Using the formula, Area = (3.1416) x (id radius) x (id radius), we obtain the following cross sections: 151/48 inches = 2.07 square inches; 131/44 inches = 2.19 square inches; 171/48 inches = 2.53 square inches.
Plugging each of these values into equation 1, we find the selection of peak torque becomes (in the same order of pipe sizes), 4,173, 4,415 and 5,100 rpm. Based on an intention to provide a torque boost around 4,000 rpm, 151/48-inch-diameter primaries appears to work. In accord with our previous comments about primary-pipe length, extending these primaries will increase torque below 4,000 rpm at the expense of torque above this point, which is an additional tool to manipulate a torque curve about its peak (see "Torque Peaks").
While this method will not predict header-pipe area as precisely as some contemporary computer-modeling programs, it can be a valuable quick-and-dirty tool when making decisions about header choice or application of sets already on hand.
ConclusionThere is much more to the science of exhaust-system tuning and headers that space does not allow us to include. It's worth noting once again that the final combination of parts must take into account all the components as a system, rather than looking at the headers as a separate entity. Any engine will make its best overall power when treated as a complete system.
Terms to RememberBlowdown pressure-This is a pressure history developed from the time of exhaust-valve opening to the point where cylinder pressure equals exhaust system pressure. It's also energy created by the combustion process and related to the potential speed at which exhaust can be removed from the cylinders. The diameter of primary pipes figures into the effectiveness of blowdown pressure. Fast-burn engines tend to have higher blowdown pressure than slow-burn engines. The presence of excessive blown-down pressure suggests some power was lost to the atmosphere by way of the exhaust system, the result of not allowing cylinder pressure to do all its work before the exhaust valve opened.
Backpressure-Restrictions that limit the amount of net exhaust flow can create backpressure. This is a limiter to the efficiency of a given exhaust system. Increases in backpressure can be traced to contaminated fresh air/fuel charges, which reduce combustion efficiency and cost power. Exhaust gas recirculation (EGR) with emission-controlled engines tends to create conditions similar to those caused by backpressure.
Volumetric efficiency-This is a measure of cylinder filling efficiency, based on a ratio of actual air capacity to ideal air capacity, expressed as a percentage. Accordingly, actual air capacity relates to the mass of air in a cylinder at any given rpm. Ideal capacity relates to total cylinder volume. For example, a VE of 85 percent (at a specific rpm) means a cylinder is being filled to 85 percent of its physical volume. Volumetric efficiency and torque are directly tied to each other. All else being equal, an increase in VE typically nets a gain in torque.
Combustion contamination-The presence of noncombustible material in a mixture of air and fuel contaminates the combustion process, reducing efficiency and power. Failure to efficiently remove exhaust gas from engine cylinders can lead to combustion contamination.
Wave motion-In a given exhaust system, depending upon multiple conditions, there are sets of waves or pressure excursions representing changing energy levels within exiting gasses. This wave energy can be used to increase (or reduce) exhaust system efficiency. The speed of such waves and their effect upon a given header design is influenced by pipe dimensions, particularly diameter.
Torque PeaksThis chart estimates the peak torque rpm of five engine displacements based on three different primary-pipe diameters. The rpm levels are derived from the formula given in Matching Headers to Objectives section below.