One of the most satisfying aspects of engine building is selecting the parts that go into the build, and coming up with a combination that is right on target for the intended application. When that new bullet is finally assembled and comes to life, it's kind of like final exam time-and the results are your report card. The actual goals for an engine are as varied as the intent of the builder, with success measured by an engine that performs to its potential, and does what it is supposed to do. It doesn't matter if the engine project is an 8,500-rpm drag racer, or a torque monster for you dump truck, it isn't going to make the grade unless the camshaft is the right type, and ground to the needed specifications.
The camshaft's job is to control the timing and motion of the valves, and how those valves are orchestrated has everything to do with the way the engine will behave. A misapplied camshaft leads to disappointment every time, while nailing the cam selection will often provide more from an engine than you might expect. With so much on the line, it is understandable lots of guys find spec'ing the cam for a build a nerve-racking process. We can think of a few cam "gurus" who make their living hanging out on Internet message boards recommending cam specs and serving as middlemen to real cam companies.
So, what does it take to make an appropriate camshaft selection? First, you've got to have an understanding of the types of cams available, and the general characteristics and applications for a given type of cam. Next, you'll need a firm understanding of the various specifications used by manufacturers to describe their cams. We'll go into all of this in detail here, however, this information is useless if looked at in a single dimension, or in isolation. To fully integrate the cam selection process, you have to consider the entire build and the various constraints involved in the engine application. Things like the expected longevity and durability of the engine, the cost, the intended application, and operating characteristics of the engine must also weigh into the final cam selection. Much of the decision making will hinge upon the other complementary components comprising the engine assembly, such as the cylinder heads, compression ratio, valvetrain, and rpm capabilities of the bottom end.
Finally, you'll need to consider what you want the cam to do, whether it's rattling windows downtown on cruise night while frying the tires at will, or hitting dead-smooth while delivering years of high-powered open-road service. Either way, you're going to have to know camshafts to get the right cam for the job.
Flat Tappet Vs. Roller
There are two basic camshaft types, flat tappet and roller. At one time, rollers were strictly the domain of all-out race engines, but in recent years the roller cam configuration has virtually displaced the flat tappet in production engines. In the realm of aftermarket cams for V-8 engines, flat-tappet camshafts are still the most common type, however, the popularity of roller cams continues to grow. As the name implies, a roller camshaft uses a tappet with a roller wheel as the cam follower, and it simply rolls over the camshaft lobe. In contrast, a flat tappet appears to be simply flat at the lobe interface. Nevertheless, a closer inspection reveals that the geometry of a flat tappet is considerably more complex. The face of a flat-tappet lifter actually has a large radius of curvature, while the lobe is ground with a taper. This allows the tappet to actually skate while it rotates in the lifter bore over the lobe, rather than scrub across the lobe as commonly thought.
The very geometry of the lifter's contact to the lobe is quite different between a flat-tappet and a roller. A roller's contact with the lifter is linear, while a flat tappet presents a geometric plane to the lobe's surface. The result is that the motion imparted by the lobe is very different between the two types. You might be impressed by the broad "squared-off" lobes of a roller, and imagine the tremendous gain in resulting valve action, compared to a "pointy" flat-tappet lobe. A roller profile requires a much broader lobe to provide a motion similar to a flat tappet, since the nose of the lobe works across the entire diameter of the flat tappet lifter over peak lift, while the roller rises and drops off with the profile. Where a roller really has an advantage is in velocity, since the flat tappet is limited in velocity by the diameter of the tappet, whereas the roller is not velocity-limited by geometry.
A roller has a far greater ability to tolerate spring loads, so as the application becomes more demanding, a roller will allow for the required spring loads to maintain valvetrain control. A flat tappet's durability is greatly affected by spring load, and there is a definite limit as far as how much spring force can be used before it is all over. With very aggressive profiles, high lift, and high rpm, more spring load is typically needed for valvetrain control, and a roller becomes the natural choice. These days, rollers have become very popular even in less demanding applications. With a roller, there is an increase in area under the lift curve due to the increased velocity at the higher end of the lift range. The result is higher lift and better use of the flow rates from today's cylinder heads. It adds up to a performance advantage. A roller also does away with the need to break-in a cam (as flat tappets require), and greatly reduces the possibility of premature cam failure.
Solid Vs. Hydraulic
When contemplating camshaft types, the first characteristic to consider is whether the lifter is a solid or a hydraulic. Both roller and flat tappets are readily available for either solid or hydraulic applications. Hydraulic camshafts have been the norm for OEM V-8 engine applications for decades, and for good reason. The hydraulic mechanism continuously self-adjusts for zero valve lash, resulting in a quiet and durable valvetrain with no maintenance or precision adjustment required. The self-adjusting nature of a hydraulic lifter lends itself to a simplified valvetrain, often with no provision for clearance adjustment.
In a higher-rpm, high-performance application, the very hydraulic mechanism that works so nicely in a production engine can pose a limitation on power. The hydraulic mechanism in the lifter can become a source of instability in the valvetrain as a result of numerous factors present in a serious engine. The list of common causes of instability is long, including flex and deflection in the valvetrain, high spring loads, insufficient spring control, inertial effects from rpm, increased direct and side loading from aggressive camshaft profiles, or any combination of these factors. A solid lifter can be subject to all of these same problems and conditions, however, there is no hydraulic mechanism to introduce a source of control loss.
The bottom line is when the engine is turned up for serious performance, gaining stable and controlled performance from a hydraulic becomes more difficult, and at some point a solid becomes a better choice. Nonetheless, keep in mind that hydraulic camshafts are very suitable for moderate performance applications, and are routinely run to 7,000-plus rpm in well-developed engine combinations. For street performance use to 6,000-6,500 rpm, a hydraulic system can generally be used without too much trouble.
Comparing camshaft specifications between a hydraulic and solid, keep in mind that the specifications are derived from different standards. This, along with the effects of valve lash, precludes a direct comparison of the specifications. Looking at COMP Cams' specifications, for instance, solids are generally rated at a .020-inch tappet rise checking height, while hydraulics are rated at .008 inch. Even the .050-inch duration numbers are not directly comparable, since the clearance in the lash negates a portion of the duration of the solid. As a rough rule of thumb, a solid requires about 10 degrees more duration at .050 inch to produce roughly the same event duration at the valve.
Filling The Gap
As mentioned, when it comes to maintaining valvetrain control as the engine application becomes more demanding, solids have traditionally been the cams of choice. When considering this decision in relation to roller cams, the intended engine application becomes an important factor. Hydraulic roller lifters have been steadily gaining in popularity for a number of years now, with retrofits available for many engines that were never originally equipped with a hydraulic roller. These cams offer many of the performance advantages of a solid roller such as faster lobe velocity, no break-in, and the ability to accept more spring force than a flat tappet. The downside, as with any hydraulic, is the high-rpm stability problems that can occur by virtue of the hydraulic mechanism. A solid roller, in contrast, eliminates the hydraulic mechanism, but in a street-bound roller application, longevity of a solid roller lifter can become a concern.
Many of the problems associated with running a hydraulic lifter are exacerbated by rpm, high valvetrain loads, and aggressive, high-lift cam lobes-all of the things we want for building horsepower. To overcome some of the difficulties associated with hydraulic instability in the lifter, COMP Cams has designed a line of Short-Travel Race Hydraulic Roller Lifters, which cut the hydraulic plunger travel to a minimum. A factory hydraulic lifter can have nearly .300-inch of piston travel, which provides a tremendously large playground for lifter instability. With these new lifters, the travel is held to just enough for proper lifter adjustment and function, but the range for false motion is held to a minimum. For applications where the reliability of a hydraulic is desirable, but you want to approach the rpm and valve action of a solid, the new Short Travel Hydraulics from COMP seem to fill the gap.
Preload & Valve Lash
Valve lash and lifter preload are both terms related to the adjustment of the valvetrain. In a solid lifter installation, a certain amount of clearance is required to prevent mechanical binding of the valvetrain, while allowing for the expansion of engine components with temperature. The lash provides this clearance via an adjustment typically made at the rocker arm. Camshaft manufacturers provide a recommended lash setting, listed on the cam card for a given cam. A solid cam's lobe profiles are ground specifically for lash in the range of the recommended specification, with a clearance ramp at the initiation of the lift cycle. The clearance ramp gradually takes up the lash to ease the transition into the lift portion of the profile.
In contrast, a hydraulic relies on its internal piston balanced against pressurized oil to remove any slack in the valvetrain system. In order for the hydraulic system to work as intended, the internal piston of the lifter must be within its range of travel at all times. While a typical hydraulic lifter has substantial plunger travel, most camshaft manufacturers favor a setting at the upper portion of the plunger travel. The lifter preload is a specification measuring how far the pushrod is adjusted into the plunger travel while the cam is on the base circle. Typical recommended lifter preload settings are in the .020- to .040-inch range.
We know from the lift specification how far the valve is opened, but it is just as important to know how long the valve-open event lasts. This measurement is referred to as duration, and is referenced by degrees of crankshaft rotation. Essentially, the duration measures the degrees of crank rotation from the time the lifter rises to begin the valve opening event, until the event is completed by dropping the lifter back to the start position. Although duration as defined above seems very straightforward, the actual procedure used for taking the measurements complicates matters. This complication centers on the checking height.
The checking height simply means at what amount of lifter movement off the base circle the duration measurement is actually recorded. The precise moment of the initiation of lift is not easily discernable, so a certain lifter rise specification (checking height) is used for duration calculations. Generally, camshaft duration is given in advertised numbers (sometimes called gross duration), and duration at .050 inch. The difference is the actual start and stop lifter rise specification over which the duration is recorded. With advertised (gross) duration numbers, there is no clear consensus among manufacturers on what lifter rise to use for making the measurement. For instance, when rating hydraulic cams, some manufacturers would use .006 inch as the checking height, while others would use up to .012 inch. Clearly, with the greater checking height, the duration measurement begins later and ends earlier, making the duration seem lower, while the opposite is true at a lesser checking height. With no standardization of checking height, it is difficult to compare camshafts from various manufacturers.
In order to provide a standardized means of specifying duration, decades ago, camshaft manufacturers reached a consensus to list the duration at a standard checking height of .050-inch tappet rise. Since the duration at .050 explicitly defines the checking height, using this duration specification ensures that a known yardstick is used for deriving the duration numbers, allowing meaningful comparisons of duration.
As more duration is added, all else being constant, every valve event is extended, improving cylinder filling at higher rpm. At high rpm, cylinder filling is limited by the ever-decreasing time factor, and a longer duration period will help compensate to a point. Again we must consider cylinder head flow, cubic inches, and the cylinder head's cross-sectional area. A long-duration cam will only take the engine so far in terms of additional top end power and rpm whenever the engine is rpm-limited by the flow capacity and velocity of flow in the cylinder head ports.
Again, keeping all else constant, more duration will make the engine less efficient at low rpm, with more overlap dilution of the induction charge, and the later intake valve closing reducing the trapping efficiency. Therefore, a long-duration cam will cause rough running at idle (lope) and a loss of low-rpm torque. The latter of these factors can be compensated for to an extent by using a higher static compression ratio. Keep in mind that as duration is increased, the cam designer is able to incorporate more lift in the profile. So for a high-rpm, high-powered engine with good cylinder heads, induction, exhaust, and compression, it all comes together for more power.
Lobe Separation Angle (LSA)
Lift and duration adequately describe a given lobe of a camshaft, and indeed we can map the profile of the entire lift curve of an individual lobe by just plotting lift versus duration, however, a running cylinder requires two lobes, an intake and an exhaust, and these events must happen at the appropriate time relative to each other in an operating engine. While many enthusiasts will only look at the lift and duration specifications, these specs tell nothing of the missing piece of the puzzle, the phasing of the intake and exhaust lobes relative to each other. To define this important aspect of the cam's design, we use the lobe separation angle, sometimes called the lobe displacement angle (LSA) or spread.
The lobe separation angle is the simple angle between the peak lift point of a cylinder's intake and exhaust lobe pair. This direct angular measurement at the cam is specified in degrees at the cam, in contrast to degrees of crankshaft rotation, which turns at twice the camshaft speed. As you might imagine, the relative phasing of the intake and exhaust valve events has a serious affect on engine performance. A camshaft with a lobe separation angle of zero would have the intake and exhaust valves opening and closing at the same time, and that clearly won't work. In fact, typical lobe separation angles run in a range from about 102 to 116 degrees, with most performance aftermarket cams ground between 106 and 112 degrees. Even within this relatively narrow range of common LSAs there is quite a dramatic difference in how the engine will behave when comparing a narrow LSA such as 106 degrees, to a wide one such as 112.
Let's look at how a change in LSA will affect the valve events (assuming a fixed installed centerline). As the separation is narrowed, the exhaust opens and closes later, while the intake will open and close earlier. The later exhaust closing and earlier intake opening directly adds to the cam's overlap, and the overlap effects are significant (See: Overlap). Rather than individually looking at all the subtle interrelated effects created by a change in lobe separation angle, it is more useful to just bottom-line it here. As the lobe separation is narrowed, expect the cam to exhibit a nastier idle with more lope. Typically, once "on the cam," the peak torque and horsepower are improved, however, the engine will drop off more quickly past peak horsepower rpm.
|EFFECTS OF LOBE SEPARATION ANGLE
Of all the specifications related to a camshaft, "lift" is the easiest to understand. Lift simply refers to how far a valve is opened off the valve seat, with the specification given in thousandths of an inch. Where does the lift specification come from? The gross lift is the cam's actual "lobe lift," multiplied by the rocker ratio. As a cam lobe rotates from the base circle to the ramp, the lifter is displaced upwards by the eccentricity of the lobe until the point of maximum lift is reached. This action at the lobe of the cam is called the "lobe lift," and the lobe lift is generally given as a base specification on a cam card or in a catalog. The lobe lift isn't the same as the amount of lift at the valve, since valve lift is the product of the lobe lift and rocker ratio. Typically, the factory rocker ratio for most production V-8 engines ranged from 1.5:1 to 1.75:1, depending upon the engine type, but aftermarket rockers are available in a wide selection of ratios. Usually the lift given in manufacturers' catalogs is based upon the cam's lobe lift multiplied by the engine's original rocker ratio.
While the gross valve lift specification for a given cam is generally derived from the lobe lift and factory rocker ratio, the actual lift delivered may vary substantially from this spec. Clearly, if the rockers are a ratio other than stock, the valve lift will change. The gross valve lift can be determined for any rocker ratio by simply multiplying the lobe lift specification by the rocker ratio. For instance, a cam with .320-inch lobe lift will provide .480, .512, .544, or .576-inch lift with rocker ratios of 1.5, 1.6, 1.7, or 1.8:1, respectively.
So is the gross lift as given above exactly the true lift at the valve? Not necessarily, since the actual ratio any given rocker delivers may be somewhat off the quoted ratio specification, and subtle valvetrain geometry factors can also have a measureable affect on the true valve lift. The only way to precisely know the valve lift is to take a direct measurement using a dial indicator on the valve stem with the valvetrain fully assembled and adjusted. Still, this being the case, the gross lift as calculated by the lobe lift and rocker ratio is generally a good approximation when considering lift.
Now the big question: How much valve lift is right for my engine? Generally, more lift equates to more power and torque, however, the trade-off is component life, reliability, and the ability to control the valvetrain at higher rpm. It is important to consider the cylinder head's flow characteristics when contemplating lift, and target a valve lift that is well matched to use the head's capabilities. Your returns diminish rapidly when using higher lift on a stock-style head that may see no flow increase or even a drop in flow as the lift is increased. Conversely, applying less-than-optimal lift on a high-flow aftermarket cylinder head capable of very good high-lift flow simply is not taking advantage of the flow potential presented by the cylinder head.
Installed Centerline Angle (ICA)
All of the camshaft specifications listed so far-lift, duration, and lobe separation angle-are attributes of the camshaft itself. The installed centerline angle specification (ICA), in contrast, is a specification that describes how the cam is installed in the engine. What we are referring to here is the camshaft timing, or phasing, and this is measured relative to the crankshaft, and given as a point in degrees of crankshaft rotation.
When a cam is installed, it must be phased to operate the valves in the correct relationship to piston position. A simple reference is provided by the marks on the timing chain that will normally put the camshaft in the ballpark. Since the installed centerline angle will have a direct affect on every valve event relative to piston position, the true installed centerline will have a significant impact on the running characteristics of the engine. The process of "degreeing-in" the camshaft measures the installed centerline.
When a camshaft is installed at a position (in degrees of crank rotation) that is equal to the lobe separation angle, the cam is said to be installed "straight-up." For instance, a cam ground on a 108-degree lobe separation angle will be "straight-up" when installed at an intake centerline angle of 108 degrees after TDC. This will also put the exhaust centerline angle at 108 degrees before TDC, and split the overlap event evenly over TDC. This "straight-up" or "split-overlap" camshaft position is the zero reference for the cam position, and any advance or retard is relative to a "straight-up" installation. Advancing the cam will move all of the events to an earlier position relative to crank rotation, while retarding the cam will have the opposite effect. Advancing the 108-degree LSA cam from the example above by 4 degrees will put the intake installed centerline angle at 104 degrees ATDC, and the exhaust at 112 BTDC, and move each valve event 4 degrees earlier relative to crank position.
Normally, we find positive effects from moderate amounts of cam advance, while retard can produce negative results. Consequently, most aftermarket cams are ground with a small amount of advance relative to the keyway or pin, usually 4 degrees. An advanced position favors low-rpm operation, helping idle quality, cylinder pressure, low-rpm torque, and vacuum. Retarding the cam deteriorates these characteristics, and in many instances may not show any benefit at higher rpm.
Overlap is a measurement of duration, given in degrees of crank rotation, specifying the period during which both the intake and exhaust valves are both open off their seats. During overlap, there is open through-flow between the intake and exhaust ports of the engine, and this can be a very useful situation for the production of power. As with any duration measurement, the actual checking height used to determine the overlap will directly impact the measured overlap specification, so the checking height needs to be known to make meaningful comparisons of overlap.
It is useful to touch on the dynamics of overlap, to gain some perspective of how it affects the engine's output. In a running engine, the exhaust flow within a header system produces significant scavenging force in the form of negative pressure at the exhaust valve. This is a combination of the inertia of gas flow and reflected pressure waves. These forces combine to clear the cylinder and chamber of residual exhaust gasses, and draw intake charge to improve cylinder filling. The downside is that the negative pressure generated by both inertial and pressure wave effects are rpm sensitive. At low rpm, the open area (overlap duration and lift) can work in the opposite direction, causing reversion and the subsequent lope, rough idle and even misfire.
The amount of overlap is not something that can be altered independently in a camshaft design, but rather it is determined by the duration and lobe separation angle. The flow area during overlap is also directly influenced by the overlap lift, and as the overlap duration increases, so does the lift during overlap, multiplying the effect. As duration is increased, and/or the lobe separation is narrowed, overlap increases as a direct result.