Headers are one of the most common and basic of your bolt-on mods, however very little is understood about how they actually work. How and why headers work is an arena full of tall tales and myths from old hot rodders and Internet keyboard gurus. I can easily understand why this is true, as I have yet to see very many accurate stories in the media on how headers actually work. Most of us know that headers can produce substantial amounts of power on an engine with very few negative compromises. Properly designed headers work so well in producing extra power with no negative side effects - such as mileage losses common with other mods - that they are a rare, win-win modification. This makes them a mod that's almost essential for any serious engine build-up.
For space, cost and catalytic converter light-off reasons, most cars come with a crude, cast iron, log-exhaust manifold from the factory. A log manifold is simply a tube with stubby legs connecting the exhaust ports to the main tube. This is a cheap way to make a mass produced part. A log manifold is also good for conserving exhaust heat to quickly light off a catalytic converter during cold starts. It is compact, hugs close to the cylinder head and preserves valuable under-hood space for today's crowded engine compartment. All of these attributes are important to an OEM engineer. However, a log manifold is detrimental for power production, which is often a secondary concern to the OEM designer. A few of the newer cars come with more efficient tubular exhaust manifolds, most of which have short runners and crude stamped transitions. These aren't the greatest for performance, but are better than a simple log. Most of these are double walled and have insulation sandwiched in between the walls to speed cat light off.
Header BasicsLet's first start off with the most basic of basics. By the enthusiast definition, a header is an exhaust manifold fabricated from tubular sections of pipe. Full radius mandrel bends are preferred so the pipe's tight radiuses will not be crushed down. Each individual exhaust port is treated to its own separate primary runner of close to equal length, instead of merely dumping into the shared main pipe of a log manifold. The equal length, or close to equal length, primary pipes converge at a single, larger diameter point or collector. The collector then leads to the main exhaust pipe.
An old hot-rodder tale is that headers produce more power by reducing backpressure. Intuitively, when looking at a kinked-up stock log manifold this seems correct, but backpressure reduction alone is not the main reason why headers work so well. Case in point: There are a few cars like the D15/D16 Honda engine or the DOHC Ford/Mazda V-6 engine where adding headers to an otherwise stock engine produces very little additional power despite a really crappy looking stock exhaust manifold. You might wonder why this is so.
Headers make more power three ways: by using gas column inertia pulse tuning to help evacuate the cylinder during the exhaust stroke; acoustic pressure wave resonance tuning to create a low pressure reflected wave rarefaction pulse during the overlap period; and, finally, a reduction in back pressure. If the first two reasons sound like a passage from "Hooked on Physics," check out the following simplified analogy. Some say a well-built engine's sound is music to the ears. Well this really is true. Basically a header is a glorified musical instrument. A header is tuned much like how an organ pipe is tuned, but it's tuned to make the right conditions in the cylinder to make power instead of a specific note on a musical scale.
Pulse TuningThe optimal length for the header's primary tube is one that has a fundamental note corresponding to the degree point in crank rotation of the 4-stroke cycle when the exhaust valve opens. At this point, the piston is traveling downwards and is near the bottom of the stroke. When the exhaust valve opens, a high-pressure pulse of hot expanding exhaust gas travels down the exhaust port at approximately 300-350 feet per second. This wave of hot, moving, high-pressure gas has mass and inertia of its own, which pulls a period of suction or a low-pressure rarefaction behind the main wave of the pulse. This first negative pressure wave helps evacuate the cylinder of burnt exhaust as the piston nears Top Dead Center (TDC) and slows down.
Depending on the engine, the pulse can have a positive pressure of around 70-90 psi at the valve when the exhaust valve opens; and anywhere from 5 to 15 psi at the end of the primary tube, with the low pressure rarefaction behind the pulse being anywhere from 1 to 5 psi of negative pressure. This low-pressure rarefaction follows several milliseconds behind the initial high-pressure pulse. You want to have the low-pressure rarefaction to be in the area of the exhaust port as the piston approaches TDC on the exhaust stroke to help evacuate the cylinder of burnt exhaust gasses as the pumping action out of the cylinder from the piston traveling upward decreases. The primary pipes length and diameter controls this as the speed of the pulse is fairly constant and the length and diameter govern at which rpm this phenomena works the best, hence the analogy of tuning like a musical instrument. A header, just like an organ, is tuned by the dimensions of its pipes. This is how the first power-producing phenomenon works.
Acoustic PressureThe second reason works by pulling as much fresh fuel/air mixture into the cylinder as possible during the overlap period. The overlap period is in between the end of the exhaust stroke and the beginning of the intake stroke in a four-stroke engine, where both the intake and exhaust valves are open at the same time for a few degrees of crankshaft rotation as the piston travels around TDC. Engine designers use overlap to help the engine breathe better in the mid and high rpm range. Overlap helps blow the last bits of stale residual exhaust gas out of the cylinder while allowing an inrush of fresh fuel and air mixture. Less burnt gas and more fresh fuel and air mixture mean more power. It is best if the low level vacuum or rarefaction can be maintained past the initial low pressure front in the primary pipe to help scavenge the cylinder of burnt exhaust gas and pull in a fresh charge of fuel-air mixture during overlap. A well-designed header uses acoustic energy to maintain low pressure near the exhaust valve during the overlap period.
As the initial pulse of high pressure, high energy gas leaves the end of the primary tube and is diffused in the larger diameter header collector, the point where all of the primary tubes merge, a pressurized pulse of sound energy just like a musical note is generated, much like that of an organ. Some of it travels down the exhaust pipe and out into the open air. This is exhaust noise that you hear. The rest of it is reflected back up the primary pipe towards the exhaust valve. This sonic pulse has quite a bit of energy, as your ears can attest to if you have ever been around uncorked race cars.
This reflected sonic pulse initially travels up the primary tube at sonic speed, which is usually around 1100-1900 feet per second in thin, hot, exhaust gas - causing a slight rise in pressure at the valve. The pulse travels into the combustion chamber and is then reflected back down toward the open end of the primary pipe trailing a rarefaction behind it. If the pipe is of proper length and diameter, this reflected wave can be exploited to lengthen the amount of time that the condition of low pressure exists around the exhaust valve well into the overlap period, causing a further improvement in scavenging.
This explains why some engines are relatively unresponsive to headers while others really respond to a properly designed header. The Ford and Honda Engines we talked about earlier have very little overlap in their stock cam profiles so the headers could not improve scavenging very much. On the other had, its possible for a proper header to make over 100 hp on a high-strung, fully built, all motor class drag car over a poor header or a stock exhaust manifold.
Back PressureThe third reason why headers works is pretty obvious, by reducing backpressure with straighter, less kinked tubes with smooth bends. Thus the flow of exhaust is less impeded out of the engine. As we discussed in my last column, an engine is a giant air pump, and less restriction means more pumping efficiency. In addition, the long individual runners of a header prevent the exhaust blast of one cylinder from blowing into the next cylinder in the firing order that happens to be on overlap. To the surprise of most people, this third reason is often the smallest contributor to power gains.
Because a header is tuned like a musical instrument, it can only be optimized to produce the greatest scavenge-improving vacuum in a band of several hundred rpm. In a nutshell, without going into a lot of math, some general guidelines for selecting a header are that shorter primary runners and/or bigger in diameter primary runners are better for top-end power. This has to do with travel time of the main initial exhaust gas pulse and the reflected low-pressure acoustic pulse in relationship during the engine's overlap period. Just like a piccolo is a higher pitched instrument than a clarinet, a shorter, fatter primary pipe is better for higher rpm, as the pulses have a shorter distance to travel. Conversely, a longer and/or thinner in diameter primary tube is better for lower rpm for the same reasons as above. The cams' duration and overlap timing also has a lot to do with the optimal dimensions of the header's pipes as well. Generally the later the opening point of the exhaust valve, the shorter the header primary pipes must be.
The way the primary pipes that come from each cylinder gather together is also important. This area of convergence, or the collector as it is called, is critical for proper header function. It must be of larger diameter than the primary tubes, because it must be large enough to acoustically represent the end of the pipe for tuning reasons; and it must be big enough to support the flow from all the cylinders without creating excessive backpressure. Usually, the collector is just a junction where all of the pipes are stuffed and welded into a larger pipe that may or may not neck down into the final size of the exhaust pipe. A well-designed collector pairs cylinders opposite each other in the firing order, so an exiting pressure pulse from one cylinder will not hamper the evacuation of the next cylinder, which is on the overlap part of the power stroke. In a typical inline four-cylinder, that would mean paring cylinders 1 and 4, and 2 and 3.
The best collectors are called merged collectors. This is a collector where the primary tubes are paired together in a smooth taper. Merged collectors usually produce a wider powerband and sometimes more top-end power. The wider powerband is due to the more gradual propagation of waves from the smoothly joined tube ends. With smoothly merged tube ends, the flow in the pipes is less turbulent, thus creating less backpressure, and increasing flow velocity. Not too many production headers are merged, due to the difficulty in fabrication, but most headers found on racecars are.
Many headers presently available for popular sport compacts are of the tri-Y design. For street cars, tri-Ys are usually the best as they are forgiving to differences in camshaft design and other tuning factors that the header builder has no control over - unlike a real racecar designer who knows exactly what's in his engine. Tri-Ys also promote a wide power band. A tri-Y design pairs the opposite cylinders in the firing order together in a short "Y", and then brings the two pairs of "Ys" into a single collector, hence the name tri-Y.
When a pulse travels down the primary tube of a tri-Y header to the collector, it mostly goes down the main branch of the primary. When it reaches the collector, the reflected wave also travels back up the main primary to the exhaust valve and back out again. However in a tri-Y, the branch that goes up to the opposite cylinder acts like an interference branch, since the exhaust valve is closed for that cylinder, it creates a pulse and an assisting negative pressure wave of its own, slightly out of phase with the main pulse. This increases the bandwidth of rpm so that the additional scavenging is effective by making the pipe less sensitive to rpm-induced changes in pitch.
The pipe becomes "in-tune" for a longer band of rpm, widening the engine's powerband at the expense of slightly reducing peak power over a 4-1 design. This is because some of the pulses' energy is dissipated in the interference branch. The main pulse is not as strong and the scavenge effect is not as effective for the tri-Y. Peak scavenging efficiency is compromised for having good scavenge over a wider range of rpm. That's why many full race engines, where peak power is important, use 4-1 designs; while many headers that are designed for a wide powerband or for applications where the final cam and engine specifications are unknown, like street engines or rally engines, use tri-Y headers.
We feel, for the most part, the majority of street performance freaks are better off either with a tri-Y or a 4-1 with either long runners, small runner diameters (or both) and a merged collector.
For street cars it's essential that the headers you purchase have provisions for all of the vehicle's stock O2 sensors, EGR fittings and any other emission controls that the vehicle originally had fitted to the exhaust manifold. Most modern emission controls do not rob any wide-open throttle horsepower. The common EGR valve, which reduces toxic oxides of nitrogen, closes and has no effect at wide-open throttle. Most air-injection devices only usually operate either on cold start or under closed-throttle deceleration on most modern cars. Removing these controls does not help power, and pollutes the air. This is not good for a street car, as we must all do our part to help keep our planet clean.
Just because your headers have provisions for all of your smog equipment, don't assume that it is street legal. Due to the intelligence of some of our local government agencies, unless an aftermarket part is CARB-approved with a CARB EO number, it is not legal in some states no matter how clean the gasses coming from the tail pipe are. So if avoiding a smog certification hassle is important to you, either check your local laws before installing or make sure that the part you buy has a CARB EO number. Your parts dealer should be able to answer that question.
You can generally expect a power gain of about 5-15 hp at the wheels from a well-designed header on most cars, depending on how bad the factory exhaust manifold was. If you drive the same, not exploiting your newfound power too frequently, you can expect better mileage with a header due to the improved pumping efficiency it produces. In buying a header, look for thick-wall mild-steel tubing, at least 16-gauge, and preferably 14-gauge. If you can afford it, buy a stainless header - stick to 304 stainless. The increase in life is well worth the extra cost. Rust and heat resistant ceramic coated or stainless steel primary pipes are preferred for longer life. Look for thick flanges also, as these will resist exhaust leaks and last much longer.
When it comes to purchasing a header for your vehicle, you really get what you pay for. There are a bunch of companies on the Internet selling copycat rip offs. These companies copy proved designs from established manufacturers, and produce the copies overseas from inferior materials. Companies like these steal established companies' R&D efforts for tuned design and fit. Although these parts seem like bargains, they use inferior grades of metal - the stainless steel may be of poor grade 400 series. These headers are prone to failure and when they put established companies out of business you can expect the innovation in our industry to die as well.
So this time around we explained how a header works. In our next edition we will delve into some formulas to determine a close approximation of what header will work the best for your engine and how to design a custom header for your particular application.