Bigger is better, baby, and the hot rodder's battle cry is nowhere more in vogue than with the part-time engine-builder hooked on big-lift cams. But there's much more to the science of lobe crafting than just picking a big valve lift number and stuffing it in the engine. Camshafts may be simple-looking devices, but the cam is actually the brain for your engine. The heads determine the flow capacity, but the cam is what really determines the ability to make power. If you haven't yet run through the cam basics story, do that first. Once you've got a grasp of the terms and basically how a cam works, this story will make a lot more sense.
Roll OnThe new car companies were the ones who made hydraulic roller cams popular. Ironically, the OEM companies moved to hydraulic rollers in an effort to reduce internal friction and improve fuel mileage. We're going to show you why these cams will help you make more power. It has to do with the way the roller cam contacts the lobe of the cam. To begin with, flat tappet cams are generally built out of cast iron, specially ground, hardened, and sent out the door. This is why they are relatively inexpensive at around $100 to $150 apiece. Roller cams must be built out of hardened steel, and most current street rollers now come with an iron gear installed to make it compatible with a stock distributor gear. This better material and extra machine work all costs money. That's why roller cams are two to three times as much as a flat tappet. Plus you have to add in the cost of roller tappets and the attendant accessories. We'll deal with all those variables in the story on converting to roller cams.
So why go through all this extra effort and expense? Because roller cams can make good power, and they do so while maintaining decent street manners and throttle response. The trick is in some interesting physics that occurs when you employ a roller tappet. First, let's look at a flat tappet style cam.
Flat tappet cams are limited in velocity by the diameter of the lifter. Basically, you can only smack open a valve with a flat tappet lobe so quickly before you run the edge of the lifter into the lobe. The larger the lifter diameter, the quicker you can move the lifter. Small-block Chevy cams are limited with a relatively narrow lifter that's 0.842 inch in diameter. This limits the amount of lift you can generate with a given number of degrees of duration. In other words, to make more lift, you need to add duration. The problem with added duration for street cars is that this moves the peak torque rpm point higher in the rpm band, hurting low-speed torque and making the engine less streetable.
With a hydraulic roller camshaft, the limitation is that the roller cannot accelerate as quickly off the seat compared to a flat tappet lobe. However, as lift increases, the roller can move the lifter at a higher velocity, creating more lift sooner in the mid-part of the opening and closing sides of the lobe, generating more lift without radically increasing the duration. Our comparison chart of two cams with the exact same duration at 0.050 makes this point more clear. Basically, you're able to generate about an additional 0.030 inch of valve lift with a roller cam with the same duration at 0.050 numbers as a flat tappet hydraulic.
Of course, this also means you will need an engine combination that can take advantage of this. A relatively stock 350ci engine with weak heads and a near-stock exhaust and low-rise intake will not benefit nearly as much from a more aggressive roller cam. Conversely, a 383 with a good set of heads, compression, excellent intake, a 750-cfm carburetor, and a set of headers with a free-flowing exhaust is a combination that could take advantage of a hot hydraulic roller cam.