Mistakes made while speccing out the valvetrain can cost a substantial amount of output. A
Editor's Note: The potential for controversy dictates listing the author's qualifications to write about this subject. What we are presenting cannot be found from generally accepted sources. It is the result of Vizard's 40 years of original research on the subject. This includes extensive testing that few in the industry can rival. Several of Vizard's cam testing sessions, such as for Crane 1985, lasted as long as six months. This involved three small-block Chevy 350-, 383-, and 406ci engines. These engines, with two sets of heads, had some 8,000 (yes, eight thousand) combinations of cam events and rocker ratios run through them. Similar tests for Europe's largest cam company (Kent Cams) resulted in superior valve event cam designs for Ford Pinto and Mini Cooper engines. In this field, Vizard is a university lecturer and an industry consultant with an exemplary race success record.
Fig 1: A high-performance or race engine relies heavily on an exhaust-driven induction cyc
It seems that if I want to get into hot water, all I have to do is start writing about camshaft Lobe Centerline Angles (LCA). But let me tell you, as a one-man show, I really understand the advantages of being able to compute something quickly and accurately instead of adopting a time-consuming trial-and-error method on the dyno. These days, I don't flush cams through a motor to find out what works best; in fact, I haven't done that for 15 years or more. Now I spend just 15 minutes with a self-generated program, computing what is required with deadly accuracy. One fact that all my cam testing has indicated is the starting point for any cam spec should be the LCA. The other point that thousands of tests have brought home is that the engine's characteristics dictate (and nearly limit) what the optimal LCA should be. It is not, as is so often believed, the engine builder.
Fig 2: Here is what would be seen looking at a pushrod V-8 cam, end on: 1) Intake lobe lif
To get a better understanding of how the optimal LCA varies with engine specs, let's establish a few criteria. First and foremost, we are dealing with a normally aspirated engine that has an effective intake and exhaust system (i.e., non-restrictive). Second, it is very important to understand that gas flow, velocity, and pressure wave characteristics around TDC during the valve overlap period have more to do with the success of the induction stroke than any other factor. If these characteristics are not correct for the intake duration involved, the result will be a loss of output virtually throughout the entire rpm range. Be very aware that if the intake charge does not get going in the first half of the intake event, there is nothing that can be done to make up for it in the second half.
We usually think of our engines as four-cycle units, but in reality a race engine is a five-cycle device and has two distinct induction phases (Fig. 1). With a well-tuned exhaust, we find that the strongest draw on the intake port is brought about by the negative pressure created by the exhaust-not, as is so often supposed, the piston going down the bore. To utilize any exhaust effect, the overlap has to be right and proportionate to the total duration the cam will eventually have. All this leads to two conclusions of significant importance. The first is that the LCA has to be right. Secondly, it is not an adjustable tuning entity.
Fig 3: Getting the LCA right-what's it worth? In this graph, the torque difference (we are
To drive home the point that the first half of the induction stroke is the most important aspect toward a good cylinder fill, let's further consider what we are dealing with here. A typical Detroit two-valve-per-cylinder pushrod engine suffers from lack of valve size per cubic inch of displacement. When the intake opens, it needs to do so as quickly as possible to offset the lack of size. Obviously, there is a limit to how fast it can be opened, so getting ahead of the game by opening it earlier so that it is further off the seat during the piston induction phase would seem like a good idea-and it is to an extent. If this early opening is used in conjunction with a tuned exhaust that can pull on the intake earlier, then the intake charge has a longer and stronger pull on it. This means the valve is flowing earlier into the intake cycle, and the intake port does not have to be quite as big because it is utilized more effectively for a longer period of time. This results in a smaller port getting the job done, hence a higher port velocity to continue filling (or overfilling if the job has been done right) the cylinder at the other end of the induction stroke. From this you can see that getting things right in the overlap periods affects everything that subsequently happens in the rest of the induction stroke.
At this point, I have to find a way for you to apply what may be learned here without hauling in 50 pages of math. I think the best way to do that is to start off with a known entity that is fairly common to many engine builders and work from there, explaining the degree to which certain changes affect the optimal LCA. To best understand what's going on here, we need to recognize the common thread throughout. This thread consists of the intake and exhaust port velocities that exist in and around TDC during the overlap phase. Essentially, the port velocities of both the intake and exhaust have to be sufficient to keep things flowing in the right direction. Remember, air is not some near massless entity that has little momentum. A typical school gym holds 50 tons of air, and even the small amount within the ports of a high-performance engine can have considerable kinetic energy just waiting to be converted to pressure energy to better fill a cylinder.
As a starting point, let's consider a 350-inch small-block Chevy equipped with heads that possess the commonly used 2.02- and 1.6-inch intake and exhaust valves. Also, let's assume this engine has a good intake along with adequate carburetion and a set of headers feeding into an exhaust system with negligible-to-zero backpressure. For a compression ratio, 10.5:1 is it. For rocker ratios, our baseline engine uses 1.6 for the intake and 1.5 for the exhaust. The cam concerned here has a flat-tappet design. Assuming the flow figures (especially off the seat and at low lift) are typical for a set of heads with a good seat job, the optimal LCA will be 108 degrees in at 4 degrees advance (Fig. 3). This number holds well for cams from about 270 degrees of off-the-seat duration through approximately 300 degrees. Below 270, there is a tendency for the optimum to tighten up by about a degree or so; above that number, it widens by a similar amount.
This Kaase Ford head has 2.1-inch intake valves. If the LCA was optimal for a 2.02 intake
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.
Stroker cranks are a popular way to get cubes, but to get the best from a stroker deal, th
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.
Higher ratio or fast-off-the-seat rockers, such as these Gold Race rockers from Crane, cal
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.