 |
Its obvious from the shape of a hydraulic-roller cam lobe compared to a flat-tappet that the roller offers more duration at the higher lift. The flat-tappet cam lobe is more pointed while the roller cam nose is more blunt, which means the roller is holding the valve open at higher lift longer. Thats a good thing. |
The biggest problem with hydraulic-roller cams is that the tall lifters are heavy. The added weight makes it difficult to control at high rpm without using stiffer valvesprings. Unfortunately, heavy springs tend to collapse hydraulic lifters at high rpm. AirFlow Research sells a Hydra-Rev kit for the small-block Chevy that adds rpm by adding spring pressure to control the lifter body. |
Because a roller lifter can accommodate a more aggressive lift curve, a roller cam can produce much more lift than a flat-tappet cam for the same duration. |
4 Mechanical-roller cams offer even more lift and area under the curve than hydraulic rollers, but can suffer from durability problems if used in high-mileage applications. Comps Endure-X cure is to aim lube directly at the lifter roller. Excessive spring pressures are still the ultimate problem for big-lift cams on the street. |
Roller cams require a much harder steel surface than flat-tappet cams because of higher loads. In the early days of roller cams, this meant you had to run a soft silicon-bronze gear that wore out quickly. Now, most cam companies offer iron gears on the end of the steel shaft to accommodate a standard iron distributor gear. |
This illustration shows how a flat-tappet lifter will dig into an aggressive roller profile, which is why flat-tappet cams cannot offer the same profile as a roller cam. |
The new darling of the camshaft world is the hydraulic-roller cam. GM made it fashionable back in the late 80s when Chevy put hydraulic-roller cams in production Corvettes and 87 Camaro engines. At first, the performance community viewed this innovation as a low-performance option, but cam companies soon released performance versions of these cams that are now making serious horsepower. We decided to take a look at what makes roller cams so powerful and popular.
Before getting into the specifics, lets take a look at some basic cam tech to find out why a roller works so well. A hydraulic-roller cam offers much more aggressive lift-curve capabilities compared to a flat-tappet cam. In order for a flat-tappet cam to generate as much lift as a roller, it requires more duration. Flat-tappet lifters will actually dig into the lobe flank if the lobe is designed too aggressively. Roller tappets do not suffer that problem, so the designer can put much more lift into a roller-cam lobe. The accompanying Profiling graph makes this easier to understand.
Curves Ahead
If you compare a hydraulic-roller cam to a flat-tappet hydraulic cam with similar duration at 0.050-inch numbers, the hydraulic-roller cam will always have a longer seat-duration figure. While the roller configuration allows faster ramp acceleration, it also suffers from slow acceleration off the seat compared to a flat-tappet cam. Therefore, the advertised-duration numbers are slightly longer.
This means that a hydraulic-roller cam with the same duration at 0.050-inch lift as a flat-tappet hydraulic will not idle exactly the same. If youve read this months story on overlap (The Overlap Chronicles, pg 90) then you know that more advertised duration increases the amount of overlap, which will cause a somewhat lumpier idle. This is not a big problem, but worth noting if you are considering swapping in a hydraulic-roller cam. If a smoother idle is important, the roller cam could easily be ground with a wider lobe-separation angle (110 to 114 degrees for example) to reduce the overlap.
Powerful Profiles
At first it might appear that the real key to a hydraulic-roller cam is the additional lift. This is true, as we point out in the Profiling sidebar. Adding this additional duration above 0.200-inch tappet lift is really aimed at increasing airflow. The true advantage of a hydraulic or mechanical roller is that the profile can hold the valve open longer during the time that the cylinder head offers the most potential flow.
While the right hydraulic-roller cam will improve power even on an engine with stock heads, the real potential lies in combining a roller cam with an engine equipped with a set of good-flowing cylinder heads. While this includes monsters with huge ports, you should not overlook even mild heads like the iron Vortecs. These heads offer outstanding mid-lift flow numbers in the 0.200- to 0.400-inch valve-lift range. Slide in a roller cam with longer 0.200-inch duration and avoid long advertised duration and a late intake closing and you have a recipe for incredible power in an engine that is still very streetable.
Consider that the Vortec head does not offer killer flow numbers above 0.500-inch lift. What it offers instead is outstanding flow between 0.200- and 0.500-inch valve lift. This is where the hydraulic-roller cam shines. Combine the two and you have a powerful combination. Of course, there are other heads that also offer this same kind of great mid-range flow potential, but you can buy a complete pair of Vortec heads for under $450. Use the money you save with the heads to purchase a hydraulic-roller cam package and youre on your way to big-time power.
Ups and Downs
There are some great reasons for stepping up to a hydraulic-roller cam package, but all is not perfect in the roller-cam world. One glance at the pricing sidebar will reveal that this better technology comes at a hefty price. This price includes adding in more expensive roller rocker arms, stronger pushrods, and better valvesprings. If you plan to bolt a hydraulic-roller cam into a pre-87, non-roller-cam block, the retrofit lifter kits can get expensive. This will also require a thrust bearing on the front of the cam to prevent cam walk.
This isnt required with a flat-tappet cam since the lobes are cut at a slight angle to offset the cam thrust movement thats inherent in a roller cam. This is just one more piece to the hydraulic-roller cam investment portfolio thats important to know.
One way to control this additional expense on your next engine buildup is to start with an 87-or-later hydraulic-roller cam block. For example, you can pick up one of these inexpensive short-blocks as the starting point. This allows you to use the factory hydraulic lifters and retaining system, which can be less expensive than the aftermarket retrofit kits. The Hot Deal sidebar prices out using the GM Performance Parts HOT cam and lifter kit. You can also add to the great deal using a GMPP factory hydraulic-roller cam gear and chain setup that costs under $40.
Bottom Line
As usual, if you want to make more power, its gonna cost more money. The good news is that a properly selected hydraulic-roller cam offers the potential to make more horsepower and more torque without sacrificing much in the way of street manners. Assemble the right combination of parts and you might just find yourself suffering from massive traction problems. Thats a good problem to have.