. The exhaust opens (EO) 55 degrees before BDC and closes 25 degrees after TDC. The way we quote the events of this cam is: 25-55-55-25. The overlap, when both the intake and exhaust are open, is indicated here and, in this case, is 50 degrees (25 + 25).
9-4. Duration No. 1 is at a solid lifter’s lash point. The lash point at the lifter is the lash at the rocker divided by the rocker ratio. Arrow No. 2 is the so-called advertised duration and is usually 0.006 inch for hydraulics and 0.020 inch for solids. The red arrow indicates the duration at 0.050 inch.
It is important to understand this rate-of-change factor, so we can move toward understanding the remaining motion factors mentioned earlier. The “tappet acceleration” is the rate of change of velocity, and “jerk” is the rate of change of acceleration, and so on. It is important that each of these factors changes progressively; otherwise, the valvetrain system suffers spurious vibrations and at some point goes into resonance. When that happens, the valve motion seriously deviates from that intended by the cam form. For instance, if the acceleration instantly went from zero to some relatively high value, it simulates the valvetrain being hit with a hammer. Such a move can hardly be expected to produce a quiet or otherwise functional valvetrain.
There is a simple experiment to demonstrate how this works, and you can do it on your next drive. When you want to bring the car to a stop, put on the brakes by applying, say, 40 pounds (or whatever) of force to the brake pedal. Do not change the applied force until after the vehicle has stopped. What you are doing here is slowing the car at a constant deceleration. When the car stops, there is an instant change of acceleration from some value (say, 0.2 g) to zero. Notice when the car stops, you are jerked by that instant change in acceleration.
When we drive, we avoid that jerk at the stopping point by letting off the brake pedal progressively as the vehicle comes to a stop. If you practice the art of smooth driving, you would be progressively applying the brake so, at both ends of the stopping procedure, the change in deceleration between zero and peak is a smooth progression. If you understand the dynamics of this example, you are well on the way to understanding the interrelations of displacement, velocity, acceleration, jerk, etc.
The whole point of refining the cam’s profile in this manner is to make its operation smooth. Doing so considerably reduces the propensity of aggravating the valvetrain as a whole or any of its components at their primary resonant frequencies. Some of the advantages are the need for less spring to control the valvetrain to its design RPM, less valve-to-piston clearance without hitting the piston, less chance of spring surge below peak RPM causing valve bounce, and so on. We should add valvetrain longevity to that list as well.
Hi-Perf Four-Cycle Engine
The type of engine we are dealing with here is traditionally known as a four-cycle engine. While that may be largely true for many daily-usage production engines, with exhaust systems of less-than-adequate flow and scavenging capability, it is certainly not true for a genuine high-performance engine. Given that a length-tuned exhaust system can produce a very strong negative-pressure pulse at the exhaust valve, we see that there is an opportunity to produce an induction phase wholly independent of the piston’s motion down the bore.
Here is what the motion data of a typical cam looks like. The red curve is the lift from the base circle on up. The black curve is the lifter velocity (rate of change of lifter displacement) in inches per degree. The blue curve is the acceleration (rate of change of velocity) in inches per degree per degree (usually seen as inches per degree2 ). The green curve is the jerk (rate of change of acceleration, or inches per degree3 ).
9-5. Starting at cycle No. 1, the exhaust-generated vacuum starts the intake charge moving into the cylinder way before the piston even starts down the bore. As the crank rotates farther, we get to cycle No. 2. This is normally considered the charge-inducing stroke. In an ideal situation, cycle No. 1 has cleared the combustion chamber and put a considerable amount of kinetic energy into the incoming charge before the piston starts down the bore. The result is an engine that can achieve a volumetric efficiency well over 100 percent. The bottom line is: A good exhaust system is worth a lot of extra torque, horsepower, and, best of all, extra mileage. But to make all this work as intended, the cam must generate the right events around TDC.
For a well-tuned system, the exhaust pulse can be super effective over as much as 4,000 rpm. In terms of intensity, it can be far stronger than the suction caused by the piston traveling down the bore. A really good two-valve race engine making upward of 135 hp per liter can have as much as 150 inches of water suction on the intake valve at TDC, while the piston, at peak power, has only about 20 inches. Any more than that and it is a strong indicator that more cylinder head airflow is needed.
Having returned to the subject of airflow, let’s focus on that and investigate how cylinder head airflow and port velocity should be managed for best results.
Airflow Dynamics
After years of studying the subject, I realize that one basic rule emerges as the most important to appreciate if optimal output is to be seen from any high-performance engine. The question we need to ask ourselves here is: which of the gasmoving cycles is the most influential in terms of generating output? Referring to our five cycles in Illustration 9-5, we have numbers 1, 2, and 5 to consider. While none of them is without its own degree of importance, we do need to establish a priority.
Up until maybe as late as the 1980s, the delayed intake closing after BDC was considered the biggest influence in making power. Although this was a widely held belief, the reality is that it is the overlap period that is the number-one influence on the cam’s success. This does not mean that more is better. What it does mean is that it is very important to have the right amount of overlap for the RPM involved and the right proportion of the intake and exhaust duration occurring within that overlap. Getting the overlap right for the engine geometry/flow characteristics concerned means first taking optimal advantage of exhaust-driven combustion-chamber scavenging, and then having the intake valve as far open as possible at the start of the piston-driven intake stroke.
This 302 Ford 5.0 Mustang makes 475 streetable hp. This was mostly due to a strong interaction between cam events and exhaust.
Be aware that there is much more to cam selection than just plugging in a bigger cam. When buying a cam, consider that one with the right events because it costs no more than one with the wrong events.
Tuned-length intake systems look high-tech, but at the end of the day it’s the exhaust working with appropriate cam events that makes for results.
The more accurate the cam is spec’dout, the more important it becomes to time it in right. Chapter 10 explains why.
Cylinder pressure tests and subsequent dyno testing prove that the suction caused by the exhaust, and what the piston does on the intake port in the first half of the induction cycle, is critical. If this is not done right, there is no chance of rectifying the situation in the second half of the stroke. Trying to fix an inadequately filled cylinder in the second half of the induction cycle is a bit like bolting the stable door after the horse has left.
Having made that point, it should be evident that opening duration, which is the most-often-used cam selection parameter, is actually the wrong way to go. Since the huge depression caused by the exhaust brings about the single strongest action on the intake port, it follows that the overlap to allow this to happen must be the most important element of all the opening and closing events to get right. This being the case, it demotes duration as a deciding factor to several places down the list.
Based on this, we can then say that the number-one selection factor is overlap. Following this, the number-two selection parameter is the lobe centerline angle. On a single cam engine, the LCA has to be ground into the cam. But for an engine with separate cams for the intake and exhaust, the LCA is adjustable by means of the timing on each of the cams in relation to the crank.
It could be said that duration is the third most important factor but, in reality, we cannot independently adjust it, so it’s really only a by-theway deal. Here’s why: After the overlap and LCA requirement (which we deal with soon) has been determined, we find the duration is now fixed. In other words, X overlap with Y LCA can only produce Z duration. Assuming a single-pattern cam, the duration is the sum of half the overlap, plus the LCA, multiplied by 2.
An example looks like this: Let’s say the overlap required is 54 degrees and the LCA is 108 degrees. Half of 54 is 27. The 27 plus 108 is 135, which, when multiplied by two, comes to 270. That duration is the only one that gives the overlap and LCA called for. From this, you can see that the duration is totally dependent on the overlap and the LCA being used.
Choosing a Cam
Up to this point, you probably were buying into a cam selection procedure based on duration. Even if you did not know exactly what you were doing, you could at least have made a decent ballpark guess if duration was indeed the sole deciding factor. As of now you have probably learned that what you thought you knew was in fact relatively useless, and the procedure to select a cam is based on variables you might not have previously considered. This means if you had little doubts about your ability to select a cam before you started this chapter, you probably really have them now. But in the next chapter, we fix that.
Having the correct events is just the start of the cam selection procedure. At the end of the day, the entire valvetrain needs to be spec’d-out.
Shown here is the real-life lobe centerline angle for a small-block Chevrolet roller-follower cam.
Assembling the components for an ultra-functional valvetrain is all a question of attention to detail. A point to note here is that any valvetrain is only as good as the valvespring. Assuming a smooth cam profile, the rest of the job is very much making sure that spurious vibrations do not get the better of things; this is where aluminum components work well.
As unlikely as it may at first seem, the crank damper is actually part of the valvetrain and can strongly influence its dynamics.
Written by David Vizard and Posted with Permission of CarTechBooks
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