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The Truth Behind The Subaru EJ-Series Engines - Tech-Knowledge

Are they as weak as tuners claim?

Scott Tsuneishi
Feb 4, 2011
Photographer: Owner

Once known simply as the automotive company that built modest commuter cars like the Brat ('78 to '87), Subaru (a division of Fuji Heavy Industries) became internationally known for their boxer engines. Beginning in 1989 and continuing through to the present day, Subaru's horizontally opposed EJ-series motors are the mainstay engines of their model lineup, with all EJ-series engines sharing a 16-valve, flat-four horizontal configuration with displacements ranging from 1.5 to 2.5 liters. So why has Subaru continued their love affair with the boxer engine?

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Utilizing a boxer engine configuration similar to what was once used in older-model Volkswagens, Subaru has continued use of the boxer engine with good reason. The Subaru boxer engine is a unique engine that's anything but typical. Engineer Karl Benz patented the flat-four engine in 1896. In 1971, inspired by airplane engine design, Fuji Heavy Industries, through the Subaru brand, released their interpretation of the flat-four. Because the engine was configured to be inline with the transmission it had a lower center of gravity to help minimize body roll in comparison to inline-four and V-type engines, which have a higher center of gravity and are prone to side-to-side vibration. This design provided a lighter weight and compact, economic design for Subaru. In all, there have been 23 engine variations of the EJ motors, including the '11 WRX STi model that develops 305 hp and 290 lb-ft of torque.

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Subaru engines have always been known to be well engineered that are both powerful and torquey, but Subaru engineers left a lot to be desired when it came to the EJ205 ('02 to'05 USDM WRX), EJ255 (USDM WRX '06 to present) and EJ257 ('04 to'07 USDM WRX STi) turbocharged models. This month, we get in-depth with three of the top Subaru tuners in the world, as they help dispel myths and speak their minds about the infamous EJ-series engine.

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Yimi Sport Tuning of Santa Clarita, CA, has been serving the Subaru community since 2003 as a one-stop shop for enthusiasts of all makes with parts, installation, and tuning, and also custom fabrication and welding services. Yimi Sports not only caters to customer cars but are avid participants at various road race events such as Super Lap Battle and Import Tuner EVO vs. STi Shootout, including this year when they took home the title of fastest Subaru.

Q: Focusing on the EJ205, EJ255, and EJ257 engines, there's been a lot of debate on their strengths and weaknesses and how they were never designed to handle an elevated horsepower level. Is there a secret recipe that can better educate Subie owners on how to build their engines to increase horsepower output and longevity?

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A: The EJ-series motors are good motors, and we would not call them "weak" per se. On a stock motor, the two primary weaknesses are the stock cast hypereutectic pistons and oiling to the rod bearings. The pistons (particularly on the EJ255 and EJ257) will crack if subjected to knock/detonation events. Obviously, with proper ECU tuning the engine can be kept away from the knock threshold, making the pistons not as much of an issue.

Oiling to the rod bearings is the other primary issue. We typically don't see very many problems with the EJ255 and EJ257 in this respect, but do see it more on the older EJ205. Fundamentally there isn't a big difference between the 2.0L and 2.5L motors in terms of the design of the oiling system but the EJ205 is at least 5 to 9 years old now and some have not been treated to regular maintenance with quality fluids. From the factory, the allowable range of bearing clearance is pretty broad and if you don't use quality synthetic oil and change it often it can lead to problems. The same can be said of any motor, so while not a strong point of the EJs, it should not be painted as a "bad motor" because of it.

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We don't believe in "secret formulas," but rather common sense. In the case of making EJ motors last, it boils down to proper ECU tuning by a reputable tuner who clearly understands the logic of the ECU and its interaction with the engine, along with regular maintenance and using quality fluids in the car.

Q: There's been much discussion that the newer Subarus are difficult to tune. Is it true that the '07-to-current Subarus require tuning with every aftermarket component installed or risk a potential loss in horsepower?

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A: It's not the engine that is more difficult to tune, it is the ECU that has gotten more complex on the newer cars. As long as the tuner understands the functions and logic of the ECU, the newer cars are no more or less difficult to tune than the older cars. Ideally you'd always want a custom tune to be performed with the exact combo of parts that are going to be run for the foreseeable future.

Unfortunately, Subaru chose an unfortunate ECU tuning strategy in more recent years (mostly '07 and newer) for Federal emissions compliance. Basically there is a long delay in the switchover from closed to open loop operation that forces the car to run very lean fueling even as load/boost rises. At that point, knocking/pinging/detonation is almost unavoidable and the resulting concussive pressure wave will crack the piston ring lands. In large part, this is what has caused some people to believe that the EJ motors are weak, but the underlying cause is actually in the ECU tuning.

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Auto Master AMR, located in Mamaroneck, NY, is a family owned and operated establishment with over 50 years and three generations of experience. AMR is known to build some of the world's highest horsepower cars, from nitrous oxide to forced induction, and currently holds the record for the world's quickest and fastest '05 STi (7.75 @ 175 mph) and '08 STi with a quarter-mile record of 10.54 @ 142 mph.

Q: What are some of the drawbacks of the WRX and STi engine's internal design, and the limitations on how much horsepower they can handle?

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A: The Subaru EJ-series engines are put together very well from the factory. Though not equipped with forged internals, the factory engine in its stock form can handle double its factory power output. A majority of engine failures are caused by uneducated tuners who don't understand how the vehicle and its engine work, along with not knowing the consequences of using low-quality parts to handle their horsepower expectations. The Subaru EJ-series motor is unlike any OEM engine on the market. We at AMR have pushed these motors, with our internal forged components and performance parts, to over 1,000 hp with no failure point. The bottom line is an educated tuner, high-quality performance parts, and an educated driver will equal a reliable vehicle.

Q: What's a common misconception when building an engine?

A: The most common is believing the block needs to be sleeved. Most people jump right to them at 400+ whp but sleeves are not needed until over 550+ whp.

Q: What are the most overlooked issues that cause mistakes when building engines to handle an increase in horsepower?

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A: Plain and simple, it would have to be checking proper clearances and tolerances.

Crawford Performance specializes in designing, building, and campaigning high-performance vehicles in various racing motorsports including but not limited to time-attack, drift, rally, and endurance. Crawford is recognized around the world for its successful customization of the Subaru Impreza STi, including building the infamous Gymkhana 1 and 2 cars driven by Ken Block. The combination of high-end components, extensive R&D, and their resistance to up selling the customer has created the trust Crawford Performance currently enjoys in the Subaru community. It probably doesn't hurt that Crawford is the only company factory backed by Subaru offering built motors.

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Q: "Forum specialists" who offer bad advice on how to build engines are flooding the internet. What sort of bad advice have you heard from customers or read on the forums that made you cringe?

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A: We at Crawford Performance will start this by stating the number one issue responsible for the damage to Subaru engines is the free software available on the internet to modify the tuning parameters within the ECU. There is plenty of bad advice going around but the most common one I hear is "the bigger or looser the piston or bearing clearances, the better the engine will perform." Another is about larger pressure/volume oil pumps are always better because the factory oiling system on the EJ motors are inadequate, when in fact, a high-pressure oil pump can cause oil cavitation while lowering the ability for any oiling system to work efficiently. Another falsified fact is using a lower-temp thermostat will make your motor run cooler in hot weather but in reality it will have zero effect on increasing your engine's cooling capability. Other falsified facts include tuners believing that installing 1mm oversized intake and/or exhaust valves over the stock size valve seats will increase cfm airflow, and the best of all that makes me laugh every time I hear it is "you can make 500 whp safely on pump gas."

Q: What are the stronger points on the turbocharged EJ platforms?

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A: The boxer motor is well balanced because of its design and it's because of this design that it delivers plenty of torque compared to other four-cylinder motors on the market. I have tuned a lot of motors over the years and the EJ loves to make power in stock form, there's no need to change internal components to get it to rock!

Q: You came across an interesting finding on the factory water pump. Would you care to disclose this information?

A: The water pump (front, right) is a cast turbine that flows more water, whereas the OEM piece (rear, left) allows for water to bypass the blades and does not flow as much water. It is one of our trade secrets of engine building and you are first to hear about it.

Engine Building Tips From the Pros

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If there's some magical formula on how to properly build an EJ engine, chances are our top tuners have found it! Follow along as they offer a simple how-to guide to help point you in the right direction whether you're planing on building a 300 whp, 400 whp, 500 whp, or 500+ whp engine.

Quirt Crawford Of Crawford Performance

300 whp: To obtain this horsepower level, the EJ motor only requires an upgraded intercooler, larger sized fuel injectors, and a retune of the ECU-no internal engine modifications are required. We highly recommend the use of an air/oil separator (AOS) on any turbocharged EJ motor to eliminate oil from coating both the intake tract and the inside of the intercooler, which causes a reduction in cooling efficiency that can cause detonation.

400 whp: There are no internal engine modifications needed at this power level. The car would need a larger turbo, intercooler, fuel injectors, fuel pump, AOS, and retune of the ECU.

500 whp: At his point, we suggest replacing the OE rods with a set of stronger, aftermarket forged rods, along with forged pistons, head studs, and the external components listed under 400 whp.

500+ whp: The same engine components are needed in the 500-whp package but you will probably need a larger turbo, and of course, proper tuning.

Albert And Robert Bonamici Of Auto Master AMR

300 whp: The EJ20/EJ25 engines can handle 300 whp maximum with stock internals with no further modifications other than a proper fuel management tune.

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400 whp: We suggest a set of forged AMR pistons and billet rods to safely boost up the engine.

500 whp: The same components previously mentioned in the 400-whp package but also include our AMR 9/16-inch "Big Boy" head studs, AMR plate hone, and O-ring the block to increase sealing of the heads to the cylinder block.

500+ whp: Previously mentioned items with the addition of a line-hone to ensure proper bearing clearances, AMR billet crankshaft, and ductile iron sleeves for additional cylinder strength.

Paul Yim And Paul Leung Of Yimi Sport

300 whp: No modifications are required to the stock block, assuming all the bolt-on parts and ECU tuning are working in harmony.

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400 whp: For an EJ205, you should be doing a basic build on the block that would include forged pistons, rods, good bearings, and head studs. Obviously, quality machine work and tight tolerances/quality control are crucial. For an EJ255 and EJ257, all the block needs is a set of forged pistons. 400 whp (particularly on 91 octane pump gas) is when cylinder pressures are high enough that a random knock event can take out the stock pistons. The rest of the internals are more than up to the task.

500 whp: We suggest adding forged rods, aftermarket rod and main bearings, and ARP head studs to the EJ25 along with making sure that the oiling system and cooling system are in 100 percent operating order.

500+ whp: For over 550 whp, we recommend sleeving the block, as the stock sleeves can become a failure point. For really high-horsepower applications, pinning the mains and oversized head studs and/or an O-ringed cylinder head may become necessary.


Crawford Performance
Oceanside, CA 92056
Yimi Sport
Santa Clarita, CA 91350
Auto Master AMR, Ltd.
By Scott Tsuneishi
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