An engine has often been compared to an air pump; the easier it is to get air in and out of the engine's cylinders, the more efficient the engine will be. That is why most performance modifications like conical air filters, cold air intake systems, headers and cat-back exhaust systems are aimed at making the intake and exhaust systems less restrictive. These parts increase the flow of air into the engine and ease the flow of exhaust gas out of the engine. Among these components, headers may be the most difficult to install, so before you start taking your car apart, read on to find out what headers do, the different types and how much performance they can add to your car.
Headers take the place of your exhaust manifolds, the first part of your exhaust system between the cylinder heads and the front exhaust pipes. They are constructed of individual tubes, one for each exhaust port, that all meet in a larger tube called a collector. Headers are installed because the stock exhaust manifolds usually flow poorly. However, as we will see, the quality of manifold exhaust flow varies from vehicle to vehicle. Regardless of the manifold, we recommend that before you install headers you make the rest of the exhaust system less restrictive. This usually means installing larger diameter, mandrel bent pipes and definitely means using more free flowing mufflers. It doesn't make any sense to put headers on if the exhaust flow is only going to be choked further down the line.
Headers are most beneficial to cars and trucks equipped with cast-iron log-type exhaust manifolds. A log-type manifold is just a big tube bolted to the exhaust ports to collect the exhaust gas. It doesn't take up a lot of space in the engine bay and does the job of getting the exhaust gas back to the exhaust pipes, but it doesn't do much for performance. The problem is that all the exhaust pulses dump into the same space. On a V8 engine, one cylinder on each bank fires within 90 degrees of crankshaft rotation of another cylinder on the same bank. These two cylinders exhaust almost simultaneously into the tube. When the exhaust gas has to compete for space like this in the manifold, you get restriction.
An exhaust header has individual tubes for each exhaust port, allowing each exhaust pulse to flow unobstructed from the cylinder head. However, some factory exhaust manifolds also have individual tubes. The most sophisticated of these and the most well thought out of this type are found on performance cars. Cars with individual runner manifolds will not benefit from headers as much as those with the log-type manifolds, but that doesn't mean they can't be improved upon. The tubes on headers are usually larger in diameter than the runners on an exhaust manifold and the tubes are mandrel bent for gradual radius bends. Once again, the goal is to reduce restriction and increase flow, but headers have still another benefit.
Well-designed headers produce a "scavenging" effect. When an exhaust gas pulse exits a header tube into the collector, a negative pressure wave is created that travels back up the header tube to the exhaust port during valve overlap. This negative pressure helps pull any remaining exhaust gas from the cylinder and also helps draw the incoming intake charge into the cylinder. This, of course, is very beneficial to engine performance but the tubes must be long enough for it to be effective during a useable rpm range. This brings us to our next topic, header types.
The aforementioned scavenging effect is most pronounced on long tube headers, however these headers are usually more expensive, the most difficult to install and take up the most space. Long tube headers require that the exhaust pipes be cut and flanges attached that will connect with the header collector. The tubes on short tube headers may not be long enough for effective scavenging, however they generally fit more easily in the engine compartment and some are designed to connect to the existing exhaust pipe flange, so no cutting or welding is necessary. The size of the header tube also affects flow and scavenging, with larger tubes generally better for high rpm, and smaller tubes for maintaining exhaust gas velocity for low end torque.
Most headers are of the 4 into 1 design, where all 4 pipes end together in the collector, but there are also tri-y headers. On a tri-y header, the opposite cylinders in the firing order are paired in a short "Y", and then the Ys combine in the collector. Tri-Y headers increase the scavenging effect and provide a wider powerband.
Headers come in plain steel, stainless steel and with ceramic coating. Steel headers will not last very long unless they are coated. The paint they are delivered with is just something to keep them from rusting in the box; they must be painted with high temperature paint before they are installed. Even that kind of paint won't last however, and they must be periodically recoated or they will rust away. Stainless steel headers will not rust but they will usually turn blue near the head flange. Ceramic coatings are durable and can withstand higher temperatures. A quality set of headers will have a substantial head flange to prevent exhaust leaks and all connections will have high quality welds. Finally, an engine works as a system, and the effectiveness of individual components is therefore very dependent on the other parts. While long tube headers are generally better for maximum performance than short tubes, the amount of power increase either type can provide depends on the camshaft design, intake system, cylinder head ports, and how restrictive the stock exhaust manifolds are. In general, a quality set of headers should provide an increase of approximately 10 horsepower, and if you're restrained with your right foot, you may even see an increase in fuel mileage.