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Secrets Of Camshaft Power

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Tappet Dancing

Flat Tappets
Hydraulic flat-tappet camshaft and lifter systems are the most popular configuration for street applications. They provide quiet operation, low maintenance, easy installation, great response, and good power. But hydraulics can "pump up" at high rpm, leading to rapid power loss caused by valve float.

Solid flat-tappet lifters offer a stiff system that can more easily maintain control at high rpm. They require periodic valve lash adjustments, but these can be minimized with good rocker adjustment locking devices. For street use, the crossover point between hydraulic and solid lifters is somewhere between 6,000 and 7,000 rpm, depending on the engine's specific valvetrain configuration and weight.

Mechanical cams usually need about 8-10 degrees more duration to have a comparable power band to a hydraulic lifter cam in the same engine. Also, a mechanical cam's gross valve-lift figures don't include lash, so the recommended lash must be subtracted to come up with the theoretical valve lift.

With flat-tappet cams, the maximum velocity allowed by the tappet before the contact point between the tappet and lobe skates off the edge and causes failure is directly proportional to the tappet diameter. A larger diameter tappet allows the use of a profile with higher maximum velocity. Profiles designed with higher maximum velocity can have more area and more lift for a given duration than similar profiles with less maximum velocity. Most GM applications use a 0.842-inch lifter foot diameter, but Fords and Chryslers use 0.875-inch and 0.904-inch, respectively. This gives these engines a theoretical advantage (albeit at the cost of a slightly heavier lifter) when restricted to a flat-tappet profile if the profile is ground to take advantage of it.

Roller Tappets
Tappet diameter becomes irrelevant with roller lifters. Solid roller lifters allow much higher velocities than flat-tappets and can tolerate the increased spring forces necessary to maintain valvetrain control with these extremely aggressive designs. The typical powerband of flat tappets is 3,000- to 3,500-rpm wide, yet roller lifters usually have a 4,000-4,500-rpm wide band. This is because rollers can hold the valve on the seat longer, then open it quicker. However, the initial departure from the valve seat is slightly slower than a flat tappet because of geometrical limitations. At some point, as rollers are designed for quicker and quicker acceleration off the seat, the designer must go to an inverted ramp profile. There is a limit to how much inversion is possible before the flanks become too difficult to grind. Overall, the increased area permitted by the roller's higher average velocities more than compensates for its slower initial acceleration. Lifter wear was the main drawback to rollers, but new lifters are being introduced that provide greatly increased durability. Currently, the main drawback is cost.

Hydraulic roller lifters provide many of the same advantages as solid roller lifters. However, they are more rpm-limited than hydraulic flat tappets. This is due to the hydraulic roller's higher overall weight, which makes it hard to utilize the more aggressive potential of rollers and maintain stability over 6,500 rpm without relying on very high spring forces that tend to collapse the hydraulic plunger. Further development may lead to improvements in this area, but cost still remains a problem.

Crane Cams
1830 Holsonback Drive
Daytona Beach
FL  32117
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