Nothing determines the "personality" of an engine more than the camshaft. Think of the cam as a mechanical computer, calling the shots on when and how the valves open and close. That computer is “programmed” by the grind imparted on it when the cam is manufactured, and it communicates that knowledge to the valvetrain with every cycle of the combustion process. To describe the characteristics of a cam lobe, we have specifications, namely lift and duration. The further specifications of lobe separation angle, overlap, and installed centerline angle provide more information on the cam’s design. Trying to select a camshaft without understanding these specifications is a blind shot at best. We don’t have the space here to delve into remedial cam card reading, but we fully explain each of these specifications in an accompanying piece (keyword search: "camshaft basics") at www.PopularHotRodding.com.

Assuming you know your basic camshaft specs, the very first decision to be made when contemplating cam selection is the actual type of cam to use. Here we have four basic choices: solid roller, hydraulic roller, solid flat tappet, and hydraulic flat tappet. Each has its advantages and disadvantages, but there is a great deal of crossover in their appropriate applications. Making an informed decision on what type of cam is right for you requires knowing the differences in their characteristics.

Hydraulic vs. Solid
Mechanically, hydraulic cams feature a lifter with an adjustment mechanism that will self-adjust to a zero lash setting. The upshot here is that valvetrain adjustment becomes an automatic function, while providing for quiet operation. In contrast, a solid-lifter cam requires a small amount of mechanical clearance in the system, provided by the valve lash. The lash must be checked periodically and adjusted as required to maintain the required clearance. Since the lash is a clearance, there is a greater amount of noise transmitted from the valvetrain. The amount of noise varies from almost indiscernible, to fairly intense, depending upon the amount of lash and the design of the lash take-up portion of the cam profile. For some enthusiasts, the sound generated by a solid cam is actually part of the appeal.

At what rpm point can we expect the practical potential of a hydraulic to be a limiting factor? That answer depends on a wide range of variables. The major factors at play here start with the cam lobe design, and include the spring load, valvetrain stability and inertia, the stability and load of the springs, and valvetrain weight, including the springs and valves. If instability is generated anywhere in the system from the cam to the valve, the hydraulic mechanism in the lifter will begin to lose control. The rpm point at which these problems begin to occur varies widely based on these factors. A Gen III Hemi engine, for instance, with a good smooth cam profile can run with stability on a factory hydraulic roller lifter to 7,200 rpm, with the stock valvetrain. We have seen stock-style big-block Chevys with heavy ⅜-inch stem valves fall flat due to lifter instability at under 5,500 rpm with hydraulic roller setups. The situation with hydraulic flat tappets follows a similar pattern.

Solid-lifter cams lack the hydraulic circuitry in the lifters, providing a solid, mechanical link from the cam to the valve. Without the hydraulic system, a solid eliminates the potential for high-rpm valvetrain instability originating at the lifter. This makes a solid lifter better suited for high-rpm performance. So, is a solid cam a guarantee of rpm capability? Only if the rest of the valvetrain, particularly the valvesprings, allows the engine to reach its ultimate rpm potential. Solids are the only way to go in applications running very high rpm.

To put a benchmark on the general rpm breakdown for solid versus hydraulic applications, a typical hydraulic should be very functional to 6,000 rpm with moderate preparation. Solids are generally the way to go if the peak rpm is in the 7,000-plus rpm range, while the 6,000- to 7,000-rpm range can be considered the crossover. Hydraulic lifter performance and rpm potential can be increased with a well-developed valvetrain and lightweight components, and the use of limited-travel hydraulic lifters.