EJ255 intercooler
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Best Subaru EJ255 Mods

Chandler Stark

Meet Chandler

Chandler is an automotive expert with over a decade of experience working on and modifying cars. A couple of his favorites were his heavily modded 2016 Subaru WRX and his current 2020 VW Golf GTI. He’s also a big fan of American Muscle and automotive history. Chandler’s passion and knowledge of the automotive industry help him deliver high-quality, insightful content to TuningPro readers.

For those looking to build out their EJ255 powered Subaru, the top mods are a downpipe, ECU tuning, cold air intake, and an intercooler. Subaru used the 2.5 liter flat-four EJ255 four-cylinder engine from 2006–2014 inside various models, including the Impreza WRX, Forester XT, Legacy GT, and Outback XT. Depending on the year and model, Subaru rated the EJ255 at 210-265 horsepower and 225-245 lb-ft of torque. While it is not as popular as its more powerful EJ257 cousin, the EJ255 is still a very capable motor for builds. Let’s go ahead and take a look at the top Subaru EJ255 mods to increase engine performance. 

EJ255 intercooler
EJ255 Build (Credit: Antoxa2584x/Reddit)

Subaru EJ255 Basics

First, let’s start off with the basic nuts and bolts of the EJ255. It is a 2.5 liter, horizontally opposed flat-four motor with four-cylinders. The valve train is a dual overhead camshaft (DOHC) setup with four-valves per cylinder for 16 valves total. There are three different versions of the EJ255 that Subaru released. The first version lasted from 2004–2006 in some models, and used B25 heads, had a 8.2:1 compression ratio, and did not have variable valve timing. 

Starting in 2006 and lasting through 2014 in most models, version 2 of the EJ255 had 8.4:1 compression, D25 heads with smaller combustion chambers and a secondary air induction system, and variable valve timing on the intake camshafts. As for version 3, Subaru only used this on the 2010–2012 Legacy, and it had 9.5:1 compression, E25 heads, and variable valve timing on all four camshafts. 

Over the years, the EJ255 has earned a solid reputation as one of the top performance motors of the 2000s. It is not quite as highly regarded as its cousin the EJ257 that Subaru used inside the STI, but it’s a very similar motor. The main differences are the cylinder heads, compression ratio (STI is 8.2:1), the EJ257 uses VVT on both camshafts, and the EJ257 uses stronger connecting rods.

How Much Power Can the EJ255 Handle?

When building the EJ255, one of the most important questions to ask is how much power it can handle before you need to start upgrading the internals. Subaru motors are notorious for melting pistons and throwing rods. They can also be very susceptible to engine detonation or knock. There is a pretty wide variety of opinions regarding the issue, but most people agree that staying on the stock turbo is pretty safe for the engine.

On the EJ255, the stock turbo is going to max out around 300 wheel-horsepower for most cars. At that power level, the EJ255 is a very stout engine that does not need any internal modifications. If you plan on swapping your turbo, however, you’ll want forged pistons and rods at 400 wheel-horsepower, and head studs at 500 wheel-horsepower. 

EJ255 Supporting Mods for Your Builds

Before we get into the recommendations for the best EJ255 mods, let’s briefly discuss supporting mods. In addition to the mods we’ll go over below that add horsepower and torque, you’ll also want these upgrades that help your engine work with your new mods to produce the healthiest outcomes possible. 

Your standard EJ255 supporting mods are a 3-port electronic boost controller, turbo-heat shield (2006–2007 models), post-MAF hose upgrade, bigger fuel injectors, and a cat-back exhaust. These mods won’t add performance, but they will help with things like cooling, boost control, increasing fueling, and making your exhaust sound better. These mods are optional, but many tuners recommend them even on smaller builds, as they can help prevent catastrophic failure. 

The Best Subaru EJ255 Mods

Our top 4 EJ255 mods are:

  • Downpipe
  • Tuning
  • Cold Air Intake
  • Intercooler

1) Throwing on a New Downpipe

EJ255 downpipe
EJ255 downpipe (Cobb Tuning)

The first step for any EJ255 build is going to be replacing the factory downpipe with an aftermarket version. The OEM downpipe is heavy, restrictive, has a factory catalytic converter, and does not flow very well. While it’s adequate for stock performance levels, it’s one of the biggest impediments to increasing horsepower. 

Replacing the OEM downpipe will net around 15-25 horsepower and torque and really makes the engine come alive. In addition, it will also help the turbo spool up quicker, which brings on boost faster and increases torque. A downpipe adds significant volume to the engine as well, and makes the “Subie-Rumble” very prominent. 

If you do plan on upgrading the EJ255 downpipe, you will definitely want to get that 3-port boost controller. The stock 2-port system will not keep up very well with the decrease in backpressure and is prone to overboosting. You will also want to look into ECU tuning. Upgrading the downpipe without compensating via ECU tuning will result in detonation and overboosting. This is a quick recipe for catastrophic engine failure, so make sure you get the engine tuned too. 

Best EJ255 Downpipes

There are a ton of different downpipes for the EJ255 out there. The most popular tend to be from Invidia, Cobb Tuning, and Perrin. All of these will perform pretty similarly, though they will vary in terms of catalytic converter placement and type. You have the option of either standard or bellmouth designs, but it doesn’t really make a difference on the stock turbo. 

2) Subaru ECU Tuning

After upgrading the downpipe, you definitely need to look at an ECU tuning solution for the EJ255. ECU tuning will ensure that all of your mods play nice with the engine, and will ideally stop catastrophic damage from occurring. It will also help you get the most performance out of your mods, increasing horsepower and torque. 

Subaru motors are notoriously finicky and do not like random mods being thrown on them. Pretty much whatever you do, if it’s a downpipe, intake, turbo, or really anything, you’ll need to tune. Without tuning, modded Subaru engines are prone to detonation and pre-ignition, overboosting, and eventually complete failure. 

The Cobb Accessport For EJ255

By far, the best, easiest, and most user friendly ECU tuning solution for Subarus is the Cobb Accessport. The Accessport makes flash tuning an absolute breeze and is the industry standard for Subaru tuning. Most reputable tuners utilize the Cobb Accessport exclusively, and Cobb Tuning provides incredible backend and user support. 

3) Cold Air Intakes

Ej255 cold air intake
EJ255 Cold Air Intake from GrimmSpeed

After upgrading the downpipe and finding a tuning solution, the next step for your EJ255 mod list is a cold air intake. The factory intake is not the best and looks awful. They are somewhat restrictive, and their use of a panel filter instead of a cone-shaped filter is far from ideal. Getting an upgraded cold air intake allows your engine to suck in a greater amount of air with less restriction. Aftermarket intakes also usually shed a few pounds in weight by getting rid of or replacing unnecessary parts like the snorkel and heavy plastic airbox.

If you plan on getting an intake, you 100% need to figure out a tuning solution. Subaru used a mass airflow meter (MAF) to measure the amount of air entering the engine. The ECU calculates the proper amount based on the expected size and flow characteristics of the intake. If you upgrade to a new intake the ECU calculations will be off. This often results in lean air-to-fuel ratios, which induces detonation and can cause engine failure. Tuning is the only way to solve this problem.

Best EJ255 Intakes

Like downpipes, there are tons of different intakes on the market for the EJ255. Typically, the most popular are from Cobb Tuning, Mishimoto, Perrin, and GrimmSpeed. Importantly, you’ll want to get an intake with a full airbox, as it will cut down on intake temperatures and help the MAF scaling be more precise. With an upgraded intake, you can expect 5-15 horsepower and torque increases. 

4) Upgrading the Intercooler

For our final EJ255 mods entry, we suggest getting a better intercooler. The intercooler is extremely important for a turbocharged engine, but unfortunately the stock unit is lacking. Intercoolers’ job are to cool the hot compressed air after it leaves the turbocharger and before it enters the engine. Colder air means a higher oxygen content, which means more horsepower and torque. It also helps cut down on detonation and pre-ignition by keeping cylinder pressures down, too. 

If you are considering upgrading the intercooler, you have a few different options. The stock location is known as a top-mount intercooler, and it places the intercooler directly on top of the engine. For the WRX, this allows the hood scoop to feed cool air over the intercooler to help it cool down. Many people like the use of the scoop and keep the stock location when they upgrade.

The other option is to get a front mounted intercooler. A front mounted intercooler puts the intercooler in front of the engine and radiator. This gives it access to the coldest air possible, significantly reducing intake temperatures. Front mounts are often the best option, though they are also more expensive and require more extensive intercooler piping. 

Best EJ255 Intercoolers

For the best EJ255 intercoolers, you definitely have your fair share of potential picks. Some of the most popular and best performing are from Mishimoto, Perrin, GrimmSpeed, Cobb Tuning, and Process West. These include both top and front mounted options, and all of them perform extremely well compared against each other.

Additionally, many people will also retrofit an EJ257 STI intercooler onto their EJ255. This is a cheaper route to go, and the STI intercooler is still an upgrade over the EJ255 version.

Best Subaru EJ255 Mods Summary

During its run from 2004–2014, Subaru’s EJ255 engine built itself a solid reputation as a performance engine. Stock, it produces 210-265 horsepower and 225-245 lb-ft of torque, but it has so much more potential when modded. With just some basic bolt-on mods and tuning, you can make 300 wheel-horsepower with your EJ255.

The top 4 best Subaru EJ255 mods are a downpipe, ECU tuning, cold air intake, and intercooler. With all of these mods together, you can achieve at least 300 wheel-horsepower on most EJ255 builds. Power delivery is very smooth, comes on quick, and lasts until high in the power band. If you’re really serious about getting crazy, you can even look at swapping your turbo, but that’s an article for another day. 

Have you started building your EJ255 powered Subaru? Let us know what mods you’re choosing in the comments below!

Best EJ255 Mods FAQ

What are the best EJ255 mods for the WRX, Legacy, Outback, and Forester?

The top 4 best Subaru EJ255 mods are a downpipe, ECU tuning, cold air intake, and intercooler. With all of these mods together, you can achieve at least 300 wheel-horsepower on most EJ255 builds.

How much horsepower can the EJ255 engine handle when modded?

A stock EJ255 engine without any internal modifications can withstand 300-350 wheel-horsepower. From there, you’ll want forged pistons and rods at 400+ wheel-horsepower, and head studs at 500+ wheel-horsepower. 

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