EJ257: An Overview of the Changes Over the Years (2004-Present)


On a quest of STi erudition!
EJ257: An Overview of the Changes Over the Years
2004 - Present
(USDM models)


In 2004, Subaru said:
"The 2.5-liter turbocharged powerplant is the largest displacement engine ever installed in a WRX STi model and is exclusive to the North American market. It is the most powerful 4-cylinder engine available in the US. Until now, STi-badged Subaru vehicles had been available only in select world markets.
The success of the WRX prompted strong demand from American consumers for an even more extreme-performance version of this car. As a result, the U.S.-market Impreza WRX STi gives up nothing compared to models offered in Europe, Japan and other markets and in some ways offers more performance."


-2.5L (2,457cc / 149.9 Cubic Inches) Horizontally Opposed Boxer
-DOHC 4 cylinder, 16 valve
-Enlarged Air-Intake Hood scoop with Water-Sprayed intercooler: manually operated hood-scoop water sprayer provides additional cooling to the intercooler
,use distilled water, do not overfill if freezing is possible
-Drive-by-wire throttle
-Aluminum alloy pistons
-Cast iron cylinder liners
-Reinforced "semi-closed deck" alloy block
-Forged high-carbon steel connecting rods
-Resin composite intake manifold
-Cast iron exhaust manifold
-Electronic Drive by Wire throttle
-Bore: 99.5 x 79mm, 3.92 x 3.11"
-Redline: 7,000 RPM

-B25 (Single AVCS)*

-IHI VF39 Turbocharger
-The VF39 utilizes a thrust bearing design and the P18 exhaust housing
-The turbo is water cooled and the bearing is oil cooled

-300 HP @ 6,000 RPM
-300 lb/ft TQ @ 4,000 RPM

-Maximum Boost 14.5psi

-AVCS (Active Valve Control System) variable valve timing optimizes engine volumetric efficiency through the rev band
-Sodium filled exhaust valves

Tune Up Specs:
-Direct Ignition Iridium plugs, #NGK IFLR6B
-Firing order: 1-3-2-4
-Ignition timing at idle 17*+/- 10*
-Idle RPM at park: 700 +/- 100

-91 octane minimum, high octane is important!

-12V, 90A. Battery 48ah


No revisions added to the 2004 engine.


No revisions added to the 2004 engine.


-Addition of a Secondary Air Pump: is an air injection system that injects air post combustion for a predetermined amount of time after the engine is started. Its only active on startup for a very short time (15-60 seconds) and is inactive after that point. It was introduced in 2007 in the STI's and now most Subaru models have them. The air pump assembly is solely there to combat stringent emission laws. It has no impact whatsoever on fuel efficiency or performance. The total system consists of an Air pump, 2 valves and a crossover pipe.


-V25 (Single AVCS with Air Pump)*

-IHI VF43 Turbocharger
-The VF43 utilizes a thrust bearing design and the P18 exhaust housing. The primary difference between the VF43 and the VF39 used previously on STI's seems to be the stiffness of the wastegate. The VF43 has a stiffer wastegate designed to reduce boost creep issues. The VF39 is still prone to cracking at the wastegate, but occurs far less than the VF39.

-293 HP @ 6,000 RPM
-290 lb/ft TQ @ 4,400 RPM

-The exhaust valves are no longer sodium filled


-There is no longer an intercooler sprayer system
-Aluminum alloy intake manifold
-Redline: 6,700 RPM

-W25 (Dual AVCS)*

-IHI VF48 Turbocharger
-The primary difference between the VF48 and the VF43 used previously on STI's is the more advanced bearing design.

-305 HP @ 6,000 RPM
-290 lb/ft TQ @ 4,000 RPM

-Maximum Boost 14.7psi

-Dual ACVS implemented. Dual AVCS adjusts both the intake and exhaust camshafts, independently.

Tune Up Specs:
-Direct ignition Iridium spark plugs, #NGK SILFR6A


No revisions done to the 2008 engine.


No revisions done to the 2009 engine.


No revisions done to the 2010 engine.


No revisions done to the 2011 engine.


No revisions done to the 2012 engine.


No revisions done to the 2013 engine.


Tune Up Specs:
-Ignition timing at idle 15*+/- 10*


No revisions done to the 2015 engine.


No revisions done to the 2016 engine.


No revisions done to the 2017 normal STI engine.

However, in 2018, the STI Type RA was introduced. This special Record Attempt variant was limited to 500 units at the cost of $50,000.

STI Type RA Engine:
-Reinforced pistons
-High flow air intake system
-High flow exhaust system
-Sport Tuned Engine Control Unit

STI Type RA Heads:

STI Type RA Horsepower/Torque:
-310 HP @ 6,000 RPM
-290 lb/ft TQ @ 4,000-5,200 RPM

STI Type RA Compression/Turbo:
-Maximum Boost 16.2psi

STI Type RA Valves:
-Sodium filled exhaust valves

STI Type RA Tune Up Specs:
-Direct ignition Iridium spark plugs, #NGK ILFR7H

I did some more research and found out the following information:

A bone stock 2018 STI Type RA on the dyno made:

-295 HP to the wheels @ 5,600 RPM
-306 lb/ft TQ to the wheels @ 3,700 RPM

A bone stock regular 2018 STI on the dyno made:

-266 HP to the wheels @ 6,000 RPM
-263 lb/ft TQ to the wheels @ 4,700 RPM​

Math checks out that a regular STI in stock form makes 87.2% of the factory rated horsepower to the wheels and 90.6% of the factory rated torque to the wheels. With these percentages, I figured out that the STI Type RA in stock form makes roughly:

Estimated Horsepower/Torque:
-349.7 HP @ 6,000 RPM
-335.8 lb/ft TQ @ 4,000 RPM​

Therefore, the STI Type RA engine is underrated from the factory by roughly 40 HP and 45 lb/ft TQ.


The 2019 engine has literally been imported from the 2018 STI Type RA! All 2019 STI's will come with that engine! This is extremely crucial!!! Never has the base STI model been so revamped before with these much needed improvements. 2019 STI owners receive a vast majority of the upgrades, of the $13,000 premium price of the 2018 STI Type RA, at the base price of just $37,000!

* = Still need to research more information.

This is going to be a future sticky thread. This is going to be a massive and ambitious overview on the changes done of the EJ257 over the years. This will take a lot of time and thorough research until it gets completed and will constantly be edited with new information. This will be based off of the USDM engines from 2004 to present time.

I think most of my information will come from cars101 since they have proven time and time again that they have endless information on Subaru vehicles. Edit: I have found various errors in some of their information. I have looked through the owner's manual PDF's and other sites to verify what is and is not correct.

It's becoming evident very quickly with the more I search that there isn't much information out there regarding these changes over the years... This is a surprising discovery...


STI Type RA Sources:


On a quest of STi erudition!


On a quest of STi erudition!

I looked at that thread as well, but I wanted to know the revisions done to the engines specifically throughout the years. I'm going to be doing a similar thread on the transmissions from 04-present next.

Also, I'm fairly certain there is a lot of information that I could not find. It's somewhat hard to believe only a single revision was done between the 2009-2018 engines...


On a quest of STi erudition!
I'd like to know what is the difference from the W25 to the new N25 heads

Since I don't have much knowledge on the internals of the engines, I wasn't aware of some information. Apparently:

2004-2006 STI heads = B25 (Single AVCS)
2007 STI heads = V25 (Single AVCS with Air Pump)
2008-2018 STI heads = W25 (Dual AVCS)
2018 Type RA/2019+ STI heads = N25

Is this correct?

I'm not finding any technical specifications online... I think I'm going to have to email SoA.
I know I've seen D25 too. I am not familiar with heads other than the W25 and when buying oversized valves, cams, springs, seals, etc. I remember seeing on iag website of head work they had N25 from Type RA/newest STI and went to SOA website and cross referenced part number the heads were different but if I remember correctly part numbers of cam sprockets, cams, valves, springs, seals, etc. were identical except for the head gaskets.


New member
I have the IAG stage 1 N casting heads, the cams are different all 4 have avcs oiling holes, and there is an additional port on each front exhaust side of the cam cap, IAG said all 4 cams have avcs and the exhaust side would need to be blocked off for them to work with my 2014.


New member
Turns out I was mistaken, I was under the impression my car had dual avcs but it does not thats why I thought there was a difference.