Solid Vs. Hydraulic Lifters - Battle Of The Bump Sticks

Without a doubt, solid cams carry a certain mystique. In the musclecar days, solids were factory-fitted in some of the hottest iron out of Motown, including the early street Hemi. There was a little extra status when laying out the engine specs and telling the boys, " . . . it's got a solid cam." Actually, all cams are solid. The real difference is in the lifters, with a corresponding change in cam-lobe profile to accommodate the requirements of the lifter. Solid cams have a reputation for higher rpm power, and for some, the image of a race-only piece. Solids require periodic adjustment, make noise, and are just a little different from what the average guy is running. For some, that's reason enough to want to run one.

We, however, were more interested in seeing what differences could really be found in power output. First, we offer a rundown of the differences between solid and hydraulic lifters, and why there may be power to be gleaned.

Hydraulics
Since the '50s, with a few notable exceptions, hydraulic lifters have been a Detroit norm. Hydraulic lifters self-compensate for valvetrain clearances, giving the consumer years of maintenance-free service. While hydraulic lifters themselves are much more complex than standard solid tappets, the accompanying valvetrain could be built much more simply and at a lesser cost, doing away with the provisions for valvetrain adjustment. Simple dirt-cheap, one-piece, stamped-steel rockers were the inevitable result. Best of all, the travel in the hydraulic mechanism soaked up variations in production tolerances with ease, undoubtedly streamlining the production process, eliminating the need to set valve lash at the engine plant and down the road in service. Hydraulics self-adjust to zero lash. They provide unrivaled quietness, a primary goal in OE engine design.

Hydraulics Or Solids For Performance?
All-out racing performance was never on the agenda when hydraulic lifters were conceived. However, the vast majority of performance cams sold are unquestionably hydraulic grinds. Some of the same attributes that made them a favorite with Detroit hold favor with many enthusiasts. Since most engines were initially set up with hydraulic cams, hydraulic performance cams are usually the most cost-effective replacement choice. Making a switch to a solid grind can come with quickly escalating costs, most often requiring the upgrade to adjustable rockers and compatible pushrods. Along with the cost, quieter operation and never having to adjust the valves make the hydraulic a tempting choice for dual-purpose applications.

Hydraulics work extremely well in moderate rpm applications-the range of most mildly modified street engines. Move up the performance, though, and the very hydraulic mechanism that makes them oh-so-sweet in a milder application can create problems. Why? Under the stresses of high rpm, the hydraulic piston, which serves to zero-out the clearances in normal operation, can either pump up or bleed down. These are two very different phenomena, both of which can hinder hydraulic-lifter performance.

All hydraulic lifters can absorb a small portion of the cam's lift profile in running, through fluid bleeding past the lifter's plunger piston during the lift cycle. In stock or mild street applications, absorption is likely negligible. Highly aggressive cam profiles and spring loads in a radical street or racing application can strain the hydraulic lifter's mechanism to the point where some performance potential is lost through absorption. Lifters with tight internal clearances and valving most accurately follow the cam's profile, and are termed stiff.