When we talk about acceleration rates and high lifter velocities, it's worth mentioning that a good set of valvesprings are also essential to ensuring you make all the power the cam can provide. If you remember from last month, we tested a 454 H.O. big-block Chevy that had always frustrated us in trying to make decent power with that hydraulic roller cam-equipped engine. The simple addition of a better set of valvesprings was worth power across the entire power band of the engine, peaking with an over-20hp increase at the top. This was strictly the result of improved valve control, which is a result of the higher valve velocities created by a hydraulic roller cam. The point here is that all these pieces fit together to create the power.
How much power is a hydraulic roller cam worth over a flat tappet? As this story is written, we're in the middle of testing a hydraulic roller cam in our Chevy 350 H.O. engine compared to previous tests with a flat tappet cam. The cams are not exactly the same at 0.050-inch tappet lift, but they're close. You can read the story in this issue, but the hydraulic roller was worth over 25 hp more at the top, while torque improved throughout the entire rpm curve from 2,500 to 5,800 rpm. When you can pick up power all the way through the rpm curve, you're definitely going to run faster. What's even more impressive is that this engine responded this well with only a pure stock pair of iron Vortec heads fitted with a set of Comp's new beehive valvesprings.
ConclusionThe beauty of a flat tappet hydraulic cam is that it is simple, inexpensive, and if matched properly with the engine combination, will offer up excellent power. But if you're looking for that next level of street performance, hydraulic roller cams are the hot ticket.
Roller Vs. FlatThis is an intake lobe comparison of a flat tappet hydraulic versus a hydraulic roller lobe, with the same duration at 0.050 numbers for the flat versus roller camshafts. There are several interesting things going on here. The most obvious point is that the roller cams offer significantly more valve lift. The Comp hydraulic roller generates 0.026 inch more maximum valve lift than the Comp flat tappet hydraulic while the advertised duration is 8 degrees longer. The Crane roller also offers the same duration at 0.050 with almost 0.050 inch more lift than its comparable flat tappet cousin.
Perhaps the most critical point in this discussion is the duration at 0.200-inch tappet lift numbers. Note that the Comp hydraulic roller cam offers 8 more degrees of duration (145 vs.137) between the 0.200-inch lift points on the opening and closing sides of the lobe. The hydraulic roller's added duration keeps the valve open longer compared to the flat tappet, which means the intake port can stuff more air into the cylinder. In essence, it's like installing a longer duration camshaft without sacrificing the lower-speed torque of a longer advertised duration cam.
It's also worth noting that the hydraulic roller's advertised duration numbers are longer than the flat tappet lobe. This reveals the net effect of the flat tappet cam's higher initial acceleration rate off the seat. We'll get more into that with the next sidebar, but you can see it here in the numbers since the hydraulic roller cam's advertised duration is 8 degrees longer than the flat tappet even though they offer the same duration at 0.050-inch tappet lift. The roller's slower initial acceleration is also reflected in the "HIT" numbers. This is not always true, however. Some Crane hydraulic rollers offer very short advertised duration numbers.
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