In the world of engine hop-up components, headers are truly a win-win-win deal. They can deliver measurably better torque, horsepower, and mileage. When swapping out factory iron manifolds for headers, a typical peak-to-peak gain (even for a smog-laden motor) is about 25 hp, and roughly the same for lb-ft of torque. Having said that, the low-speed torque can go up as much as 40 lb-ft with headers that are optimized for the engine. For the average hot rodder there is no downside, and for the racer they are essential. What they are not, though, is foolproof. Headers can deliver a great deal in terms of return on investment—if the headers are spec’d out properly. When it comes to the right spec, it’s all about dimensions.
Shown here are the critical...
Shown here are the critical lengths and pipe diameters that need to be sized to suit the engine spec and the rpm band to be most used.
The critical dimensions we need to concern ourselves with are primary diameter, primary length, collector style, secondary diameter, and secondary length. The combination of all these dimensions working in concert is what is commonly referred to as a tuned system. When the system is tuned correctly not only does the exhaust leave the cylinders in an unhindered fashion, but also pressure waves below atmospheric pressure act on the cylinder during the valve overlap period to effectively scavenge the combustion chamber of the last remnants of the exhaust. This action, in turn, causes the intake charge to start into the cylinder well before the piston starts its induction stroke. In other words, there are in effect two induction events. The first is the exhaust-driven one, and the second is the piston-driven one.
This chart works well for...
This chart works well for high-performance heads in as-cast or ported form. To determine the primary inside diameter, you will need a good ballpark figure for the exhaust flow at the peak valve lift. Using this figure, go up the chart until you reach one of the colored lines (green for street, purple for street/strip, blue for race). At that point go across the chart and read off the pipe ID required.
So how does this exhaust-driven induction stroke come about? In essence, it is relatively simple. When the exhaust valve is initially opened, the cylinder pressure is still relatively high at some 70 to 120 psi. Opening an exhaust valve with this kind of pressure behind it generates a very strong positive (high) pressure wave that travels down the exhaust pipe at the speed of sound (some 1,300 feet/second in a hot exhaust environment). When this positive pressure wave reaches the end of the pipe (or any substantial change in cross-sectional area), it is reflected and undergoes a reversal such that a positive pressure becomes a negative (below atmospheric) pressure wave.
The negative pressure wave now travels back up the exhaust pipe, and if it arrives at the exhaust valve during the overlap period, it will suck the remaining exhaust out of the combustion chamber and start the intake charge moving into the chamber. The trick is to match lengths to the operating rpm of the engine. By coupling up cylinders and adding the effect of a secondary collector length, the range over which the exhaust can scavenge the cylinder can be made broader to the tune of some 4,000 rpm. With a well-tuned system, the degree to which the exhaust can “suck” on the cylinder to start the intake charge on its way can be very substantial. On a well spec’d high-performance street engine, on up to an all-out race engine, this suction can amount to as much as 7 psi, and that’s way more than the suction caused by the piston going down the bore.
Primary pipe length is usually considered to be the most important dimension to get right. While this may be so in most instances, it is not the case for a two-plane—cranked V-8 typical of Detroit products we commonly drive. The suspected reason for this is the unevenly spaced firing pulses seen at the collector, but whatever the reason, it does (for once) all work in our favor, as we’ll see later.