Understanding The Complex Theory Behind Subaru’s Stout Boxer Engines

There is no arguing the prowess and power potential of the Boxer engine. Patented in 1896, Boxer engines have been utilized by Volkswagen, Porsche, and Subaru. Porsche has one of the most legendary racing portfolios in motorsports, each accolade owing to the Boxer. Subaru has taken its Boxer engine design to numerous World Rally Championships and road circuit victories.

We will be examining the development of Boxer engines, how they are built and operate, and why Subaru sticks with Boxer engines in their vehicles to this very day.

The Encompassing Idea Behind It All

A Boxer engine positions its cylinders in two banks, 180-degrees apart from one another and on each side of the crankshaft, respectively. As a result, the engine sits at a lower center of gravity compared to other V engine setups, which in turn reduces vehicular body roll and enhances handling and maneuverability.

Due to the engine’s lower center of gravity, a Boxer engine provides less body roll during cornering.

We spoke with Dominick Infante, National Manager of Product Communications at Subaru of America, who gave us a bit of insight about Boxer engines.

“It was developed in the early 1960s, when our engineers found out that they should apply a front engine, front wheel drive layout to expand the interior space of the vehicle,” Infante said.

The biggest advantage is vehicle stability thanks to Subaru’s all-wheel drive layout, which is highly contributed by the Boxer engine.
– Dominick Infante

The Subaru 1000 was the first front-wheel-drive Subaru produced that was classified as a compact car by the Japanese government, which started in 1966. Interestingly, previous models such as the Subaru 360 were rear-engine, rear-wheel-drive propositions.

Being the first production Subaru to utilize a Boxer engine, the engineers at Fuji Heavy Industries learned a considerable amount of information with the experiment.

“With a transverse engine layout, the length of driveshafts are different between right and left, requiring the use of a joint part, and in turn causes some vibration,” Infante said. “On the other hand, with the longitudinal inline engine, it brings bad weight distribution from a long front overhang. As a result, the engineers needed to change the configuration. Since the V-type engine configuration is theoretically not good in balance, they decided to adopt the Boxer engine setup, which has good inherent balance.”

According to Fuji Heavy Industries, Subaru has kept the same formula all these years because of the engine configuration’s reliability. The pistons in V-type engines move in a way that creates wear and tear on the engine as well as reduce fuel efficiency.

In a Subaru Boxer engine, the pistons face opposite each other in a side-to-side symmetrical layout. The opposing pistons work to cancel out the inertia force of each other, resulting in less vibration, superb rotational balance, and a smooth feel when approaching the high rev range. The engine’s flat, low-profile configuration enhances driving stability and handling performance. Over a long period of time, Subaru has committed to maximizing the advantages of its Boxer engines and continued to enhance them to power its vehicles.

Click on the image to see Boxer pistons in motion.

Furthermore, opposing banks of pistons in Boxer engines reach top dead center at the same time as compared to V-type engines where the piston movements alternate from bank to bank. A Boxer engine’s vibrations are essentially negated by the side to side movement of the pistons, reciprocation, and ignition forces.

Inline and V-type engines cannot reproduce the same result without constructing complicated crankshaft counterweight and dampening systems. With each opposing cylinder associated with its own crank throw in a Boxer engine, their axis is offset from one another, resulting in reciprocating torque.

Complexity With Precision: How The Boxer Is Made

Boxer engines experience a complex production process that truly separates them from the pack. After the aluminum block is initially cast, it is sent to a special assembly line where complex and expensive proprietary machines create intricate parts, which are etched with a QR code to ensure part number accuracy.

The engine consists of several parts as the pistons are located on each side of the engine. The piston heads are then assembled symmetrically from the crankshaft, which results in the engine being lower than a majority of the mechanical components that drive the automobile.

Angled split connecting rods bring the engine together with ease, but this takes twice as long to assemble since the heads are quite complex. A QR code system is used to ensure top quality parts are being used during the production process.

The assembly of a Subaru Boxer engine takes approximately 4-1/2 hours at the Subaru Plant in Gunma, Japan. According to Subaru, they have assembled in excess of 11 million horizontally opposed engines for the market worldwide.

1024px-Boxer_Sports_Car_Architecture_engine

A look inside the Boxer engine shows its engineering complexity.

Boxer Advantages

Subaru continues to be a leading manufacturer with Boxer engines simply because it complements their vehicles better than any other configuration.

Subaru has been producing Boxer engines for more than 40 years. These powerplants are now more compact, use minimal amounts of petroleum, and are environmentally friendly due to advances in engine technology.

Boxer engines provide quick handling response and flat, confident cornering during spirited driving. With the pistons laying flat in a boxer engine, it helps the vehicle run smoother as each time a piston cycles, the motion of the opposed pistons cancel out the vibration.

However, the only drawback to Subaru’s late model Boxer engines is it requires extensive costs for production and development. But such was not always the case. Shinroku Momose is regarded as the father of the Boxer engine in Japan. He was the mind behind the 1966 Subaru 1000s EA52 engine, which measured 977 cubic-centimeters and was the first generation Boxer engine offered by Subaru.

A Look At A Few Notable Subaru Engines

The EJ20 engine seen in a 2006 Subaru Forester.

EJ20

The EJ20 is the second generation 2.0-liter Boxer engine produced by Subaru in 1989. A successor to Subaru’s EA engine, the EJ became a pillar in Subaru’s engine line with its 16-valve, horizontal flat-four cylinder design.

This engine was versatile in that it was available for single or double-overhead camshaft configurations. The EJ20 was also manufactured in naturally aspirated and turbocharged trim, producing power outputs ranging from 96 to 320 horsepower.

The EJ207 is used in Subaru Impreza WRX STi’s 1998 to present day and serves as homologation models for the World Rally Championship. The differences that separate this from the EJ20 are a semi-deck block, a higher rev limit, red intake manifold, and an inlet under the manifold.

The EJ207 engine in WRX STi's are something of legend.

EJ25

The EJ25 is a 2.5-liter Boxer engine that has compression ratio between 9.5:1-10.0:1 naturally aspirated and 8.0:1-9.5:1 turbocharged. Used in the ’98 Impreza RS, ’96-’99 Legacy/Outback, and ’98 Forester, the EJ25 had a naturally aspirated power rating of 165 horsepower at 5,600 rpm and 162 lb-ft. of torque at 4,000 rpm.

One hiccup this motor had was the DOHC valve architecture because the spark plugs are more difficult to service compared to the SOHC configuration. As a result, the DOHC EJ25s were fitted with platinum spark plugs as a preventative course of action.

However, EJ25 short blocks are the groundwork for reliable racing engines. The EJ25 is a popular engine for swaps simply because of its power potential. The EJ25s 2.5-liter turbocharged Boxer engine won International Engine of The Year award in 2008 for its blend of high-performance coupled with advanced technology. The International Engine of The Year awards was established in 1999.

FA20

The FA20 Boxer engine, developed from the FB engine, had common design goals, namely a reduction in the engine’s weight while retaining the same durability. The FA20D is high-tech. It features both direct and port injection and Subaru’s AVCS variable valve timing system.

Utilizing a shallower oil pan and a shorter intake manifold, the FA20 engine sits even lower than Subaru’s EJ engine, giving the FR-S/BRZ optimal balance and a lower center of gravity.

Used in the Subaru BRZ, the FA20D is a 2.0-liter flat four-cylinder boxer engine that has a 12.5:1 compression ratio and a power output of 200 horsepower at 7,000 rpm and 151 lb.-ft. of torque at 6,400 rpm. This engine is also standard fare in the Scion FR-S.

Using its own direct fuel injection and twin-scroll turbocharger, Subaru introduced the FA20F in 2012, but was only available for the 2012-up JDM Subaru Legacy 2.0GT. It didn’t get used in the United States until the 2014 Subaru Forester XT, which is rated at 250 horsepower and 258 lb-ft. of torque.

The most powerful version of the FA20 resides under the hood of the 2015 United States Subaru WRX, which has 268 horsepower and 258 lb.-ft. of torque with a 10.6:1 compression ratio.

Designed to give the driver the best driving experience possible, the FA20 engine series serves as a motor built for spirited driving. These terrains include mountain and hill climbs as well as tight and narrow roads. The combination of a light car and a light engine result in a nimble, capable automobile that has shot the FR-S/BRZ into the spotlight. Let the FA20F/BRZ swaps commence.

Tuning The Boxer Platform

All the engines listed above are stars in the tuning community. To gain some insight into the pros and cons of tuning the Boxer, we contacted Aaron O’Neal of English Racing. English Racing is a tuning shop based in Vancouver, Washington that has a great deal of experience developing flash tunes, full engine builds, and everything in between for popular Subaru platforms, as well as 4G63 and 4B11 Evos, and the R35 GT-R.

From an architecture standpoint O’Neals says, “flat cylinder motors are naturally dynamic balanced and therefore do not require balance shafts nor counterweights on the crankshaft. It decreases windage as a result because the crankshaft is far smaller in dimensions than a comparable inline or V motor. They are compact longitudinally which is nice, and though they are wider than a comparable inline motor mounted longitudinally, they are comparable if the inline is mounted transverse in that they occupy a space frame rail to frame rail.”

It could be said that tuners make their living on the disadvantages of a given platform. By optimizing the weakness, the tuner creates a superior performing product. To this end O’Neal is quick to point at three recently cleared hurdles that have reinvigorated the Subie scene.

“A decent intake manifold with proper length runners and a decent plenum is a fairly new offering. The Cosworth piece was nice in its time but the Process West billet manifold is what the motor has needed .”

“Secondly, the turbo manifold and piping is excessively long in stock configuration. Turbo kit manufacturers in the last few years have placed the turbo low and in front of the motor which cures two problems. One, it keeps center of gravity low. Two, and arguably more important, more heat goes into the turbo optimizing performance and the tuned exhaust length is closer to what it needs to be, which is shorter. Lastly, the rear ports on the Subaru heads are asymmetrical compared to the fronts and this leads to various issues which only can be lessened with a good head porting job.”

O’Neal categorizes these tidbits as “shot from the hip” opinions about the Boxer and that there is plenty more to talk about. He says it seems that the powerplant is finally coming into its own but notes it has been the awkward little brother compared to other import four-cylinder platforms.

The Subaru 1000 engine that started the Boxer engine revolution.

Overall, Subaru has stuck with the same formula since the 1960s and has seen fantastic success. With its reliable and powerful engines, the automaker has engineered an intoxicating combination that has also taken the tuning scene by storm.

The EJ207 engine is tried and true, capable of plentiful power potential and exceptional weight bias and distribution. The EJ engine series remains a staple in the rally racing and the street arena and we don’t see it changing anytime soon.

So if you’re looking for a great handling automobile that is reliable and powerful, look no further than Subaru’s ingenious Boxer engines. To learn more about what Subaru is up to next, check out Fuji Heavy Industries website here.