How headers work

Feb 10, 2018 10:41 AM - 751 Views

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How headers work

Headers can provide more than just sending the exhaust to the back. A properly designed header will actually increase the efficiency of the engine, helping to draw more air and fuel into the combustion chamber through a process called scavenging. This process is very similar to a stove pipe or chimney.
Under the right circumstances, the chimney draws air from the room and releases it out the top, this is called drafting. As wind blows across the top of the chimney, a low-pressure area is created inside, drawing air from the room below, which feeds the fire, generating more heat production.

If the chimney is too small, the effect will be reduced and smoke fills the room. If the chimney is too large, smoke fills the chimney, but the lack of drafting reduces the amount of air feeding the fire.
Engine exhaust works on a similar principle, with mechanical pressure coming from the engine side. As the combustion process ends, the exhaust valve opens, releasing the pressure into the exhaust system.
The air moves from the high pressure chamber to the low pressure exhaust. The rate at which the exhaust gasses move is directly related to the initial pressure inside the header (and exhaust system as a whole).
This is called “blowdown” and is the difference between cylinder pressure and exhaust system pressure. Excessive blowdown pressure means less gas moving into the header on its own, requiring mechanical expulsion (pump action, i.e. piston movement). Reducing blowdown increases the rate of gas movement in the initial stage of the process.
Once past the initial blowdown expulsion, when the pressure between the combustion chamber and exhaust system have equalized, the piston takes over, providing the pump action needed to expel the rest of the exhaust and push it along its way through the exhaust until the piston reaches TDC (top dead center) and the pump action ceases.
The exhaust gasses do not stop when the pump action is finished. The inertia of the hot gasses keeps it moving through the pipe. The exhaust pulse is a not a single action, one blast and it’s over, the repeated pulsing functions like a waveform, just like sound waves. If it were blasted into open space (as in no header or manifold) it would have no positive effect on the engine.

The basic principles of exhaust flow are fairly simple; getting the maximum exhaust from point A to point B is where things get interesting. There are many factors at work in the scavenging and blowdown process. Primary tube length and diameter are the main components in the fight for horsepower and torque. From here, we will discuss the mechanics of headers in terms of short, mid-length and long primary tubes.
“Effective header primary tube and component geometry attempts to take advantage of two distinct forces occurring inside the header to increase performance” said O’Neil, “One is the kinetic energy of the gas stream and the resultant low pressure area behind it, and the other is the considerably greater energy of sonic finite amplitude waves that originate upon the opening of the exhaust valve.”
In terms of primary length, amplitude wave tuning is limited to long tube headers. Short and mid-length headers are simply too short to take advantage of the length of the waveform. The most common use for shorty and mid-length headers are cost and clearance.

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