Proving the Point
Once the idea of independent variable camshaft testing entered our minds, we had no other choice but to turn our dreams into reality. Our 383ci Smeding Performance motor set the stage for a high-performance foundation and would allow us to run various cam tests (this engine will ultimately be part of our Project POS Camaro). The Smeding engine was originally equipped with a hydraulic flat-tappet camshaft; we swapped over to a hydraulic-roller setup for lifter-reuse reasons. Smeding offers its crate engine combinations with the hydraulic-roller option, but since we didn't originally order it that way we had to change ours around. This required new lifters, lifter tie bars, longer pushrods, a cam retainer plate, and the roller camshaft itself. The Smeding small-block crate engine features a GM-style camshaft core using a recessed nose that allows the cam retainer plate to hold the camshaft in place. Read ahead to find the method we used, which called for a standard-style cam core outfitted with a cam button placed inside the timing-chain center hole. This cam button takes up slack >> between the camshaft nose and the timing-chain cover, making sure the camshaft will not walk out of the cylinder block. Either way, all small-block Chevy roller camshafts require a retainer to hold the cam in place.
When making the testbed camshaft selections, we looked for grinds that would show considerable power changes between duration and LSA. We called the Lunati tech line for some expert camshaft advice and a helpful representative recommended we come up with our own custom grind. The actual figures of these camshafts were not chosen to boost the power of the 383 but picked to show what happens when specific camshaft variables are altered.
Our first camshaft called for 0.326 inch of lobe lift (0.489 at the valve) and 215/224 degrees of intake/exhaust duration measured at 0.050-inch lift and founded on a 106-degree LSA. This camshaft was small for a 383 but would soon prove a valid point. Our next camshaft choice called for the exact same grind with the exception of the LSA. Here we added 8 degrees of LSA (quite a bit) for a total of 114 degrees. Obviously, we would be scrutinizing LSA power differences between the first two cams. The third camshaft added slightly taller lobes and drastically longer duration points, with 0.510/0.525 inch intake/exhaust lift and 232/242 degrees of duration at 0.050-inch lift.
After a few warm-up pulls on the dyno at Vrbancic Brothers Racing, we set the testing points between 2,500 and 6,000 rpm and let the motor rip. The low-end and midrange torque numbers were strong with the first cam, and the horsepower posted a steady 405hp peak at a rather low 5,000 rpm. We made several more pulls and found the camshaft would barely pull 6,000 rpm. During one pull, the engine just nosed over at 5,700 rpm and wouldn't rev any higher. Making 415 lb-ft of torque at 2,500 rpm was impressive, but the low 12 inches of manifold idle vacuum >> and its inability to carry the power curve much past 5,000 were not.
With the second cam, we changed nothing except for a wider LSA. If theory held true, we would see an increase in idle vacuum and perhaps even a little more low-speed torque with a slight loss of peak horsepower. Well, torque fell slightly across the curve and we lost some horsepower too. Engine idle vacuum came up to 14 inches and the cam pulled well beyond 6,200 rpm, but this wasn't worth the loss of power. Torque and horsepower had diminished because of the increase in camshaft overlap. Less overlap means the valves will close sooner and affect upper-end peak power. Torque usually increases, but the short duration of this camshaft didn't >> create enough overlap to bleed off a detrimental amount of cylinder pressure. Therefore, at low rpm, torque was already maximized, and when we further decreased overlap, we hindered the torque by closing the valves too early. A solid street/strip compromise would be a few more degrees of duration with an LSA of 110 or 112 degrees.
By now, our technical spirits were tickled, and we couldn't wait to dive into the next camshaft swap. The third camshaft sported slightly more lift (0.510/0.525 inch) with a lot more duration (232/242 degrees at 0.050) all ground on the wide 114 LSA. Ideally, a separate lift and duration test would have been nice, but every degree of 0.050-inch duration is typically worth much more power than 0.010 inch of lift.
This time we expected to see a shift in the power curve to produce improved peak power at the expense of some torque. As it turned out, we saw a huge loss of low-speed torque, a slight loss of midrange torque, and a moderate gain in horsepower almost 600-rpm higher in the power curve. Engine-idle vacuum also fell considerably, forcing us to raise the idle and take a steady reading of 10 inches at 300 rpm higher.
What happened? Increased duration kept the valves open longer to raise the rpm and peak power of the engine. The large loss of torque occurred due to the bleeding off of excessive cylinder pressure below 3,000 rpm. As rpm increased, the torque falloff wasn't as great and we posted a peak number that was 13 lb-ft down from the first test. Had we been chasing more top-end power, a narrower LSA would have boosted peak horsepower while allowing peak torque to flatten. In this case, valve overlap played a major role in the mannerisms of our Smeding 383 ci.
What It Means
According to our test, the first camshaft would have made a great street machine choice. There was plenty of vacuum to run accessories and enough torque to pin the passenger against the seat. As for the short rpm curve, this cam would work well in a towing rig. Ideally, we'd add a few more degrees of 0.050-inch duration combined with a slightly wider LSA (somewhere around 110 to 112 degrees).
The second camshaft featured a wide LSA that hurt the engine almost everywhere in the powerband due to a small duration effect. This cam was merely a test subject and would work better if altered. We'd recommend the third cam for a street/strip car with some gear and a loose torque converter. The torque below 3,000 rpm would hurt street performance while the extra 20 hp up top would add a few miles per hour. Once a small-block cam reaches the 240-degree, 0.050-inch duration figures, we'd recommend foregoing streetable torque and tightening the LSA up to something like 108 or 110 degrees to watch horsepower climb. In any case, it's important to remember that these cam events were chosen for testing purposes only. We did not include the grind numbers because the Lunati catalog has many others that are capable of producing better results. When the time comes to purchase a camshaft, use this story to grasp the principles of camshaft selection and contact your favorite cam company. The answers are out there.