The most basic shape in nature is the circle. While each lobe on a camshaft is actually an eccentric, it begins and ends with a circle, which is where we start. A camshaft lobe really has only one job – to convert rotary or spinning motion into linear or back-and-forth motion, which is what the eccentric does. Place a follower against the base circle of a cam. Now spin the lobe and watch the follower move up and down in its bore. How quickly that follower creates lift and how long it creates lift before returning to the lobe’s base circle is what we are investigating in this chapter. We then take that information and combine it with the rest of the lobes on the cam and see how they all work together to make the combustion process more powerful.
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Before we get too deeply into this, let’s first look at the two basic types of camshafts used in the typical small block Chevy—the flat tappet and roller lifter style. Flat tappet cams are the senior members of this exclusive club. They were the first type of cam used in the earliest 265-ci Mouse motor. In this case, the face portion of the lifter slides along the lobe as the camshaft turns. The second style of cam is the roller lifter version. Those without a historical perspective may assume that the roller lifter and cam design is a relatively new invention. But in reality, some of the earliest known internal combustion engines used roller cams and lifters, dating at least as far back as the 1911 Stearns flathead engine and probably even earlier.
Within each of these two cam families are subsets of mechanical and hydraulic lifter cams and lifters. Mechanical cams (whether flat tappet or roller) use a very simple lifter. They are solid with just a radius cup in the top of the lifter for the pushrod. These cams require a designed-in clearance into the system to account for growth as the engine expands on its way to its normal operating temperature. This is one reason for the clearance or “lash” spec that accompanies any solid lifter camshaft.
The second style, the hydraulic, is far more popular for street engines. It incur porates a small piston inside the lifter body that rides on a cushion of engine oil. This small chamber is filled with engine oil, creating a pressurized area that automatically compensates for clearance changes in the engine during operation. Instead of clearance used for mechanical lifters, hydraulic cams require preloading the lifter used to compensate for these changes in clearances. We get more into that in the chapter on cam installation.
Lobe Prospecting – Duration
Now let’s condense that camshaft down to one simple lobe and dissect it into its more simplistic components. A lobe has a few simple points worth mentioning, starting with the base circle. The lobe begins to rise off the radius of the base circle; this is referred to as the opening flank. The peak or top of the lobe is referred to as the nose. As the lobe continues to turn, we reach the closing flank that eventually transitions back to the base circle.
The most basic measurement of any lobe is the amount of rise, or lift, created at the nose. This is called lobe lift. Cam or lobe lift is the amount of tappet rise eventually multiplied by the rocker arm to create valve lift. Rocker arms are covered in detail in Chapter 6. When the cam designer creates lift in a lobe, it is not created instantly. In other words, the cam lobe is not square. Instead, it has a slope that gradually pushes the lifter up to its maximum lift. The amount of time the lifter is raised off the base circle is referred to as duration and is expressed most often in crankshaft degrees.
In the small-block Chevy, and in all four-stroke engines, the camshaft is driven at exactly half of the engine speed. This is easy to see in the cam drive system in a small-block Chevy where the cam gear has exactly twice the number of teeth as the crank gear. To make it easier to work with the numbers, most cam specs are expressed in degrees of engine rotation. Given that, the amount of time that the lobe rises off the base circle, then is described in degrees of crankshaft rotation. For example, a typical small-block Chevy intake lobe may be expressed as having 270 degrees of advertised duration. This means that if you were to place a dial indicator on a lifter with a 360-degree wheel bolted to the nose of the crankshaft and you measured the amount of time (in engine degrees of rotation) that the lifter was off the base circle, you would see the crankshaft turn 270 degrees. It’s not quite that simple because of specific checking points that must be used. We’ve addressed that in the accompanying sidebar on advertised duration versus duration at 0.050 tappet lift.
The important distinction to retain here is that the camshaft is turning at half engine speed yet duration is expressed in engine degrees. At first, it may appear that the intake lobe is open for 3/4 (270 of 360 degrees) of an entire rotation, so how can that work? To explain this, we have to go back to the basic four-stroke cycle. In order to have enough time to complete all four of the intake, compression, combustion, and exhaust events, the engine must rotate through two complete revolutions, or 720 degrees. So if we have a camshaft with 270 degrees of 720 degrees rotation, it is only 37.5 percent of total engine rotation per four-stroke cycle, which works very nicely.
So now that we’ve covered lift and duration, lobe centerline is next. For every lobe with a given number of degrees of duration, there is a mid-point or halfway point in that event. This can be easily expressed in a line drawing of a lobe splitting it right down the middle. Cam designers use this midpoint to help them position both the intake and exhaust lobes in relation to each other. Again, this position is expressed in crankshaft degrees. For example, the intake lobe of a typical small-block performance camshaft has an intake lobe centerline positioned at 106 degrees ATDC (after top dead center). Note that this is the position of the intake lobe centerline as positioned in the engine. This shortcut is used because it’s the easiest way for a typical engine builder to ensure that his camshaft is positioned properly in the engine. If the cam is correctly ground and installed, this is the number the engine builder comes up with when he degrees the camshaft in the engine. How to degree a cam is shown in a later chapter.
As you might have already assumed, the exhaust lobe also has a lobe centerline figure, most often expressed as a given number of degrees ABDC (after bottom dead center). This figure is generally not used very much in typical installation or checking procedures, except to help determine our next detail in our somewhat complex world of camshaft information. This is something called lobe separation angle, or lobe displacement angle. Simply put, this is the number of degrees (expressed this time in camshaft degrees) separating the intake and exhaust lobe centerlines. The easiest way to understand this is to look at the layout of the intake and exhaust lobes in the drawing. It looks like two adjacent hills or camel humps. If you study this drawing carefully, it does more to explain how camshafts work than literally thousands of words of explanation.
If you look at the cam graph, you can easily spot both the exhaust and intake lobe centerlines at (or near) the peak of their respective lift curves. The distance between those two centerline points is expressed in cam, not crankshaft, degrees. Of all the camshaft specs, lobe separation angle is the only camshaft spec expressed in camshaft degrees. This is because the definition takes the total number of crankshaft degrees of separation and divides it by two. By doing so, this changes the number to camshaft degrees because the cam spins at half crank speed.
An important point to make when discussing lobe separation angle is that the cam designer establishes the angle. Once that relationship is established by grinding the cam, the lobe separation angle cannot be changed unless a new camshaft is machined. This makes engine development and tuning somewhat difficult with any cam-in-block pushrod engine because changing the lobe separation angle requires a new camshaft. If we were working on a dual overhead cam (DOHC) engine, changing the lobe separation angle is merely a matter of moving either the intake cam or the exhaust cam centerlines, or both. In fact, the General Motors DOHC 4.4L Northstar V8 (among many other engines foreign and domestic) uses variable valve timing to move either or both the intake and exhaust valve cams relative to each other over an incredible range of motion.
Advance or Retard
Before we get to the next level of camshaft discussion, it’s important to go over camshaft opening and closing points. Several options are available to do this. First, let’s look at advance versus retard. In those classic bench-racing sessions that you’ve no doubt been a party to, you’ve probably heard a tuner talk about advancing or retarding the camshaft. Advancing a cam merely means that the engine builder desires to open both the intake and exhaust valves sooner in the fourstroke process. For example, let’s say that the intake lobe centerline on a particular camshaft has an intake centerline of 110 degrees ATDC. By physically changing the position of the cam in relation to the crankshaft, the engine builder can begin the process of opening and closing the cam sooner in relationship to the Number One piston at TDC (top dead center). By advancing the camshaft four degrees (for example), he now has an intake centerline of 106 degrees ATDC instead of 110 degrees ATDC. This may seem backwards, but if you study the cam graph, you can see that 106 degrees ATDC actually is opening the intake valve sooner.
While we’re on the subject, let’s go over the effect that moving the camshaft has on engine operation. Typically, advancing a camshaft begins the process of each valve event earlier in the fourstroke cycle. By starting everything sooner, this generally improves low-speed and mid-range power while hurting highRPM power. When you retard a camshaft, this delays the beginning of the fourstroke cycle, and typically tends to improve top-end power at the expense of low-speed and mid-range power. The most important effect of advancing or retarding the camshaft is that this moves the cam’s intake closing point. Of the four intake and exhaust opening and closing points, the intake closing point is by far the most important. Establishing when the intake valve closes in relationship to TDC does more to shape the engine performance curve and the RPM point at which peak torque occurs than any other single cam factor. This is discussed in much more detail in the sidebar on the four opening and closing points.
Keep in mind that changing the intake centerline means the engine builder has also moved every other opening and clos ing point in the engine. This is because he has physically moved the entire camshaft in relation to the Number One piston. If he desires to move only one or two open ing or closing points, it requires a new camshaft. The reason that engine builders may move the intake centerline is because it is a quick and easy way to evaluate if the camshaft and engine combination is close. The danger, as we mentioned, is that the tuner is moving all four of the valve events, and not all four events reinforce engine power in the same way (see the sidebar “Changing Lobe Separation Angle”).
Perhaps this is also a good place to discuss the relationship of the intake centerline to the lobe separation angle. Generally, if a camshaft is ground “straight up,” the intake centerline position and the lobe separation angle is the same number. For example, let’s say that we have a cam with a lobe separation angle of 110 degrees and an intake centerline angle of 110 degrees ATDC. More often than not, most street cam companies design their camshafts with a few degrees of advance built into their camshafts. This is easy to see on most cam cards because the cam company lists the intake centerline with a different spec than the lobe separation angle. For example, COMP Cams designs most of its street-oriented cams with four degrees of advance ground into its cams. This shows up as a 106-degree ATDC intake centerline figure on a camshaft with a 110-degree lobe separation angle. Many other cam companies do some thing similar because it’s common for enthusiasts to order a slightly larger cam than what the engine really needs for the application. By advancing the cam four degrees, the cam company in essence advances the intake closing point, which improves low-speed torque, making the camshaft a little more livable at partthrottle engine speeds.
Now we know what the lobe separation angle is, but what does that tell us? This is a very important camshaft specification because this angle establishes and defines a critical function of camshaft design known as overlap. Very early in the development of performance engines, way back in the flathead days, engine designers discovered that if they advanced the intake lobe in relation to the exhaust lobe, the intake stroke would begin sooner. The engine made more power. The why was less important.
Overlap is widely misunderstood and is worth investigating more closely since it helps determine many different engine characteristics. The basic idea is to open the intake valve before TDC. At first, this would seem counterproductive since it is the volume change from the piston traveling down the cylinder that helps move the air and fuel mixture into the cylinder. That’s all true, but the key to understanding the concept of overlap is to think in terms of the amount of time it requires to fill that cylinder. At low engine speeds, the intake port has sufficient time to fill the cylinder at a leisurely rate. The more efficiently it does this, the more torque the engine makes.
Peak torque occurs at the point in the engine’s power curve where the engine most efficiently fills the cylinder. This is also where volumetric efficiency is at its highest. As engine speed increases after peak torque, less actual time is available for the port to fill the cylinder. So the engine builder extends the duration, allowing him more time to fill those hungry cylinders at higher engine speeds.
Another trick is to maintain the same duration and advance the intake centerline. This moves both the intake opening and intake closing points earlier in relation to TDC, creating an overlap condition where the exhaust valve is just closing. What’s the advantage to doing this? First, we don’t increase the total length of time the intake valve is open. Second, we actually move the intake closing point slightly earlier, which tends to improve the mid-range torque. To take it one step further, we could even increase the intake duration, because we started it sooner, to have an even longer duration that doesn’t suffer from a very delayed intake closing.
All of this assumes that we have not moved the exhaust lobe. But what if we do move the exhaust lobe? The freedom of moving the exhaust lobe creates an opportunity to experiment with not only different intake closing points (the most important of the four valve events), but both the exhaust opening and exhaust closing events as well. Let’s say we’d like to advance the exhaust lobe. This moves the exhaust lobe centerline away from the intake, increasing the lobe separation angle and decreasing the amount of overlap. This reduced overlap improves the idle quality, but also tends to hurt the top-end power slightly. This is because it also opens the exhaust valve a little sooner; perhaps before the cylinder pressure has fully expended its energy pushing on the piston.
Conversely, if we retard the exhaust lobe, the lobe separation angle comes closer together and increases the actual valve overlap. The net effect of doing this is delay in the exhaust valve opening. An increased amount of overlap tends to hurt the idle quality; plus, it generally tends to diminish low-speed torque. At higher engine speeds, this delayed opening can improve top-end power, but that may occur only at the very top of the engine power curve. This requires the engine builder to closely examine his torque curve to see if moving the lobe is truly
beneficial. Increasing overlap by retarding the exhaust lobe may indicate that longer exhaust event duration is required in order to fully scavenge the cylinder at higher engine speeds. This is really matching the exhaust lobes of the cam to the exhaust ports, which is how you eventually make the most power on any engine.
In car magazine ads from the late 1960s and early 1970s, Ed Iskenderian referred to the overlap portion of cam timing as the fifth cycle. This was a successful attempt to draw attention to just how critical the overlap segment of an engine’s cam timing is to engine performance. For example, let’s take a small-block hydraulic flat tappet cam with an advertised duration of 270 degrees at 0.006-inch tappet lift. If we add the exhaust closing (EC) of 21 degrees ATDC to the intake opening (IO) of 29 degrees BTDC (before top dead center), we get an overlap event of 50 degrees. This is close to the amount of overlap the engine actually experiences. Now let’s look at a hydraulic cam with 305 degrees of adver tised duration also at 0.006-inch tappet lift. This cam has an EC of 43 degrees ATDC and an IO of 43 degrees BTDC for a total of 86 degrees of total overlap.
Both of these camshafts use a lobe separation angle of 110 degrees. However, we have a huge 36-degree difference in the amount of overlap between these two camshafts. This illustrates why just the lobe separation angle cannot be relied upon to truly indicate the amount of valve overlap in degrees. As you can see from this demonstration, overlap actually increases as either intake or exhaust lobe duration increases with the same lobe separation angle. That’s why you see many long duration cams with wider-than-normal lobe separation angles such as 114 degrees.
To further illustrate this point, if we took the longer duration 305 cam and widened the lobe separation angle to 114 degrees by moving both the intake and exhaust lobes equally, this would change the EC to 41 degrees ATDC and IO to 41 degrees BTDC, reducing the total overlap degrees from 86 degrees to 82 degrees. Remember that lobe separation angle is expressed in cam degrees, which means that cam specs are expressed in crank degrees. We can actually break overlap down into several areas and look at them closer to see how it affects engine operation.
If we look at just the EC side of the overlap equation, it is defined as the total number of crank degrees that the engine “sees.” It begins with the moment the intake valve begins to open until the exhaust valve physically closes. We can use 0.006-inch tappet lift if you’d like to define these areas.
Looking at Graph A, we have a total picture of the lobe separation angle. Most attention is paid to the small triangle shape in the middle, roughly around TDC. But this diagram contains much more than just this area. Note in Graph B that we’ve shaded the area affected by just the EC side of the curve. This area is the point on the EC side where the intake valve first opens until the exhaust valve finally closes. The shape of this curve is dictated by the closing-side curve of the exhaust lobe profile. The valveclosing portion of the lobe, especially approaching the seat, is a critical section of the lobe design. This is because a fast closing rate can contribute to valvebounce off the seat, especially at high RPM. This is never good for engine longevity. By advancing the exhaust lobe, you can reduce the overlap area of the curve. Retarding the exhaust lobe center line also increases the overlap area. Changing this area of the overlap curve relates to the efficiency of your exhaust port. If the port is fairly efficient, the engine may respond favorably to decreasing the amount of time the exhaust port spends open in relation to the intake.
Some engine builders feel that increasing overlap only gives the incoming air an opportunity to flow right out the exhaust rather than being captured in the cylinder and increasing cylinder pressure in the combustion process. This is especially true if the cylinder heads offer good low-lift flow numbers as evidenced on the flow bench. An engine with excellent low-lift intake and exhaust flow numbers (relative to its valve size and flow curtain area) might desire a camshaft with less overlap. This is because intake flow ends up merely flowing right out the exhaust. Conversely, a cylinder head with somewhat poor low-lift numbers could benefit from increased overlap as a way of crutching the flow curve. It starts the flow process earli er to get past the poor low-lift numbers while both valves are still open.
This gets into the art of engine building and discovering exactly what the engine wants, rather than just picking an arbitrary cam duration figure and going with that number. Accomplished race-engine builders start with a cam and then experiment with different lobe separation angle variables to come up with the best overall combination.
The intake side of the overlap curve begins when the intake valve first lifts off the seat and ends when exhaust valve closes, indicated by the shaded area in Graph C. The critical nature of the overlap portion of the timing curve is also related to how well (or poorly) the exhaust system functions. All internal combustion engine exhaust systems operate around a physical operation known as wave tuning. This occurs when a finite pressure wave is created as the exhaust valve opens. The “Wave Tuning“ sidebar explains this phenomenon in greater detail. You should read the sidebar now if you haven’t already. Timing the arrival of the reflected wave in the cylinder creates additional cylinder filling potential and the overlap period contributes to this tuning. The arrival of this negative pressure wave timed simultaneously with both the intake and exhaust valves open creates a situation known as “scavenging.” This negative pressure not only enhances the blow-down of exhaust components in the cylinder, but also gives an early “tug” on the intake tract to improve cylinder filling. The timing of this scavenging wave is directly dependant not only on cam timing and valve overlap, but also on the configuration of the exhaust system determined by header primary pipe length, diameter, and the sizing of the remainder of the exhaust system.
As you can see, many variables play significant roles in this bed of snakes that creates power in an engine. That’s why engine building continues to fascinate and attract hot rodders.
So far we have approached the camshaft as having a somewhat generic lobe profile. This works when discussing basic camshaft configurations, but now that you have a working knowledge of how all these lobes work together on a given camshaft, we can dive deeper into camshaft technology to get more specific.
The progression of lobe design has advanced drastically in the last 20 years in terms of applying serious math to the art of designing camshaft lobes. Flat tappet lobes are discussed in this chapter, saving the more aggressive roller cam lobes for the next chapter. Several limitations on what the cam designer can do lie within the basic architecture of the flat tappet camshaft lobe. In relationship to the lobe itself, all camshafts are limited in terms of lift by the amount of duration. Flat tappet cams, oddly enough, are limited by lifter diameter. Without getting into the complex math and geometry, as the lifter diameter grows, it is able to accommodate a more aggressive lifter acceleration rate. This unfortunately places the small-block Chevy at a distinct disadvantage in comparison to other engines. This is because the small-block (and Rat motor for that matter) is limited to a lifter diameter of only 0.842 inches. The small-block Ford uses a larger 0.874inch diameter lifter, while the Chrysler enjoys the benefits of an even larger 0.904inch diameter lifter. If you look at the illustration of a flat tappet lifter moving up on the opening flank of the lobe, you begin to see how a larger diameter lifter creates more lifter area to follow the lobe flank.
Within this lifter diameter situation lays the limit of lifter velocity rate. Cur rent camshafts often flirt with this limit in search of aggressive lobe designs, but only in the last few years. Let’s compare a couple of camshafts from a historical perspective. The much-vaunted L-79 camshaft was a famous factory performance hydraulic lifter camshaft for the 1965–1967 350 hp 327ci engine used in the Chevy II, among others. This camshaft was highly regarded and an extremely popular cam with Chevy performance enthusiasts because of its single pattern offering 221 degrees of duration at 0.050inch tappet lift at 0.447-inch lift using a 1.5:1 rocker ratio. With an advertised duration of 320 degrees, note that this cam combines an arm’s length of duration with rather conservative valve lift numbers.
Now let’s take a cam designed some 30 years later. We perused several cam catalogs, looking for a cam with a similar duration at 0.050-inch tappet lift, landing eventually on a COMP Cams Xtreme Energy cam with a shorter intake lobe of 218 degrees at 0.050. This cam offers a much taller 0.495-inch lift on the intake, which is a lift increase of a staggering 0.048 inches, roughly equal to using a 1.65:1 rocker ratio on the earlier cam. But the big difference is the advertised duration difference. The Xtreme Energy cam line employs a mere 270 degrees of advertised duration at 0.006-inch tappet lift. To be honest, it’s not fair to compare this to the factory’s massive advertised duration of 320 degrees because they start duration at initial tappet rise. But even if you subtract 20 degrees for the duration difference between the OEM’s initial tappet rise checking point and COMP’s 0.006-inch tappet lift spec, that still leaves a difference of 30 degrees!
This tremendous difference is due to COMP’s much more aggressive lobe design. It pushes the lifter much closer to its maximum velocity rate limit. Harvey Crane, one of the great early cam designers and the man who started Crane Cams in Daytona Beach, Florida, coined a term many years ago: “hydraulic intensity.” Hydraulic intensity refers to the time, in crankshaft degrees, that the lifter requires to move from its advertised duration starting point to the 0.050-inch tappet rise figure. Most cam designers use a gentle opening flank to open the valve more gradually, which makes life easier for the valvetrain. But the compromise is that it takes a toll on idle quality and low-speed torque since the valve is off its seat for a longer period of time. Most modern hydraulic flat tappet cams use around 60 degrees of hydraulic intensity duration to move the valve up to 0.050 tapppet rise, although cams that feature a shorter hydraulic intensity figure are available.
More aggressive camshafts like COMP’s Xtreme Energy line of cams employ a much shorter hydraulic intensity number of 52 degrees. You can actually use this hydraulic intensity number to “fingerprint” a family of camshafts. To do this, merely subtract the 0.050-inch tappet lift duration figure from the advertised duration. However, you can only compare hydraulic intensity numbers between different cam manufacturers by using advertised duration numbers with the same checking height. Otherwise, the cam with the taller (numerically greater)
advertised checking height always indicates a shorter hydraulic intensity. One tradeoff to the more aggressive hydraulic intensity number is that the valvetrain is noisier. This tappet racket is directly attributable to the more aggressive lift curve. The advantage to this type of cam design comes in if you have a given minimum idle vacuum level acceptable for street driving. With this faster ramp, you can increase the advertised duration number by a few degrees over a slower cam and still enjoy similar, if not better, idle characteristics.
Harvey Crane also coined a few other helpful terms when talking about cam measurement. Hydraulic intensity is useful for evaluating hydraulic cams just off their seat, but you can also judge a cam between 0.050 and 0.100-inch lift with what Harvey calls “minor intensity.” Minor intensity is determined by subtracting the duration at 0.050-inch cam lift from the duration at 0.100 inches. If you want to extend this kind of evaluation to mechanical cams, Harvey also came up with “major intensity.” Major intensity is computed by subtracting duration at 0.050-inch cam lift from duration at 0.020-inch cam lift. In all cases, the cams with shorter intensity figures enhance idle quality, off idle, and mid-range torque without sacrificing top-end power.
We’ve covered a large chunk of ground in this basic overview of camshaft design, but we’ve really only skimmed the surface. If you really want to get into how cam profiles are designed and built, you then become immersed in concepts like acceleration rates of the lifter using terms that would take another chapter to explain.
Written by Graham Hansen and Posted with Permission of CarTechBooks