Unequal Length VS Equal Length Header?

There are many attributes that make the Subaru WRX and STI so appealing to enthusiasts, owners, and current shoppers. Aside from their rich rally heritage, Subaru in general, has always been a unique vehicle. Subaru had led the US markets in the Japanese AWD sales and performance for many decades, however, during the 80s Subarus were an odd duck. Whether it be the wagon models, or in the mid to late 90s when the SVX debuted, they were in a class of their own. Nothing too special, but very utilitarian.

Yet in the late 90s when Subaru was really shining their bat signals in the sky, it was their efforts in rally racing. In the US, we were so used to NASCAR and Formula One, that for those of us grassroots SCCA members, watching or reading about Subarus racing through mud, snow, and terrain piqued our interests. This was a car that aside from the roll cage, astounding turbo noises, and the occasional flame blowing out of the exhaust, looked like something we could build in our own garages. When the Subaru WRX landed on US shores in 2002, we all though we were rally racers.

One of the infamous qualities of the Subaru WRX and STI models equipped with an EJ engine, is the engine’s rumble aka the Subi rumble. Subaru’s flat-four engine design creates a distinctive sound that it sounds unlike any of your average 4-cylinder Honda. Subaru like Porsche has a flat engine design where the cylinders are divided on each side that are held together by a shorter crankshaft.

For Subaru, it is the exhaust manifold design that creates that easily identifiable Subaru rumble. Now for enthusiasts who are researching mods on the internet while going down the path of power upgrades. You will always see the term UEL, or EL when reading about Subarus and exhaust systems. UEL stands for un-equal length while EL stands for equal length. Again, these are references to the exhaust manifold, or header. The length is a reference to the tubes of the exhaust manifold, or aka runners. Now let’s have a deeper look at both designs, because there are not exactly pro’s and con’s when choosing which options is right for you, there are rather gives and takes. Before we look at unequal length vs equal length, we need to take a closer look at the two.

OEM EXHAUST MANIFOLD

As you can see, the OEM design is an unequal length because you have the passenger side engine bank has an integrated-up pipe flange. And both flanges going into the cylinder heads are connected with a cross pipe. The other side of the exhaust flange on the driver side uses that cross pipe to reach over to the other side. An equal-length design would have exhaust pipes or runners that are equal in length, and would merge into the collector where your up-pipe would connect.

And for those of you who are new to mods and upgrades, the terms, exhaust manifold and header are the same. This is the first component of your entire exhaust system as it connects the exhaust ports on your cylinder head, then the exhaust gases travel out to your muffler. On turbocharged vehicles the exhaust manifold will connect to your turbo, then the turbo will shoot the exhaust gases out of a downpipe, then out to your cats, then out of your cat-back exhaust system. Same concept with naturally aspirated vehicles, the exhaust manifold will connect to a primary cat converter before getting routed back to the muffler. OEM exhaust manifolds are usually these heavy one-piece cast units where the exhaust gases come out into one narrow passage, then out to your muffler. Headers are aftermarket units where each exhaust port has their own piping runner that all merges into the turbo flange, or exhaust collector on naturally aspirated vehicles.

UNEQUAL LENGTH HEADER

GrimmSpeed Unequal Length Exhaust Header 2002-2014 WRX / 2004-2021 STI

An unequal-length header will have longer exhaust runners on the driver-side of the engine bank because where all the pipes meet on the exhaust collector will be closer to the passenger side of the engine. This is because the turbocharger is mounted near the upper corner of the transmission. These are typically a 4-2-1 design. You have (4) individual runners from each bank of the engine that merge into a collector flange that will have (2) exhaust ports, then it will connect to the up pipe. The turbocharger connects to the other flange on the up pipe that will mount the turbo on the upper passenger side corner of the engine bay. The turbo rests just above the transmission, and underneath the top mount intercooler.

Because the length of the exhaust runners are longer on the driver-side bank of the engine, this is what helps to produce that Subi rumble. And with a quality performance unit like any listed in this article, the rumble will rumble louder.

EQUAL LENGTH HEADER

IAG Performance Equal, Length Header w/Up Pipe 2015-2021 STI

As the name suggests, an equal-length header features manifold runners that are equal in length, and will typically merge in the middle, with a longer cross pipe section that will curve with the bends of the manifold as it extends into the engine bay where the up pipe will connect. Equal-length exhaust manifolds provide an even distribution for the exhaust gases to escape the cylinder heads, generating better exhaust flow with less turbulence and restriction. For those who build EJs to make big horsepower numbers (600+ and beyond), an equal-length header is usually preferred because the same length of the exhaust runners can flow more exhaust gases more efficiently when compared to unequal length.

Now sound is subjective, and equal-length manifolds will produce a more traditional 4-cylinder sound, and you won’t have that pronounced rumble. Yet the sound is still distinctive, and will be more aggressive with the right choice in cat-back exhaust systems.

UNEQUAL VS EQUAL LENGTH?

So, what should you get? An equal-length header or an unequal-length header? There are a few things to consider, and there are pros and cons for both designs. What we know off the bat is that the unequal-length header is known for producing that flagship Subaru rumble, while the equal-length can handle extreme horsepower numbers. And to be realistic, most enthusiasts are not going to build a WRX or STI to make more than 500 WHP unless they have a full commitment to race car maintenance, deep pockets, and have a fully forged bottom end with supporting mods to make the power happen. So, let’s break down the benefits and some things to consider when upgrading your exhaust manifold.

The cast OEM exhaust manifold on the WRX or STI is very heavy for what it is. To be fair, it must be to sustain all driving conditions and environments on a worldwide scale. Most aftermarket examples are made using 304 stainless steel and offer a decent amount of weight savings.
The performance options whether it be unequal or equal, will improve turbo spool as there are individual runners for each cylinder, as opposed to the OEM design where there are cast flanges that are merged by a cross pipe.
There are horsepower and torque gains as each exhaust runner will have smooth bends to allow the exhaust gases on each cylinder head to escape faster without interference. This will help maximize the performance out of your OEM turbocharger, while maximizing the flow and efficiency out of larger turbo upgrades,
For WRX owners equipped with an EJ, and aftermarket manifold/header is required when upgrading to an STI oil pan. The STI oil pan is larger, and you will not have the proper clearance needed with the OEM exhaust manifold.
Some aftermarket turbo manifolds will have their own up pipe to work with the increased piping diameter of the manifold collector size. The OEM up pipe is a restrictive piece especially in older WRX and STI models because due to all of the heat cycles, vibration, and rust in some cases, internal debris will block the exhaust path to the turbo, causing a loss in power.
With the un-equal length version, this will really amplify the “Subi rumble”, whereas the equal-length produces a more traditional 4-cylinder engine sound. Equal-length is preferred for those with high-horsepower builds; however, many have made big power numbers through unequal length examples as well.
There are also unequal-length header options for the 2015-2021 WRX models as well.
If you have power goals of above 400 HP, an exhaust manifold upgrade will be required.