There are many
misconceptions about headers, their function and the theory in which they are applied...

In any high performance application, the primary performance advantages are gained from getting in as much air and fuel into the engine as possible, and burning it completely and quickly. Once that effort is achieved, getting  the expanded combustible out as quickly and efficiently as possible, will not necessarily "make" horsepower in and of itself, but it will "allow" the horsepower potential of the inlet side to be realized in an exponential fashion. The more power an engine produces, the more important the exhaust concerns become. The optimum exhaust would include a vacuum fan sucking out every last bit of burned fuel, like airing out a smoky house after a pizza being burned in an oven, Since the combustion process makes up to 13 times more gasses than brought in, the size of the suction fan would have to be of ridiculous size. However, there is another way.   

Contrary to standard exhaust manifolds, the header's primary function is to scavenge spent exhaust from the cylinder, rather than just letting the air find it's own way out under pressure. By design, headers are 'tuned' to create peak performance in a specific RPM range, dictated by their tubing diameter, length and tube configuration. This usually happens at between 3,000 to 4,000 RPMs. Tube diameter also has an effect on the effective rpm range... the larger the diameter, the higher the rpm.

A non-turbocharged exhaust flow typically revolves around a series of specifically timed pulses resulting from the systematic bursts exiting the combustion chamber. The hot pulses surge down the exhaust tube in pressurized "bubbles" separated by low pressure vacuums. This can be best visualized as bubbles of water in a clear tube when trying to siphon water from a container. No real smooth flow can be achieved with the combination of high pressure and low pressure pulses sharing the same path without disruption. Of course, the less volume of exhaust, the less this will be a concern. Hence, low output engines will see little benefit from the increases in exhaust flow from headers. On the other hand, back pressure is never a benefit in an internal combustion engine, ever. Too much scavenging from a header "can" be a low rpm detriment, with the accelerated exhaust suction resulting in a portion of the  precious intake fuel/air mixture being partially sucked out the exhaust side when both valves are momentarily open during the exhaust stroke. High overlapping camshafts or super/turbo-charging sees this the most.

The below illustration follows the pulses from the exhaust port of the combustion chamber and into the header tube "runners" (fig.1). As the pulses flow down the tube (fig.2) the pulses begin to cool and thereby contract, which begins to create a  lower pressure in front of the next pulse coming down the tube, thus causing a vacuum. This assists in pulling more exhaust out of the engine, increasing horsepower. As the pulses race towards the collector (fig.3), the intentionally and specifically designed uneven individual tubes allow the exhaust pulses to meet sequentially in order, one after the other at almost the same time. The moment one pulse breaks it's way into the collection chamber, the expanding gases create a suction effect called "Scavenging" which assists in pulling the pulses from the adjacent runners into the collection chamber as well (fig.4). This scavenging effect transforms the individual pulses into a low pressure free flow of exhaust gasses. The scavenging effect can be best visualized by blowing over the top of an open water bottle and watching the water drawn up the bottle's neck and into the air stream.

The tuning of the header is critical as to how it will perform on a particular engine in a specific application. Overall tube length, comparative runner length and tubing size all play a major role in this final outcome. For the most part, the average header is tuned for maximum scavenging at the 4,000 rpm range, where larger tube headers raise the rpm range, and smaller tubes lower the rpm.

As power output increases with precise header tuning, performance will also be reduced both above and below the intended rpm range of the header's scavenging. The narrower the rpm range of the header, the more power produced at that point. The wider the header's rpm range, relatively less but more evenly distributed power will be the result. Typically, longer tube headers produce more bottom end power, as does smaller tubes. Low rpm designed headers typically trade off more of a loss at higher revs than gained at the lower levels.

Special heat insulating coatings are commonly used on premium headers, since hotter and inherently lighter exhaust pulses travel faster at the higher pressures down the tube. Insulating the tubes as the come out of the engine to the collector can foster performance increases reaching  up to an additional 5 to 15%.

As stated in the beginning, only Non-Turbocharged engines can benefit from headers, due to the fact that the pressure in front of the turbo negates any potential for the scavenging effects, while the turbo blades "chop up" the pulses into a "woosh" of air after the turbo, hence there are no pulses left to work with. Typically, the shortest, biggest and the most free flowing exhaust behind a turbo is best.

One of the most significant factors detracting from a header's performance, is sacrificing tuning design in favor of the required shape for the proper fitment into the vehicle's body. Only when the vehicle is built around the header, can optimum performance be fully realized.

IS EXHAUST BACKPRESSURE EVER A GOOD THING?

This is a good topic, one that is hardly ever discussed in detail. Generally you
never want a restriction when attempting to evacuate exhaust gasses. Think of how headers works... they are designed to scavenge (suck) exhaust from the cylinder. Now, why would you want to keep burned exhaust gasses in the cylinder? You would not ever want that, except for in very isolated and specific combinations to cover up a problem due to miss matching of internal components, which we will discuss later in this article.

Left over exhaust gasses are like a fire extinguisher... not only do they take up valuable room in the combustion chamber that new, fresh air/fuel can not occupy, but it also contaminants an equal portion of that fresh gas, rendering it useless as well. This is called a "residual fraction". Usually most naturally aspirated engines see about 7 to 12% Residual leftover exhaust gasses in the chamber, thus, this is why actual volumetric efficiency readings are seldom read over 88% uncorrected on the dyno.

Larger cam profiles allow the exhaust valves to hang open longer to allow the exhaust gasses as much time as possible to evacuate, at the sacrifice of low end torque and HP loss. By allowing the exhaust valve to hang open longer, the rushing incoming air of a huge overlap cam will help push out the rest of the remaining exhaust gasses for the cleanest, least contamination environment in the combustion chamber as possible. This is why many well made cams with big overlaps and lumpy idles can
still produce good bottom end torque even though their rated RPM range is much, much higher on paper. One of the unfortunate sacrifices for this is byproduct of both power and torque, is wasted fuel going out the exhaust while trying to blow out all that contaminated air as well. Think of the quandary when you burn a pizza in the oven in winter, and you open the front and back door just long enough to get most of the smoke out and save some heat, or let the doors open to completely clear the house of all the smoke, but at the sacrifice of loosing all your heat as well.

Think of the front and back door as your valves, the living room as the combustion chamber, the heat as your fresh incoming air/fuel mixture and the smoke as burned exhaust gasses from the previous combustion process. You, would be the camshaft controlling all this. Now, in what case would you
want a restriction on that door trying to get the smoke out? Never... but, there is a "crutch" you can use as a restrictive device to overcome.

IF for some reason you have TOO big of a cam by a mistaken choice during selection, and your driving style does not see much high rpm anyway, then a little more controlled restriction can keep some of that FRESH air/fuel mixture from exiting the combustion chamber in attempt to "blow out" as much of the exhaust gas as possible. The result will be more residual fraction (burned contaminated exhaust gas) left in the chamber, but also not as much fresh air/fuel mixture will not have exited either. This will certainly enhance bottom end power and torque as well as saving on fuel economy as well. BUT, once you rev it up (in the 4,000 to 5,000 rpm range), you will pay 10 fold in HP loss in the end.

Multiple disc diffusers on SuperTrapp brand mufflers allow this variation quick and easy. However, like I said, there is almost NO situation where you really would want an exhaust restriction,  but you can use it to your advantage to cover up or create a crutch for inappropriate combinations. However, if you have a big cammed engine and plan on taking it on a long, quiet journey in overdrive gear, placing a controlled restriction on the exhaust very well may enhance fuel economy as well as drivability while at that low 2,000 to 2,500 rpm overdrive engine speed. Unfortunately the SuperTrapp changes the look of the car dramatically, and usually undesirable in some cases, regardless of the benefits of adjustability.

There are many more facets and considerations to this simple explanation, but in essence it is the basics of exhaust backpressure. Actually, the origin of "...son you gotta put a muffler on dat mill or yer gonna blow er up..." came form the 50's when hot rodding was flourishing. No headers back then, and a full exhaust created a "flow" not unlike a siphon tube creates. Without it the exhaust had no natural flowing path to exit the contorted exhaust manifolds of the time, thus keeping a lot of heat in and eventually burning the exhaust valves. It sure was a lot easier to say "...you need some back pressure there son..." than it was to explain all this. 50 years later and it still stick today... and shockingly, in the minds of many automotive engine builders! Now that's the scary part.

I hope I was able to shed some light on the age-old restriction myth, as well as hopefully able to explain it well enough that this very complex and misunderstood topic can be more readily comprehended.