Test Time
With all this background tech information jammed into our heads, now it was finally time to put down the theory books and get our hands dirty. The small-block we decided to beat on was the healthy 383ci we used last month for the giant "Speed Parts Tested" cover story. The engine configuration for this test is a little different but includes 10.5:1 compression from a complete Lunati rotator assembly, a set of Dart CNC 227 heads, a Holley Keith Dorton single-plane intake manifold, and a Barry Grant 850 Mighty Demon carburetor. We started the test with the smallest cam, the Comp Xtreme Energy 284 hydraulic flat-tappet version matched up with a dual-spring package, titanium retainer, and the appropriate-length pushrods. The flat-tappet cam made respectable peak power at 507 hp at 6,200 rpm and 489 lb-ft of torque at 5,000. The beauty of a flat tappet is its decent power and great torque, all delivered at an affordable price. But now we were looking forward to ramping up the power numbers with the roller cams.
The hydraulic roller slid right in along with the taller Comp Cams hydraulic roller lifters. The taller lifters also demand shorter pushrods and, of course, a swap to a stronger set of Comp dual springs, which increase the spring load in order to help control the valves. As we expected, the hydraulic roller made more peak power than the flat tappet along with slightly more torque due to its more aggressive roller lobe design. This helps justify its increased cost. The hydraulic roller jumped the power up to 530 at 6,200 rpm while the torque also grew from 489 to 502 at 5,200. That's a solid 13 lb-ft increase of torque and 23 hp. Also note that both hydraulic cams peaked at almost the same rpm for both torque and horsepower. But all this did was motivate us to bolt in the mechanical roller.
Our 383 small-block Chevy test mule consisted of a 383 with a Lunati forged crank, rod, and piston package along with Dart CNC 227 heads, a Holley single-plane intake, and this Barry Grant Mighty Demon 850-cfm carburetor. We used Westech's dyno 1 31/44-inch headers.
By now we were getting good at swapping cams, and the motor had barely cooled down before the new mechanical roller was in place and the springs and pushrods swapped once again. With 0.016-inch lash dialed in on the intake, the duration at the valve was exactly the same as the flat-tappet hydraulic cam, yet this mechanical roller setup rocked when it came to peak horsepower. Once we pulled the throttle handle, however, it quickly became apparent that while the peak horsepower was up over the hydraulic roller, the mechanical and hydraulic roller cam torque curves in the middle were almost identical, something we didn't expect. The mechanical roller's torque peak was actually down 6 lb-ft to 496 at 5,200 compared to the hydraulic roller, but made up the difference at peak horsepower with an impressive 539 at 6,600, which is up 9 hp over the hydraulic roller.
The best way to evaluate this test is to look at the power averages for all three cams. Because all three cams were chosen with the same duration at 0.050, there's not a huge difference in power averages between the three. The mechanical roller clearly owns the peak horsepower title with a stout 32hp advantage over the flat tappet. The mechanical roller also is up 12 hp and 11 lb-ft of torque average, which is a significant difference. But let's not overlook how well the flat-tappet cam performed, especially if we factor in the additional cost of either roller cam package. Of course, there's also the hassle factor of the flat-tappet cam, with both break-in and longevity concerns with current engine lubricants. But the power difference clearly points to the best power-per-dollar choice being with the flat-tappet cam. Looking at all the data, it would have been interesting to see how a flat-tappet mechanical cam would have fared in this test.
If you look at the graph of the three power curves, this may help with the concept of which lifter style cam is the correct one to use. Remember that the easiest way to make normally aspirated horsepower is to make the same torque at a higher engine speed. If you look at the flat-tappet hydraulic horsepower curve, it tends to flatten out at 5,200 rpm, while the two roller cam curves extend peak horsepower well past 6,000. If you plan to only shift your engine at 5,500 rpm or below, there's no reason to spend the extra money for a roller camshaft since all three cam torque curves up through around 5,200 rpm are very similar. The roller cams deliver far more valve control and rpm potential to make more horsepower. Also notice how the hydraulic roller tends to drop off at around 6,200 while the mechanical roller cam continues to make power up through 6,600. We think that this slight dip in the hydraulic roller curve is probably due to pushrod deflection. Since the price difference between a hydraulic roller and a mechanical roller cam is relatively small, there are opportunities for both styles of cam, especially if your plans include a shorter-duration roller cam that is not going to spin as high an rpm as these 240-degrees-at-0.050-duration camshafts.
What this test does illustrate is how critical duration is to the power curve since all three cams, regardless of lifter design, are very close in terms of peak torque. Peak horsepower changed the most between the three cams, but most of that was the change the mechanical roller made by pushing the peak horsepower up to 6,600 rpm. This is also of concern because to take full advantage of a 6,600-rpm peak horsepower point, it's generally required to spin the engine another 400 to 500 rpm past peak power to get the most acceleration advantage out of the engine. This means twisting this small-block to around 7,000 rpm. You'd better have a good steel crank, rods, and strong forged pistons if you're gonna spin a small-block 383 to 7,000 rpm!