A component of any car or truck engine is an exhaust manifold for each bank of cylinders .In most production engines, the manifold is a cast iron assembly designed to collect the exhaust gas from two or more cylinders into one pipe. Engineers like cast iron because of its heat management properties and its superior longevity over any other type of exhaust outlet design. They also use the least metal, occupy less space and have the lowest production cost. Although these design restrictions are cost effective, they are not the most efficient choice for venting gases from the engine.
Inefficiencies generally occur due to the nature of the combustion engine and its cylinders. Since cylinders fire at different times, exhaust leaves them at different times, and pressure waves from gas emerging from one cylinder might not be completely vacated through the exhaust system when another comes. This creates a back pressure and restriction in the engine's exhaust system that can restrict the engine's true performance possibilities.
Steel exhaust headers are one of the easiest bolt-on accessories to improve an engine’s performance. The goal of headers is to make it easier for your engine to push exhaust gases out of the cylinders. It works by eliminating the manifold’s back pressure. Instead of sharing a common manifold, each cylinder gets its own exhaust pipe. The individual pipes are cut and bent so that each one is the same length as the others, which guarantees that each cylinder’s exhaust gases arrive in the collector spaced out equally so there is no back pressure generated by the cylinders sharing the collector. Therefore, you notice more power and torque from your engine, as well as better overall driveability and fuel economy.
Headers do more than just funnel spent gases out of the engine compartment less restrictively than manifolds; they draw spent gases from the combustion chamber by a process called scavenging. During the exhaust stroke of the engine, the exhaust gas is forced out of the combustion chamber through the open exhaust port by a combination of lower pressure outside the chamber and pressure from the rising piston. The exhaust will continue to be drawn out as long as the exhaust valve stays open. If the timing is off and the valve closes with exhaust left in the cylinder, you lose efficiency. Headers draw maximum exhaust from the cylinder during the duration of the exhaust stroke.
The inertia from the escaping exhaust gases moving through the header pipes creates energy pulses that form vacuums, pulling additional exhaust out of the combustion chamber so that the maximum amount of fresh fuel and air can be drawn in. The more fresh fuel and air you can pack into the cylinder, the more power the engine can make. The less energy the piston has to use pushing exhaust out of the cylinder, the faster you can go.
It’s very satisfying to listen to your engine idling and feel its powerful vibrations. But as soon as you step on the gas and hit the road, your headers come into play. They are the first stop for exhaust gases on their way out of your cylinder heads and into the exhaust stream, and they can make or break your car’s performance.
Headers come in two types- long tube and short tube. What sets them apart is the length of the primary tubes to the collector. For “shorty headers,” the port pipes actually merge into a single exhaust pipe in a much shorter distance. For long tube headers, the exhaust pipes are much longer (about 28”) and merge farther out. Because of this, each design impacts back pressure in the exhaust system in a different way.
Longer port pipes result in lower exhaust back pressure. This means improved oxygen intake and a boost in horsepower. But don’t assume that long tube headers always outperform shorty headers. It’s not that simple. Horsepower and torque output depends on the RPM range.
Factory headers, or “manifolds” are generally designed to meet the most strict emissions requirements, and be cost effective to mass produce. These manifolds are generally cast, leaving inconsistent imperfections within the inner ports. These imperfections can restrict the flow decreasing the efficiency of the engine’s exhaust flow. Their port design is typically geared towards quiet engine operation. In addition, they weigh significantly more than an aftermarket header.
However, on high-performance cars, like the Challenger R/T or SRT, manufacturers use short tube headers, or “shorties” (see photo, below). They are a great option to produce more low to mid- range power in reference to your engine’s rpm band. Shorties do a good job of emptying the cylinder of exhaust. This is especially beneficial on a big motor, like a Hemi, that produces large amounts of exhaust discharge. They, however, do not offer much ram aid, like long tubes, because they don't generate very strong negative pressure waves. Short tubes use welded mandrel bent pipes, rather than a casting. They are typically easier to install compared to long tube headers and accommodate factory catalytic converters. For emissions controlled states, short tubes are typically the only legal option.
Long tube headers are a great option for engines that build a ton of power in the mid to high rpm range. Due to their longer and narrower primary tube lengths, they require extra room inside the engine compartment. This makes them more difficult to install and may call for some costly reconfiguration (see photos, below).
Long tubes are a great option for someone who is racing and pushing their car to the redline. The goal is to produce maximum peak horsepower without worrying about things like catalytic converters and complying with emissions standards. Long tubes, however, will cause a check engine light on most modern applications, unless a custom tune is used to tune out the secondary oxygen sensors.
Long tubes keep exhaust gasses separate from each cylinder for a much longer period of time. Their length allows an elongated path for the gas to flow, thus allowing the gas to leave the cylinder with more speed because the gas has more time to gain velocity before it hits the collector.
Specifically, they work by creating a deep pressure depression that pulls really hard on the intake during overlap. This enhances the ramming of mixture into the cylinder. Getting more mixture into the engine with these reverse negative pressure waves is why long tube headers become increasingly effective as cam duration and overlap periods get longer. With a mild factory cam, where there is more dependence on efficient expelling of the exhaust than pressure wave utilization, there isn't a lot of difference between short and long tubes. It takes a decent cam, high compression and to some extent high RPMs to really get what long tube headers can deliver. The back side is they can over-scavenge the cylinder which will require enriching the mixture.
Here are photos of the modern Hemi engines with their “shorty” clam shell exhaust headers.
For racers interested in installing long tube headers, proven and trusted brand names are recommended, like: Gibson Headers, Kooks Headers, Hedman Headders, Hooker Headers, Pacesetter Headers and BBK Headers.
Also, to prevent corrosion, you should consider stainless steel or ceramic-coated (see below) headers. Ceramic headers are preferable because they run much cooler than stainless steel headers (258 degrees Fahrenheit vs. 870 degrees Fahrenheit) and maintain their out-of-the-box chrome-like appearance forever.