Breaking Through The LCA Confusion Zone

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.

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.

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.

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.