Ok, we've done header tests in the past, comparing various styles and sizes on moderate to high-powered street-style mills. In the case of the big-block Mopar, the default favorite tube size has long been a 1 7/8-inch primary pipe diameter with a 3-inch collector. This size has become the most popular choice in a high-powered street application for one reason-it has been found to work. Moving up the power ladder, say to the level of a well executed "bracket" or sportsman racing engine, or the upper realm of high-performance "street" engines, we had to wonder if there is just a little more power to be had. Would such an engine benefit by upping the ante on header tube diameter? After all, what works best on an engine in the 450-500hp range may leave some power on the table when dealing with engines in the 600-650hp class.
There are theoretical formulas dealing with the subject of pipe diameter as it relates to exhaust port flow. Use the equation (square root (cfm x .0159)), and the numbers often come in pretty close to what experience has shown necessary. Consider exhaust flow in the area of 200 cfm at 28 inches typical for a well-ported production cylinder head. Working the math shows a 1 7/8-inch pipe diameter is near optimal. Upping the exhaust-port flow into the 250-cfm range, the formula predicts a larger pipe diameter is optimal, calling for something in the range of 2 through 2 1/8 inch. just as you would expect, a higher flowing exhaust port favors a larger primary pipe diameter.
While looking at exhaust flow may help to provide a ballpark estimate of required pipe diameter, there are other significant factors that can affect the optimal tube diameter. Using port flow as the basis for sizing does have its drawbacks. Ultimately, the volume of exhaust flow will not necessarily be determined by the peak flow rate of the port. Other factors can make a critical difference in the required header tube diameter, most of which are related to the flow and mechanical efficiencies of the engine combination. Other formulas of varying complexity have been worked out to provide a basis for making a calculation of optimal pipe diameter. With these formulas, the primary variable is changed from the exhaust port flow (as used in the above example) to include factors, such as peak torque rpm, peak horsepower output, cubic displacement, and/or cam timing.
Even at that, there is a great deal of variability in what works best in a given combination. While formulas can be useful in making predictions, no formula will take into account the specifics of a header's designed layout. Significant factors here include the layout of the tubes, the radius and number of bends, collector design and size, as well as the departure radius and direction from the port exit. The only way to know for sure which headers will work best, is to try them and see. We gathered a handful of popular headers aiming to cover the most commonly used pipe diameters on big-block Mopar engines. The range included header primary pipe diameters of 1 3/4, 1 7/8, 2, and 2-2 1/8-inch step headers. Since our primary aim was to see how a higher horsepower engine would respond to the larger primary pipe diameter of the bigger headers, we dusted off our "Iron Headed Air Hammer" test engine (see Sidebar: the Mill), an engine capable of well over 600 hp.