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.