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How Boxer Engines Work

And Why You Should Care About Subaru’s New FA20

Aaron Bonk
Feb 7, 2013 SHARE
Sstp 1301 01 o+boxer engine+subaru D45 Photo 1/1   |   How Boxer Engines Work

Flat engines like Subaru’s Boxer are nothing new. The design, which gives the finger to inline and V-type engines and instead incorporates horizontally opposed banks of pistons, was patented in 1896 and has since been implemented by Volkswagen, Porsche, and most notably, Subaru. The Boxer layout, which positions its cylinders in two banks—180-degrees apart from one another and on each side of the crankshaft—has been used on everything from commercial airplanes to bikes like Honda’s early Goldwings to cars as modest as Volkswagen’s Beetle and as shameless as Porsche’s 911 Turbo, with cylinder arrangements featuring as many as 12 pistons. In the 1960s, Subaru fully adopted the Boxer design, which today remains the company’s exclusive engine configuration.

Subaru’s reason for that is simple: The Boxer’s horizontally opposed layout lends itself well to low centers of gravity and exceptional weight bias, like in the case of their BRZ and the company’s jointly built Scion FR-S where the engine’s pistons and crankshaft are positioned low and laterally centered in the vehicle in ways that would otherwise be impossible for any inline or V-type engine. The results are, more often than not, better stability and better vehicle control, which is partially what makes Subaru and Toyota’s latest creation’s so special.

How Boxer Engines Work

The Boxer design isn’t all that complicated and isn’t all that different from any other engine that takes cues from the four-stroke instruction manual. Imagine a 60-degree V-type engine where its two banks of pistons form its V shape. Now imagine a 90-degree V-type engine where its two banks of pistons form an even wider V shape. Increase that V shape by another 90-degrees and you’ve got a Boxer engine. As it turns out, it wouldn’t be an entirely cockamamie thing to say that the Boxer layout is indeed just a 180-degree V-type engine.

Boxer engines get their name from their pistons that simultaneously move toward and away from one another in a horizontal plane, much like boxers clashing their gloves together before a fight. Their opposing banks of pistons that reach TDC (top dead center) at the same time is in stark contrast to V-type engines where countering piston movements alternate from bank to bank. The Boxer’s side-to-side movement of any two corresponding pistons cancels out each other’s vibrations caused by reciprocation and ignition forces in ways that inline and V-type engines never could without complicated crankshaft counterweight and dampening systems. But that doesn’t mean that four-cylinder Boxer engines are free of complications. Since each opposing cylinder is associated with its own crank throw (unlike V-type engines where multiple cylinders share the same throw), their axis are offset from one another, which results in reciprocating torque known as “rocking couple.”

The FA20 Engine

Subaru’s latest 2.0L Boxer engine isn’t its most powerful, but tuners everywhere are beginning to understand why it might be so special. Perhaps the best thing about the FA20 is its engine cover, or the fact that it doesn’t have one at all. The missing hunk of matte-black plastic is just the beginning, though. As you’d expect, the engine is a hodgepodge of Subaru and Toyota technology. Although, at first glance, the FA20 appears to be entirely Subaru, its square configuration is very much Toyota-like and, rumor is its cylinder heads were developed by Yamaha, who’s no stranger to Toyota cylinder head development.

Subaru’s all-new 2.0L FA-series engine was designed and built specifically for the BRZ/FR-S. To help achieve the sort of handling and control its creators were looking for, the FA20 was made more compact than previous four-cylinder Boxer engines by developing a shorter intake manifold and shallower oil pan. FA20 engines even sit lower to better optimize the car’s center of gravity and overall balance, something that remains instrumental to the satisfaction of driving the BRZ/FR-S.

At the heart of the four-cylinder FA20 are an aluminum block and cylinder heads that, together with all of their internals, result in an impressive 12.5:1 compression ratio, and chain-driven dual-overhead camshafts. At only 200hp and 151lb-ft of torque, the BRZ/FR-S isn’t straight-line fast. It was never meant to be. As it turns out, 200hp, when paired with a 2,762lb chassis (a remarkably low vehicle weight by 2012’s standards) and with everything strategically positioned for optimum balance, isn’t all that far off the mark. The BRZ/FR-S was designed to provide the best overall driver’s experience, not to satiate boy racer stoplight-to-stoplight tendencies. Even so, the FA20’s power rating is nothing to turn your nose up to, especially when considering its modest engine size, the results of which are one of the most impressive specific engine outputs on the planet, measuring in at 100hp per liter.

The Intake

The FA20’s symmetrical 86mm bore and 86mm stroke gives further credence to the square bore philosophy that allows for high-rpm capabilities but without sacrificing proper heat dissipation. Subaru also implemented its AVCS (Active Valve Control System), which features variable valve timing on both intake and exhaust cams. To be sure, AVCS is partially what gives the FA20 its broad torque curve and high-rpm capabilities, all the way to its 7,400rpm redline. Subaru’s variable valve timing technology, much like other manufacturers’ systems, uses hydraulic pressure to manipulate valve timing according to engine load. Unlike previous Subaru engines, though, the FA20’s AVCS sensors and solenoids are positioned differently. The new chain-driven camshafts now allow for a smaller AVCS mechanism and a design that’s, overall, much more impact-resistant when compared to its predecessors.

The FA20’s intake feeds into a three-inch O.D. electronically controlled throttle body that’s bigger than the STI’s (2.75-inch O.D.) and will likely be the last restriction to unlocking power. The first thing to notice on the BRZ/FR-S intake is that it’s fed from cold-air utopia, directly in front of the radiator, outside of the engine bay. Once inside the intake pipe, air passes through a plastic wall with several small holes to help reduce turbulence. Once inside the air box, air passes through the actual filter element, past the MAF sensor and into the rubber hose that feeds into the throttle body. Unlike other systems prevalent today, the path is short and isn’t entirely convoluted. Although improving on the actual intake piping isn’t easy, the filter element itself isn’t terribly large and gains can be found by increasing its surface area. As with any MAF-based vehicle, changes to the intake tubing diameter and shape can create turbulence, which can cause confusion between the MAF and the ECU, which can lead to rich or lean conditions, depending on the circumstance. Aftermarket manufacturers are currently reporting gains as high as 14hp by switching to larger-diameter, tapered, molded polymer-type intake systems that increase airflow and reduce intake air temperatures.

The Exhaust

Because of the Boxer’s 180-degree orientation and traditionally unequal-length exhaust manifold, its sound remains unique and is often mistaken for a misfire by schmucks who don’t know what a proper Boxer should sound like. The primary tubes’ varying lengths randomize the travel time for exiting exhaust pulses as they make their way toward their collector. It’s these out-of-phase pulses that lead to Subaru’s unique Boxer sound.

Forget all of that, though, because the FA20’s exhaust system is where it separates itself from almost every other Boxer. It begins with a more conventional, equal-length, 4-2-1 header—another very un-Subaru-like detail—which is why the BRZ/FR-S doesn’t sound like any other Subaru and which is why the FA20 is capable of 100hp per liter. Despite all of this, the FA20’s header has two strikes against it: although it features generous 1.625-inch primaries and 1.75-inch secondaries (diameters that arguably won’t be tampered with much by the aftermarket), its merge collectors aren’t all that efficient and its built-in ceramic-core catalytic converter will stymy proper airflow. Longer primary tubes, a proper collector and ditching the cat will all improve performance.

From the header, exhaust gases pass through a short pipe that travels over the subframe, through another pipe that hosts another catalytic converter, into a mid-pipe, and finally through the muffler. Each pipe after the header and before the muffler measures only 2.118-inches in diameter and features a series of non-mandrel bends and clearance dimples, which means there’s plenty of room for improvement. Of course, the muffler itself isn’t a high-flowing core so there’s further potential trapped there. Increasing the piping diameter to at least 2.5 inches from the header to the muffler is a good place to start looking for ways to unlock the FA20’s potential. All of this is reportedly good far upwards of 15hp, according to tuners, which isn’t too bad when considering the FA20’s already impressive horsepower-per-liter ratio. Besides improved performance, most aftermarket exhaust systems, despite their larger diameters, will also shave upwards of 15lbs from the BRZ/FR-S’s already lightweight. Of course, if low emissions is your primary concern, then you’ll need to rethink everything you’ve just read since eliminating the header’s cat is the key to a proper high-flowing exhaust system.

The Fuel System

Although the brunt of the FA20 is of Subaru’s doing, Toyota is responsible for the engine’s unorthodox but effective D-4S (Direct Injection 4-Stroke Gasoline Engine Superior Version) fuel injection system, which is a combination of direct injection and conventional port injection technologies, and is derived from the company’s D-4 direct injection system that debuted in Japan in the mid-’90s. Toyota’s D-4S direct injection system works like any other direct injection system, spraying fuel directly into the combustion chambers instead of upstream, helping further cool the cylinders and create a more efficient burn. But Toyota’s D-4S direct injection system also works unlike any other direct injection system, relying on a conventional port injection configuration. While the direct injection side allows for the FA20’s high compression ratio without the consequences of detonation, upstream and working in parallel with the direct injection system, the engine’s four, 205cc/min Denso port injectors help promote complete combustion and are crucial to cold-start emissions. For the FA20’s combination fuel injection system to work properly, an abnormally high-pressure mechanical fuel pump is used that’s driven by the cams. A more conventional in-tank fuel pump is also used to transfer fuel from the tank to the mechanical pump. Once leaving the mechanical pump, fuel is pushed to each port injector. Toyota’s hybrid of a fuel injection system isn’t simple, but just might be the holy grail balance of emissions versus power.

A total of eight fuel injectors on a 200hp, normally aspirated four-cylinder means two things: One, the FA20’s capacity to provide enough fuel will never be questioned and, two, like the equal-length header, the three-inch throttle body, and the engine’s square bore geometry, it’s almost as if Subaru and Toyota developed the FA20 specifically for those of us who can’t stand plastic engine covers.

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By Aaron Bonk
252 Articles

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