Exhaust headers are among the best means to boost power on most engines. They can make a profound performance difference. How do they do this? The answer is more involved and interesting than just back pressure reduction.
Unlike exhaust manifolds, headers have individual tubes for each cylinder that join together into a collector. The tubes are of equal length and tuned.
"Tuned" means the tubes are of the proper diameter and length to perform optimally for a given engine displacement and RPM range.
Exhaust gases don't flow continuously, they flow in pulses from a given cylinder with a new pulse coming with each exhaust valve opening. Each pulse creates a point of high pressure traveling through a header tube. Between each pulse the pressure is relatively low.
Header tubes are equal length. Each exhaust valve opens at a different time - 120 degrees of crankshaft rotation apart for a 6 cylinder engine. Because of equal tube length and non-concurrent exhaust valve opening, exhaust pulses from different cylinders don't collide in the collector. The high pressure pulse from one cylinder arrives at the collector in the low pressure zone between pulses from other cylinders. This reduces overall back pressure of the system.
If the tubes where not equal length the pulses would collide in the collector and increase back pressure.
That explains the reduction in back pressure. But why not eliminate the exhaust system entirely and let the valve vent directly to the atmosphere? That would surely deliver even less back pressure. Yet ALL race cars use headers. It turns out that headers do more than just reduce back pressure. Read on.
Each pulse of gas has mass, and as it moves down the tube it develops momentum. If you suddenly try to stop the flow of that pulse by closing the exhaust valve, it will attempt to keep moving (a body set in motion will remain in motion). The result is that something of a vacuum is created behind the pulse. If the exhaust valve is still partially open,
that vacuum draws the residual exhaust gases out of the chamber, improving evacuation. This is called "scavenging" and is one of the key benefits of a header system. If the cam has a bit of overlap (intake valve and exhaust valve open at the same time) the intake charge is sucked into the cylinder, delivering a dense uncontaminated intake charge.
The diameter and length of the header tube are critical. For a given engine displacement, a smaller tube will cause the exhaust pulses to flow faster down the tube, thus increasing the momentum and the scavenging effect. Too small a tube and back pressure increases. Long header tubes provide superior low RPM performance while shorter tubes work best at high RPM.
Optimal tube length and diameter depends on displacement and the desired RPM for the power band. Hence big race engines - big tubes, small street engines - small tubes.
Bonus information for reading to the end - intake runners have a similar tuning effect! Engine components need to be balanced - exhaust, cam, intake, RPM range and displacement all have to work together.
- Chuck Moreland