Solid VS. Hydraulic Lifters
Thanks to recent innovations in reducing valvetrain mass, hydraulic roller cams are revving higher than ever. However, for sheer performance, solid lifters still reign supreme for one simple reason. "Stiffer valvetrain components provide better control as engine speed increases, and this increases the rpm limit of the valvetrain, since deflection is decreased," says Billy. Deflection robs the engine of the valve-open duration it needs as engine speed increases. Bigger cams are conducive to high-rpm performance, but as rpm increases, the loads on the lifters, pushrods, and rockers increase as well. Due to deflection, this means the engine sees a smaller cam at high rpm. "Up to about 6,000 rpm, the inherent deflection of hydraulic lifters isn't an issue, and many are used satisfactorily to about 7,000-7,500 rpm. However, even if valvetrain control is maintained to 7,000 rpm, horsepower improvements are still common when switching to a solid lifter."
Rocker Ratio"For a while, everyone was in love with big rocker ratios because they helped so much in NASCAR applications. Now we tend to use as little rocker ratio as required to get the motion we want in most applications. This reduces the load on the pushrod, thereby helping increase stiffness. Not wanting to sacrifice stiffness is what kept Pro Stock engine builders from going up to the 2.5:1 rocker ratios seen occasionally in NASCAR Cup applications; however, those ratios are still in the 1.7:1-2.0:1 range, depending on the cam journal's size. Note that up to 70mm (2.756-inch)-diameter camshaft journals are used in NHRA Pro Stock. The diameter limit in NASCAR is 60mm (2.3622-inch), a stock LS1 has 55mm journals (2.165-inch), a stock big-block Chevy is 1.949-inch, and a small-block Chevy uses a stock 1.868-inch journal diameter. The bigger journals allow roller lobes with more tappet acceleration and more lift. If we need more lift and acceleration at the valve and can't increase what we are doing to the tappet, then the rocker ratio must be increased. Again, we are dealing with a balancing act, and there are no perfect answers."