Now that you've learned about basic header design in Part 1, let's get into the tuning and how a header makes power!
|This simplified diagram shows the relationship of cylinder pressure vs crank position. The first low pressure rarefaction is due to the gas column inertia, the second lower rarefaction is due to the reflected acoustic wave|
Header Performance and Tuning
Factory exhaust manifolds tend to have fairly restrictive designs. These restrictions tend to build more backpressure and hamper the velocity and flow of exiting exhaust gases from the combustion chamber. A general assumption is that a header produces more power merely by reducing backpressure. Although this isn’t entirely a false assumption, a header produces the most power due to exploiting other events in the exhaust stream.
First, a header increases power by serving an anti-reversion function. By keeping the high pressure exhaust pulse of one cylinder on its exhaust stroke from entering the cylinder adjacent in the firing order during overlap, a header greatly reduces cross contamination of the intake charge at overlap, preventing the intake charge from becoming diluted with exhaust gases.
Second, the greatest advantage of a tubular header comes from inertial tuning, which improves scavenging, or the removal of exhaust gases from the cylinders. When the exhaust valve opens, high (5-15 psi) pressure exhaust gases blow down the exhaust port at about 300 feet per second. The hot exhaust gas has a mass and inertia and its movement down the primary pipe produces a suction, or a negative (1-5 psi) pressure rarefaction wave, that travels down the primary tube behind the pulse. This negative pressure pulse creates a vacuum, producing a low pressure zone around the exhaust valve that helps scavenge the burnt exhaust gases out of the cylinder. It is important to time this event so the negative pressure pulse arrives when the exhaust valve is open on the overlap period of the 4-stroke cycle when the exhaust valve starts to close and the intake valve begins to open.
Thirdly resonance tuning comes into effect by reinforcing and sustaining this initial low pressure vacuum during the most of the overlap period. As the last of the high pressure exhaust gas empties from the primary tube into the collector, the low pressure zone near the exhaust valve starts to dissipate but as the exhaust gas flows into the larger diameter, lower pressure collector, it produces a backwards reflected acoustic pulse like the end of an organ pipe. This pulse travels through the hot, thin exhaust gas back towards the cylinder at sonic speed, about 1100-1900 feet per second, causing a slight increase in the pressure at the valve. When the wave reaches the larger volume cylinder, it is then reflected back down the primary pipe, drawing a negative pressure rarefaction behind it. This reflected wave provides an additional bit of scavenging to the cylinder before the overlap period ends as the piston departs downwards from TDC.
Tuning a header is influenced by piping length, diameter, shape, and the thermal properties of the material used, as well as desired powerband, cam overlap, and duration. Designing the header pipes with the proper length and diameter can increase the amount of time a low pressure area exists around the exhaust valve. This will help suck out as much of the residual exhaust gas as possible and pulling in some of the fresh air/fuel mixture.
A properly designed exhaust pipe is tuned like a musical instrument and most effective at scavenging for a range of just a few hundred rpm. It is difficult to time the arrival of the wave fronts at the cylinder and collector, which is why many headers include design features such as stepped primaries, which increase the length of time the header is in tune. Other exhaust details that affect power and powerband are wave amplification and wave harmonics.
Tri-Y versus 4-1 Headers
Many aftermarket street headers use a Tri-Y design which offers the widest powerband and can adapt more easily to changes in the engine’s tuning, such as camshaft design. A Tri-Y header pairs opposite cylinders in the firing order into two “Y” shapes, and then joins both “Y”s into one collector. On a Tri-Y header, the exhaust pulse travels down the primary. When it reaches the collector, the reflected wave goes back up the primary to the exhaust valve. The exhaust valve on the opposite branch of a Tri-Y header is closed, which produces an addtional assisting wave that helps keep the primary pipe in tune for a longer period of time.
|Examples of classic Tri-Y headers. A Tri-Y is called so because the opposite cylinders in the firing order are paired together so the branch of the first Y that ends with a closed valve can act like an interference branch. A Tri-Y header typically has a broader and slightly lower powerband. This is because the interference branch makes a secondary rarefaction pule slightly out of phase with the main pulse|
|Bottom- Although Tr-Y headers are often relegated to street use due to there wider powerband and forgiving to tuning nature, Hi-Tech and a few others make race type Tri-Y's as the one shown here with merged Y collectors, megaphones, stepped primaries and stainless construction|
This widens the powerband rpm range that the scavenging is achieved. Some of the pulses’ energy is dissipated in the interference branch making the initial biggest pulse scavenging effect slightly less. This is why a Tri-Y has a wider powerband, although peak power is slightly lower than a 4 into 1 header design. Tri-Y headers are the best design for street or rally cars, which require good driveability and a broad powerband.
|DC's Honda/Acura B18C header is the classic street 4-1 design|
A 4-1 header is usually more effective at tuning for peak power and more often found on race cars or cars with highly tuned engines. On a 4-1 design, the main pulse sent down the primary is stronger, with no interference branch volume induced pressure buffering, offering more peak power but for a shorter band of rpm. Without an interference branch to buffer it, the reflected wave is also stronger. These effects generally make the 4-1 more powerful over a more narrow range. Being a simple tube, it is easier to design and tune a 4-1 header as well as predict the results of a design.