One of the bigger discussions on Subaru forums for the past decades has been centered around the design of the header, also known as a "header." More specifically, the lengths of the runners as they lead to the collector.
Whether you are a diehard "Subaru Rumble" fan or someone looking for the last drop of power out of the engine, this post will give you the answers you need to decide on a header choice; equal length (EL) or unequal length (UEL).
When you see people talking about an equal length or unequal length manifold / header they are referring to a specific layout of the pipes, aka runners, that start at the cylinder heads exhaust port and travel to a merge point known as a collector.
Stock USDM STI EJ257 Turbo Header
Exhaust gasses are expelled by pressure into the runners and travel through their individual runner tube until they reach the collector. At the collector the pulses blow through and merge into the up-pipe which feeds the turbocharger.
The factory header for most US market turbo EJ20 / EJ25 Subarus including the WRX, STI, FXT (Forester XT), LGT (Legacy GT), Baja Turbo, and Outback XT are unequal length. This includes the current 2.5L STI, but does not include the FA20 powered WRX / FXT. Naturally aspirated cars have a really badly designed, but quasi-equal-length, manifold.
Unequal Length (UEL) EJ25 Header
The header above is an unequal length model. You can see how the two left runners are much longer than the others. This saves material, weight, and has a slight benefit to turbo response in the low end of the powerband because less heat, pressure, and velocity is lost on half the header. For those that like the unique "Boxer Rumble" the unequal length manifold is the reason for that sound, but I'll explain why that is later.
The Japanese STI features an equal length manifold design on its high revving 2.0L EJ207 engine to improve some aspects of the powerband. The equal length header is characterized by having all runners be the same length, hence the name. That means the exhaust gasses travel the same distance before they merge into the collector.
AFE Equal Length (EL) FA20 Header/p>
The equal length header is a much more efficient design for a couple of reasons, but it all stems from the exhaust runners being the same length. The above Tomei header is a 4:2:1 design, meaning 4 pipes merge into 2 and then merge into a single pipe. This typically improves midrange power slightly over a 4:1 design at the cost of peak power.
The equal length manifold also reduces the cylinder temperature variations between the individual cylinders. This is important to help avoid ringland failure on cylinder #4 for example. This comes also with a normalization of exhaust pressure (backpressure) at each exhaust port which improves engine volumetric efficiency and top-end power.
Learn more about turbo Subaru ringland failure in the EJ25
For performance and reliability, the Equal Length manifold is a superior design. Unfortunately, this comes at the cost of a widely-favored and trademark sound: the Boxer / Subaru Rumble.
Sound is pressure, you're hearing the thousands of exhaust pulses being played like a musical instrument through the exhaust. The iconic Boxer Rumble has nothing to do with the horizontal orientation of the engine and everything to do with the length of the exhaust runners. The unequal length runners create a note with a strong pulse (two pulses colliding / merging) followed by a mellower note. In fact, when you record a Subaru, the microphones wave form actually will show this!
Sound Pressure Waveform Differences Between Equal Length and Unequal Length
What you are looking at is the sound pressure recorded at the end of the exhaust for two similarly setup cars. The top is my 2011 STI with the stock header. The bottom is my buddy Lukes 2010 STI with a Killer B 4:1 equal length manifold. Both of our cars have 3 turbo backs and nearly identical setups otherwise.
The bottom graph is smooth, equal, and ideal like a steady heartbeat. This is the difference in sound, its the sound of efficiency.
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