Along with a better-flowing intake and exhaust system, headers are among the first bolt-ons that any naturally-aspirated believer has got to consider. Together they help eliminate restrictions within an engine’s intake and exhaust tracts, making them essential to anyone who’s done clowning around when it comes to making more power.
What Exhaust Manifolds Do
All internal combustion engines use some sort of exhaust manifold to merge exhaust gases exiting the cylinder head’s ports into the catalytic converter, past the muffler, and into the atmosphere. The configurations are many; some feature a tubular design, some are integrated directly into the cylinder head, but most are manufactured from cast iron and feature a log-shaped layout. Log manifolds are cost-effective and lend themselves well to saving space underneath the hood. Because of their construction and shape, they also heat up quickly, which allows the catalytic converter to do its job more effectively. But log manifolds don’t do much for horsepower.
A header is an exhaust manifold, only better. And more complicated. If an engine’s exhaust only puffed once, it wouldn’t be, and figuring out a header’s ideal tube length and diameter for optimal power would be easy. Instead, a typical four-cylinder engine at 8,000 rpm spits out more than 67 exhaust pulses per second, per cylinder, complicating and muddling up the whole process.
Headers are based off of a series of tubes with smooth, gradual bends that allow each cylinder its own means of exhaust gas evacuation as opposed to factory-issued manifolds that simply gather up all of the exhaust gases near the head and dump them into what’s more often than not a turd-shaped log. Primary tubes span from the cylinder head to a collector where exhaust gases from each cylinder join before entering the remainder of the exhaust system. Like the intake side, air velocity is important, except here its own energy must be harnessed to evacuate the cylinders. Instead of doing all of this by means of positive pressure like the intake side, negative pressure generated near the exhaust valves helps suck unwanted exhaust gases from the cylinders. The hard part is figuring out a way to not just increase that negative pressure but to control it so that it occurs within the exhaust ports at exactly the right time.
Wave Resonance Tuning And More Power
Compared to log manifolds, headers can make more power in three ways, one of which is wave resonance tuning. Once the exhaust valves open, a high-pressure stream of exhaust gases forced out by the upward-moving piston begins exiting the cylinder head, creating a pressure pulse. Once that pulse reaches the end of a particular primary tube, a reverse wave travels back up, creating a low-pressure void near the exhaust valves. Wave resonance tuning by means of calculated primary tube lengths allows this low-pressure wave pulse to occur at exactly the right time—during the cylinder’s overlap period that happens as the exhaust stroke concludes and the intake stroke begins. (In case you’ve forgotten, overlap happens when a cylinder’s intake and exhaust valves are both open for a brief period of time.) When coordinated properly, the results are a suction effect that evacuates additional exhaust gases and, in some cases, can even draw the air/fuel mixture into the combustion chamber from the intake side. The process is known as scavenging.
Pulse Tuning And More Power
The length of a header’s primary tubes affects its powerband. You knew all of that, but you might not know why. The primary tube’s overall length and diameter determine how much of that negative pressure we talked about will build up and when it’ll do its job. Exhaust gas velocity doesn’t change much, so it’s the primary tube’s length and diameter that determines when all of this happens. Tuned primary tube lengths can help snake-charm additional exhaust gases out of the engine during each exhaust stroke. The optimal length is one that corresponds to the exact time in crankshaft degrees that the engine’s exhaust valves begin to open. As you’d expect, the math is complex, and different engines require different length primary tubes depending on a variety of factors, like the desired powerband.
Backpressure And More Power
Reducing exhaust gas backpressure is worth mentioning but isn’t as important as old-time hot rodders would have you believe. To be sure, wave resonance and pulse tuning are the real keys to whether or not a header makes more power. Still, backpressure does play a role. Unlike log manifolds, a header’s longer primary tubes prevent exhaust gases from cross-contaminating between cylinders, which can bungle up the evacuation process. Headers also reduce backpressure through more efficient designs, which reduce restrictions and increase pumping efficiencies—all good things if you care about stuff like horsepower.
Every header has some sort of collector where each cylinder’s high-speed exhaust gases meet and are diffused after flowing through their respective primary tubes. A properly designed collector can increase an engine’s powerband and yield more high-RPM power by decreasing turbulence and increasing exhaust gas velocity. Once inside, exhaust gases either exit through the remainder of the system, past the muffler or bounce back toward the exhaust valves as low-pressure pulses. A properly designed collector does more than serve as a junction for exhaust gases, though. Here, cylinders with opposing firing orders are paired together so that one’s exhaust pulse energy won’t interfere with another’s, further reducing cross-contamination. A good merge collector will do all of this in a smooth, tapered manner, which results in a more impressive powerband.
Reversion is bad. It has to do with the unwanted exhaust gases that travel backwards through the exhaust system when exhaust gas velocity is low and scavenging is minimal. Under those conditions, exhaust gases can enter the intake side, contaminating the combustion process. Anti-reversion headers incorporate primary tubes with a stepped design, which increase in diameter shortly after exiting the cylinder head, and a direction-sensitive cone that keeps exhaust flow moving in the right direction. When done properly, anti-reversion steps can help prevent unwanted exhaust gas reversion, resulting in a broader powerband and more torque.
Tri-Y (four-into-two-into-one) headers are one of the most popular four-cylinder header layouts, which incorporate four primary tubes that transition into two, all before merging into a single collector. The alternative is the four-into-one header in which all four primary tubes merge into a single collector. Tri-Y headers care less about camshaft profiles and overlap than four-into-one designs do and are best suited for applications where a broad powerband is desired. The Tri-Y design also lends itself to the opposed-firing-order pairing mentioned earlier. Here, when exhaust gases enter the collector, reflected waves are also sent back up opposing cylinders with their exhaust valves still shut, creating a secondary pulse that’s out of phase with the corresponding main pulse but assisting it, helping broaden the powerband at the expense of some peak power. Scavenging doesn’t happen quite as well here because of the secondary pulse’s interference, but it happens for a longer period of time when compared to four-into-one designs.
Headers with shorter and larger-diameter primary tubes are generally better suited for high-RPM power while those with longer, thinner tubes lend themselves better to mid-range output. We’ve already talked about the relationship between valve overlap and header design. As such, before choosing a header, you’ll want to consider when your engine’s exhaust valves begin to open. Typically, headers with shorter, larger tubes will respond better to engines with exhaust valves that open late. And if you have a twin-cam engine with adjustable cam gears, more power can also be found by fooling around with overlap after installing a header. When choosing a header, one with a smooth, tapered merged collector is never a bad thing. You’ll also want to consider the header’s construction. In terms of materials, 14- or 16-gauge mild-steel steel tubing is ideal as are thick flanges and the appropriate gaskets and corrosion-resistant hardware. If longevity and anti-corrosion are concerns (and they should be), consider stainless steel or ceramic-coated mild steel. Emissions legality and whether or not whatever header you’re considering features all of the provisions for any oxygen sensors or catalytic converters that your engine may require should also be considered. Finally, don’t forget that, like any engine upgrade, a header is only part of the puzzle.
Headers don’t look terribly complex but designing one that works well requires all sorts of complicated math and fancy physics. Along with an engine’s intake, throttle body, intake manifold, camshaft, cylinder head ports and exhaust system, headers are an important part of an engine’s airflow path and must be addressed and modified as a system. Matching a $1,500 four-into-one race header with anti-reversion chambers and a tapered collector to an engine with an OEM airbox will never make sense, so plan your upgrades accordingly.