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.