We could fill volumes spelling out the number of factors that affect an internal combustion engine's performance. Each component, each setting, each measurement and tolerance of every type combine to create a powerplant's performance characteristics. As we've said many times before, however, nothing defines the character of an engine more than its camshaft. Of all the measurements and specifications found in a camshaft, it's hard to underestimate the importance of the lobe separation angle, or LSA. Simply put, the LSA indicates the angle, in camshaft degrees, between the maximum lift points, or centerlines, on the intake lobe and the exhaust lobe. This figure, which is ground into the cam at the factory and cannot be changed, directly influences an engine's powerband. It's more complicated than that, of course, but the idea behind this project is straightforward. In short, we hit the dyno with our Coast High Performance 406 stroker test mule and tested three camshafts that carried identical specification-except for their lobe separation angles. How did it affect our engine's powerband? Read on to find out.
As we worked with Comp Cams engineer Billy Godbold to spec out this experiment and interpret the results, we were looking to discuss this complicated subject as simply as possible. For Godbold, this started with the term lobe separation angle. "It's doesn't mean anything except for how it affects the camshaft centerlines," he explained. "You determine the centerlines, which determines overlap, and that has performance effects." To be more specific, every cam lobe has a given number of degrees of duration, and there is a midpoint to this event. This midpoint is referred to as a centerline, and there is one for the intake and one for the exhaust. The intake centerline is used to position the cam in the engine. The exhaust lobe centerline doesn't come into play during installation or cam degreeing, but it is essential to calculating lobe separation angle. The LSA is calculated by adding the intake centerline and the exhaust centerline, then dividing by two. For example, a cam with a 106-degree intake centerline and a 114-degree exhaust centerline has a lobe separation angle of 110 degrees (106 + 114 = 220; 220 2 = 110). In fact, the cam we already had in this 406 was a standard Comp XR300HR with a 110-degree lobe separation. Our other two 'sticks were custom ground to have all the same specs as our 110-degree specimen, except that one had a 107-degree LSA and the other a 113-degree LSA.
What We Did
Dyno-tested three camshafts with identical specs except for lobe separation angle.
The effects are subtle, but lobe separation angle does affect an engine's powerband.
Only the power you give up by choosing the wrong LSA.
Or rather, almost all the same specs. The reason that changing the LSA changes overlap is because it also changes the four valve timing points found on a cam: intake opening (IO), intake closing (IC), exhaust opening (EO) and exhaust closing (EC). "The timing points are what the engine responds to," said Godbold. Intake closing is considered the most important point of the four, since it does the most to establish where peak torque occurs. An early IC improves low-speed torque, but limits high-rpm power since it also limits time for cylinder filling. On the other hand, a later IC allows more time for a cylinder to fill at high rpm, but limits low-end torque since cylinder pressure is pushed back through the intake port. Intake opening (IO) plays a big part in establishing overlap (the time when both intake and exhaust valves are open). An early IO increases overlap and can lead to a sluggish engine, since the intake charge is contaminated with exhaust gasses. A later IO reduces overlap, improves idle quality, and increases low-speed torque. Exhaust opening (EO) ranks second only to intake closing in affecting engine performance. An early EO can limit low- and midrange power by allowing torque-creating cylinder pressure to escape, but helps high-rpm performance by creating more time for exhaust gas to be expelled. Exhaust closing (EC) also affects overlap. An early EC reduces overlap, improving idle but limiting midrange power. A late EC increases overlap, which hurts idle but helps high-rpm power. All of these figures can be found on a cam card, and we've listed them here for you to puzzle over as you compare dyno runs. In short, this all brings us back to overlap. "You change the timing points with lobe separation," reiterates Godbold. So with that in mind, we present the simple version of what changing an engine's LSA does (see "Lobe Separation Angle Effects" sidebar).
So how did all this theory play out in our test scenario? Our methodology was straightforward: We ran each cam in our 406, making pulls with both Dart single-plane and dual-plane intakes mounted up. According to Godbold, the LSA changes did "exactly" what they should do in our application. "When you spread the LSA, you move out the two most important points (intake closing and exhaust opening). This makes the cam act bigger when it is ground on a wider lobe separation. Likewise, a narrower lobe separation moves the IC and EO points closer together, making the cam act slightly smaller. Hence, the 107 LSA cam was better at low rpm and the 113 LSA cam was worse at low rpm. This motor liked the overlap."
Our dyno figures bear this out. The 406 made more low-end torque with the 107 LSA cam, regardless of the intake manifold it was wearing, and held its own at higher rpm. In the same vein, the dual-plane equipped 406 had better results with the 107 LSA cam. "The dual-plane reacted to lobe separation almost exactly as it would to a smaller camshaft," said Godbold.
The 113 LSA cam, on the other hand, made less power everywhere and especially fell on its face with the single-plane intake. "I'm not totally sure if the reason is simply a result of the shorter runner length with a single-plane, or if you could trace it back to the common plenum," Godbold observed. "However, I do know for certain that single-plane manifolds have always run best with tighter lobe separation camshafts."
The 113 LSA cam may have made more power at high-rpm, but we ran into valve bounce issues with our hydraulic roller motor at 6,600 rpm, so we weren't able to find out. In most cases, though, the power given up on the lower end wouldn't have been worth the bit we gained near 7,000 rpm. The 110 LSA cam made the most peak power-609 hp with the single-plane. Its numbers were much closer to the 107 LSA cam's figures, but the narrower LSA still put out more horsepower and torque below 6,000 rpm. It's probably not a coincidence that Comp utilizes a 107-degree LSA in its Thumpr line of cams.
Godbold thinks it would be revealing to do the same test with a much smaller set of three cams in the same application. "Then I think you would see a wider power range on the wider separation cam," he observed. "I still think 107 would give you the best peak numbers, but the wider separation cams tend to fall off less beyond peak power." With smaller cams running at a lower rpm, we'd probably be able to see that. But for now, we've got a very clear demonstration of the advantages-namely a more usable powerband-of running a narrower LSA.