Be The Camshaft Expert

LCA Selection
With an overlap figure decided on, the next step is to determine the LCA. If maximum torque and power along with the biggest area under the curve is the target, then only one LCA will do the job, and it has to be the one the engine wants, not one you or someone else doing your guessing for you arbitrarily decides is the one needed.

To understand where I am going, let's take a look at a typical cam-buying scenario. A hot rodder calls up and asks for a hydraulic cam of, say, 290 degrees for his 350 small-block street driver. Now this is a fairly big cam and could involve a lot of overlap, possibly even too much for the application. What usually happens is the cam tech guy, recognizing that this cam may not be as well-mannered as the customer would like, recommends the cam be ground on a wider-than-optimal LCA. Widening the LCA reduces the overlap and tightening it increases overlap. To maintain sufficient idle and vacuum qualities, the cam tech recommends the cam be ground on, say, 112-degree LCA, which gives an overlap of 66 degrees. For a typical performance-headed 350, the optimal LCA is usually 108 degrees. Grinding the 290-degree cam, our guy is calling for on a 108 LCA results in 74 degrees of overlap. At first this does not sound like too much more, but the reality is it's the area of the overlap triangle that influences the situation, and the through-flow area of the overlap triangle goes up just about as the square of the overlap angle. This makes the 74-degree overlap a 26 percent increase, not the 12 percent you might have expected.

So spreading the LCA is a way of cutting the overlap while still retaining the duration. Unfortunately, nothing comes without a price. The downside of spreading the LCA to reduce overlap so a decent idle and vacuum are achieved has two major strikes against it. First, the piston comes further up the bore before the intake valve closes. At low speed this pushes the intake charge back into the intake manifold. Result: low-speed torque is reduced. But so long as it drives well, this may be OK if it helps top-end output. Let's investigate that. In-cylinder pressure measurements strongly indicate that there is an optimal balance between early opening of the intake (IOBTDC) and late closing (ICABDC). If the LCA is spread solely to preserve the idle and vacuum, the intake now not only opens too late, but also closes too late. Mapping intake, cylinder and exhaust pressures throughout the cycle indicates that getting the first half of the induction stroke right is of paramount importance toward making the second half optimal. In other words, if the first half of the stroke is not optimal, there are no means of redemption on the second half.

This forces us to the conclusion that for a given duration, there is only one optimal opening point and one closing. This, in turn, means, within a small window, only one LCA gives optimal results. If the LCA is spread to preserve the idle and vacuum, the price paid is reduced torque and hp. We should have gone to a shorter cam on the correct LCA, as it would have produced better results! The moral here is that if the cam had been selected on the basis of overlap and LCA first, then the duration would have been decided by these two factors, not some arbitrary decision on the part of the hot rodder. To arrive at the duration when the overlap and LCA are known, we take the overlap (in our example 66), divide by 2, add it to the LCA (108 + 33), then double it (141 x 2 = 282). That's the duration needed to satisfy the overlap and LCA requirements, and I will bet arriving at the required cam this way is a whole lot different (and far more accurate) than you have been told in the past.

How Do You Know What LCA Is Needed?
Now we come to what would normally be a real stumbling block. Exactly what LCA does an engine need for optimal results? The bottom line is that it's all related to how big a cylinder the intake valve has to feed. The bigger the cylinder in relation to the valve, the tighter the LCA needs to be, and vice versa. That is the main factor. Additionally, but to a lesser extent, we also find that as the compression ratio goes up, the optimal LCA gets wider. This is good because it means smaller valve cutouts in the pistons. For an engine in the 9 to 11:1 CR range, the LCA selection chart will, 99 times out of 100, 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.

Duration & Lift: Its Effect On Output
Now that we are over the duration hurdle and can look at it in a more realistic fashion, let's look at exactly what it delivers. Assuming the compression ratio remains constant, longer duration just moves the torque curve up the rpm range. Peak torque itself usually only increases a minor amount. The additional hp comes from the fact that the torque delivered happens at a higher rpm and power is directly proportional to torque times rpm. The graph of duration versus output shows what typically happens as the duration increases while all other factors are held constant. Although it looks like a good before-and-after test showing the longer cam's value, this test does in fact favor the shorter cam. Because the intake valve closes sooner after passing BDC, the running ratio, or dynamic compression ratio, with a shorter cam is higher. If the compression ratio with the longer cam is raised so that the same cranking pressure as the shorter cam is seen, we find that much of the low-speed torque loss is recovered. In addition to this, the longer cam will deliver a much better top end when an appropriate compression increase is made. This is an important factor, so don't overlook it. If you don't feel inclined to run a compression to match a longer cam's requirement, then stick with a shorter one, as it will produce better results.

For most V-8s with reasonable heads, the ability to raise low-speed torque with compression increases holds good to about 285-290 degrees (at lash point) of cam duration. After that, low-speed torque will drop off faster than further compression increases can recover it. A cylinder's breathing ability is not only dependent on the duration of valve opening, but also the amount of valve lift involved. The type of heads typically used on domestic V-8s are much more responsive to lift than, say, a four-valve engine.

There are two reasons for this. The first is that all two-valve V-8s are under-valved for the cubes those valves have to feed. Secondly, heads with a predominantly parallel valve design go through three phases of flow efficiency. The first, right off the seat, is a high-efficiency regime. As the valve lifts through the 0.100 inch (100 thousandths) to about 0.500 inch (500 thousandths), efficiency drops off considerably. Once the valve lift goes over a point equal to a lift of about a quarter of the valve's diameter, the flow efficiency starts to pick back up. The reason for this is that intake valve's shrouding starts to diminish because the valve is sufficiently far enough out of the seat's sphere of influence. For this reason, most of the V-8s we work with will benefit from valve-lift values equal to about 0.3 to 0.35 times the diameter of the intake valve. For a typical 2.02-inch intake, this would mean a lift of at least 0.6 inch (600 thousandths) to as much as 0.7 inch (700 thousandths). Achieving this with a short cam is something of a challenge, so going for all the lift possible consistent with reliability is worthwhile.

While doing a bunch of cam testing for Harvey Crane some years ago, I ran a test to get some kind of idea of the relative importance of duration versus lift. The results were interesting, as the nearby graph shows. The hydraulic flat-tappet cams used were 252 and 260 degrees duration. With a 1.5 rocker, the 260 cam delivered 0.44 (440 thousandths) lift while the 252 cam with a 1.7 rocker went to 0.48 (480 thousandths) lift. The intake valve lift at TDC was 0.070 for the 252 cam, and for the 260 cam and 1.5 ratio rocker, 0.073 (73 thousandths). As can be seen from the results, the peak power output from each combination was near identical, but the shorter-duration, high-lift combo made power at less rpm because it produced more torque, especially at low speed.