Boxer Rumble?

Equal Length vs. Unequal Length

What's the difference? Which is better?

One of the bigger discussions on Subaru forums for the past decades has been centered around the design of the exhaust manifold, 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).

Exhaust Manifold Basics & Nomenclature

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.

USDM Subaru STI EJ257 Exhaust Manifold

Stock USDM STI EJ257 Turbo Exhaust Manifold

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.

Unequal Length Exhaust Manifold

The factory exhaust manifold for most US market turbo EJ Subarus 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) Subaru Exhaust Manifold

Unequal Length (UEL) EJ25 Exhaust Manifold

Benefits of the Unequal Length Exhaust Manifold

The exhaust manifold 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 exhaust manifold. 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.

Equal Length Exhaust Manifold

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 exhaust manifold 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.

Equal Length (EL) Subaru Exhaust Manifold

AFE Equal Length (EL) FA20 Exhaust Manifold

Benefits of the Equal Length Exhaust Manifold

The equal length exhaust manifold 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

Which is Better?

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

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 exhaust manifold. 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.

Equal Length Sound

Unequal Length Sound

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