Flat Tappet Vs. Roller
There was a time when roller cams were strictly the domain of race engines, in the form of early solid-roller arrangements. Compared to a flat tappet, which is limited in terms of opening velocity by the lifter diameter, a roller cam allows a higher opening velocity for faster valve action and more lift for a given duration. This adds up to a distinct power advantage. The roller design also benefits from the ability to handle much higher valvespring loads, which goes hand in hand with fast valve action and high rpm. With a flat tappet in a high-performance application, the limitation on spring pressure is the loading of the interface between the cam and lifter. The engine builder has to carefully balance the required spring loads against the potential for increased wear and even outright component failure. With a flat tappet, there is always a trade-off in camshaft longevity as the valvespring tension is increased, while aggressive lobe designs and high rpm demand high spring loads to maintain valvetrain control.

As with any hydraulic, the rpm potential is the limiting factor with a hydraulic roller. Once instability in the valvetrain causes the hydraulic mechanism to lose control, power production is over, however, with good parts in the valvetrain, lightweight components, and the appropriate valvesprings, the rpm potential should be enough to meet the needs of most street applications. Add the new breed of limited-travel hydraulic roller lifters such as those from COMP, and the rpm potential can be well into solid-roller territory. For a street application, a hydraulic roller has every characteristic to recommend it, from performance to longevity. The only real drawback of a hydraulic roller is the cost, which is substantially higher than a flat-tappet cam, especially in retrofit applications.

We talked to several professional engine builders including Engine Masters Challenge champions Jon Kaase and Tony Bischoff, both of whom greatly prefer roller cams for their customers’ engines. As Bischoff says, “For a street engine, I try to emphasize the benefits of a hydraulic roller in durability and reliability as top factors over a flat tappet, and the bonus is we can make more power. I will not build a flat-tappet engine unless the customer really insists.”

The solid-roller arrangement allows very high spring loads and very quick action at the valves, making it the best choice for all-out race applications. While ideal for racing, long-term high-mileage use is not a strong point of a solid roller. The common problem here is bearing failure in the lifters, with often disastrous consequences. As Chris Mays at COMP Cams says, “You can run a solid roller on the street, but it takes a certain kind of enthusiast, someone who is willing to perform routine inspection and maintenance. The newest direct oiling lifters help in this regard, but this isn’t the cam for someone who would rather not pull a valve cover once the engine is built.” Enthusiasts looking for the ultimate in horsepower and rpm may opt for the solid roller in a street application, but it comes with a trade-off in longevity. As Rod at Isky Cams tells us: "I don’t recommend a solid roller until it is more of a weekend toy or a bracket racer or race car. You can get them to last, but it costs quite a bit to get it set up properly. For guys on budgets, it is sometimes not cost effective."

Street vs. Race
In a race application, the goals of cam selection are very clear cut: Make the most power possible within the engine’s operating rpm range, within the constraints of budget, and acceptable maintenance and longevity. Typically, the budget here will steer the selection of cam type, with a large-journal billet solid roller being the ultimate piece, but only if substantial finances make it possible. Most often in the non-pro ranks of racing, the cam selection and the entire engine combination is a compromise based on the resources available. To make the most of any combination, however, selecting the optimal cam becomes a critical factor. Most engine builders will select the camshaft specifications based on previous experience, and trial and error in testing on the dyno and racetrack. In a race application, factors such as idle quality, vacuum, and low-end torque below the effective operating range take a back seat to horsepower.

So, how would one select a cam within the above constraints to maximize engine power? Frankly, with all of the variables involved, the best resource these days are engine simulation programs such as COMP Cams’ CamQuest program, which will allow inputting detailed engine specifications and the user to juggle cam specifications while studying the projected results. Outside of years of experience, these programs are the best aid available to the DIY engine builder. Nonetheless, experienced engine builders will often just make a judgment call, mentally factoring in the targeted rpm range, head flow, compression ratio, induction system, and then draw on experience to make the call on cam specs, within the cam type dictated by the budget of the effort.

In a street application, cam selection becomes more complicated, as the end user’s goals and expectations cloud the process with dramatically varying goals. To begin with, the definition of a “street” cam is as individual as the guy behind the wheel. For some, the chop of a radical lope is the ultimate prize, while to others a powerful but smooth-running combination embodies the definitive street cam. Some are happy to pull the lifters of a solid roller each 5,000 miles for inspection in the course of routine maintenance, while others expect 100,000 miles without pulling a valve cover. Between these polar opposite objectives, you’ll find enthusiasts at every level of the spectrum. Even with the same engine combination, each will have their own definition of the ultimate cam.