Breaking Through The LCA Confusion Zone

Now we have a starting point from which to reference the effect engine spec changes may have on the optimal LCA. Here, I am assuming the cam lobes will be the same throughout our analysis of what is required. With these parameters fixed, the most influential aspect dictating what the optimal LCA may be is the low-lift flow of the intake valve in relation to the cubic inches that the valve has to feed. The more cubes the intake has to feed, the earlier it must be opened. If it is relying on any exhaust scavenging, the exhaust valve must be closed later. This means that for a given set of heads, a big-inch motor requires more overlap to produce optimal results.

Since part of the test criteria involves utilizing the same cam lobes in both tests, we can see that the LCA must be decreased if duration stays the same and overlap increases. Using the same heads with more cubes requires a tighter LCA. A good working approximation here is to tighten the LCA by 1 degree for every 4-5 percent increase in displacement. For example, if the number calculated is 103.5 degrees, go for the next whole degree tighter, as power falls off much faster on the too-wide side compared to the too-tight side.

Now for the other side of the coin-increased low-lift flow with no change in cubes. Here, the low-lift flow of the intake is 95 percent of the deal. If the flow is increased due to a more efficient valve seat form or, say, a 30-degree seat instead of 45 (30s have much more low-lift flow), or a bigger valve, then the optimal LCA becomes wider. It is not convenient to deal with this aspect in terms of flow in the space allotted here, but when it comes to spreading the LCA, a good ballpark figure based on intake valve diameter is 1 degree for every 2.5 percent increase in valve diameter. And just in case you wondered, valve area does not play into the scheme of things here because the key factor is the valve's circumference, not its area. An example here would be the installation of a 2.08-inch intake in place of the 2.02 intake. Just shy of 3 percent bigger, this would call for the LCA of our 350 to go from 108 to 109.

Before we move on, here's one last point regarding port flow in relation to the optimal LCA. If flow is increased in the high valve lift range (0.300 up), the effect on the optimal LCA is about zero.

After low-lift flow per cube, the next most influential factor is the compression ratio (CR). Increasing the compression ratio creates a smaller combustion space. One of the benefits of this is higher gas speed of the exiting exhaust at TDC. In addition to this, the rate of change of volume around TDC is increased. Also, there is less damping of the negative exhaust pulse from the exhaust length tuning. These factors combined have the effect of decreasing the amount of overlap required to get the job done with the amount of duration involved. Decreasing the number of degrees of overlap necessitates spreading the LCA when the CR goes up and tightening it when the CR is reduced. For compression increases, the situation is good because it means that valve cutouts in the pistons can be a little smaller than would otherwise be the case. For an engine in the 9.5-11:1 CR range, the nearby LCA selection chart will deliver accurate results. For each ratio above 11:1, it pays to spread the LCA about 1 degree for every two ratios of compression increase. Tighten a like amount for ratios below 9.5:1.

Stepping up the rocker ratio is often a good way to increase output with no more than a simple bolt-on modification. Higher-ratio rockers can have the effect of spreading the engine's required LCA. This means that if the existing cam's LCA is too wide, then bolting on a set of high-ratio rockers can pay a handsome dividend. On the other hand, if the LCA was such that the overlap triangle was optimal in relation to total duration, then installing a set of rockers with a higher ratio can drop output rather than increase it.

My own tests have indicated, within the ratio range of 1.5 to about 1.9:1, that for every 0.1 ratio increase in 0the intake rocker ratio, the LCA needs to be spread by 3/4 to 1 degree.

As you can see, the area of cfm/degrees is the issue we are chasing here. The foregoing gives a good starting point toward determining what is needed, but other factors still play their parts. These are less of an influence, but you should be aware of them. As long as increasing or decreasing the overlap area influences things, you should be in good shape. To put the frosting on the cake, consider that if the cam lobes used have more acceleration off the seat, the LCA will need to be wider, and vice versa. Also, do not fall into the trap of thinking that a roller cam has more initial acceleration. For the most part, a flat-tappet cam beats a roller for about 15 degrees of the total duration involved. If the cam is less than about 275 degrees, a flat-tappet cam has more opening area. The roller cam takes over at about 280, but its initially slower opening indicates that a roller needs a slightly tighter LCA under 280 and a slightly wider one over 280.

To get an idea of which way to maneuver the LCA, you need a starting point. This chart gives a good working result. Combined with the info in the main text, it should put you farther down the road toward understanding LCAs.

To use this chart, establish the number of cubes in the cylinder per inch of valve diameter. To get this number, divide the engine displacement by the number of cylinders and then by the intake valve diameter. Find that number on the vertical axis and then move across to the green line. At the intersection point, drop down to the base and read off the LCA required. Because big-block Chevy engines have angled valves, they need to have about 2 degrees less than this chart indicates.