Building a Better Boxer


Spring 2015

The many enhancements that help the SUBARU BOXER engine run more efficiently often involve relatively subtle modifications. These changes help reduce friction, ensure that every drop of fuel is ignited, and generally make it easier for the engine to operate. Here’s a look at the details.



The Tumble Generator Valves (TGVs) are positioned just above the intake ports of the engine. They’re partially closed at low engine speeds in order to create greater swirl or “tumble” in the air/fuel mixture that’s entering the engine. This swirling helps ensure an even distribution of the fuel within the mixture for easier ignition and more complete combustion. At higher engine speeds and loads (greater throttle opening), the valves are wide open to let in as much air as possible.


Over the years, the metals used in modern spark plugs have gone from nickel and copper to platinum and iridium, all to help provide a more durable spark plug. The use of this very hard and long-lasting metal, along with other design changes, allows iridium-tipped spark plugs to more easily ignite the air/fuel mixture under a wide variety of conditions, and do so over tens of thousands of miles. 


The high compression ratio in the SUBARU BOXER engine compares the volume of an individual cylinder with the piston at the bottom of its stroke to the volume of the cylinder when the piston is at its highest point, called Top Dead Center. Just a few years ago, the typical compression ratio in a Subaru engine would have been 8.4:1. The SUBARU BOXER engine has a compression ratio of 10.3:1, which helps the engine extract more energy from the air/fuel mixture, resulting in more power and better fuel economy. Even with this high compression ratio, the engine uses advanced intake and ignition controls to allow the engine to run on regular unleaded gasoline. 


If you held a piston in your hands, you easily could see the crown, or the top of the piston, the grooves for the piston rings, and then the area below that, which is called the piston skirt. In the SUBARU BOXER engine this area is coated with a special low-friction, anti-wear substance. While the piston skirt may come in contact with the cylinder walls only under rare circumstances, whenever it does, this coating will help minimize wear. In addition, the overall design of the piston rings and piston skirts helps cut friction, which helps increase fuel efficiency.


Of the three regulated exhaust emissions – carbon monoxide (CO), unburned hydrocarbons (HC), and oxides of nitrogen (NOx) – the latter is the hardest exhaust gas to control. A solution that most automakers have adopted, including Subaru, is exhaust gas recirculation (EGR), which takes the exhaust gases and directs them back into the intake tracts. EGR lowers combustion temperatures, which helps reduce NOx. Cooling the EGR gases, however, is new, and has been shown to not only reduce NOx emissions, but also help increase fuel efficiency. In the 2015 Legacy, the EGR system has a dedicated cooling system just for this purpose.

While some of these individual adjustments and improvements to the SUBARU BOXER engine may be small, they all come together to create a Subaru Legacy that is more reliable, lasts longer than ever, and uses less gas. 


An automotive engineer would tell you that the Boxer 4 engine in the 2015 Legacy has better “volumetric efficiency.” It’s easiest to think of this in terms of the Boxer 4 engine’s ability to breathe more freely – getting the air/fuel mixture into the cylinders more readily, and then helping the exhaust gases exit the engine more quickly.

So follow along as we join some air molecules as they make their way into the engine of a 2015 Legacy 2.5i model.

Into the Combustion Chambers

After zipping through the air filter and past the throttle plate and airflow sensor, our unsuspecting air molecules tumble down the intake manifold where they come across ... the TGV. If the engine is operating at lower speeds, the TGV is partially closed, but in this new 2015 Legacy, the pathway leading up to and around the TGV is much smoother and easier to navigate now. Next, a concentrated spray of gasoline from the fuel injectors joins the air molecules, and as the larger-for-2015 intake valves open, this new party of gasoline and air tumbles into the combustion chamber.

Greater Force

Once the intake valves close, our intrepid mix of molecules is squeezed together. They’re more densely packed for 2015 because the engine’s compression ratio is higher. There’s a lot of potential energy here, so when the iridium-tipped spark plugs fire, the mixture ignites and pushes the piston with greater force. 

Since the piston skirts have been treated to a special coating and the more wear-resistant piston rings have less friction, it’s easier than ever for the ignited air/fuel molecules to push the pistons, applying more torque to the crankshaft and ultimately to the wheels.

After doing this yeoman’s job, our spent molecules exit the combustion chambers and flow out of the engine more freely because of larger-diameter exhaust manifold tubes. And the process continues with fresh air/fuel molecules doing the same job.

Additional Efficiencies

Now that you’ve followed the complete cycle of engine operation, let’s consider one more 2015 change that makes the SUBARU BOXER 4 more efficient. 

A small amount of exhaust gases is diverted back into the intake manifold (EGR). The main goal of EGR is to lower certain exhaust emissions, but for the 2015 Legacy 2.5i, these diverted exhaust gases are cooled by a special radiator before going back into the engine. This lowers the temperature inside the combustion chamber, which helps the engine run on regular gas without engine “knock,” even with such a high 10.3:1 compression ratio.

It All Adds Up

All of these relatively small changes add up to make a big difference. The 2015 Subaru Legacy 2.5i with Lineartronic CVT has an EPA Combined estimate of 30 mpg. That’s 3 mpg higher than the 2014 Legacy 2.5i with the same drivetrain. It’s just another example of the ongoing efforts to lower the total cost of operating a Subaru Legacy.