Summer 2002 Forward to a Friend


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One of the symbols of the individuality of Subaru vehicle design is the horizontally opposed, or boxer, engine. The lower engine height allows for improved vehicle aerodynamics and a low hood line for improved forward visibility, while a lower center of gravity benefits cornering, resulting in exceptional handling and outstanding stability. In this story we’ll look inside at what makes this engine extraordinary.


On a Subaru boxer engine, the redesigned pistons help reduce emissions and improve fuel economy.

Recent years have brought a need for engines with better fuel economy and lower emissions. With this in mind, Subaru has re-examined even the smallest details of its horizontally opposed boxer engine, which has undergone continuous improvement since its introduction more than 30 years ago. Several years ago, this scrutiny resulted in an important change in the piston design for Subaru engines. The change in the shape and style of the piston was nothing short of a “piston revolution.”

Piston Power

The piston is one of the most important parts of the engine, and its job is tough and complex. The piston plays a role in air intake, compression, combustion and exhaust. On most engines, the pistons move up and down. On Subaru engines, because of their horizontal design, the pistons move in a side-to-side direction.

Through experimentation, the engineers at Subaru discovered that a slightly barrel-shaped piston produces less friction, vibration and noise. The engineers then focused on improving the piston skirt (the lower part of the piston that makes contact with the cylinder wall). In a radical break with convention, the skirt was made shorter and thinner to reduce weight.

Encouraged by the performance of their new piston, the engineers turned their attention to the cylinder. Normally, gas builds up between the head of the piston above the compression rings and the inside wall of the cylinder. (The piston rings fit into grooves on the outer wall of a piston, just below the head. They seal the combustion chambers and the oil rings scrape excess oil from the cylinder walls.) This unburned, compressed gas is difficult to combust because it is contaminated with leftover exhaust gases, and this reduces fuel economy. To prevent this problem, the gap between the head of the piston and the inside cylinder wall was made smaller, also making it necessary to reduce the piston pin offset and alter how the piston was weighted. These minute changes had an enormous effect on reducing the buildup of unburned, compressed gas.

The finishing touch was the dual coating process. The head of the piston was treated with alumite to prevent wear in the grooves of the piston rings, and the piston skirt was coated with molybdenum to reduce friction. The result was an all-new piston with a pleasing shape and a stylish two-tone color.

Good Looks And Top Performance

This revolutionary piston has more than just good looks, however. It improves fuel combustion and produces cleaner exhaust – that’s good news for the environment. And it significantly improves fuel economy. Running fuel economy is improved 0.5 percent and idling fuel economy is improved 2.3 percent – that’s good news for you, the Subaru owner. Interestingly, pistons with similar designs recently have been adopted by many European automobile manufacturers and makers of Formula One race cars also seeking to improve performance and fuel economy and lower emissions. Subaru piston technology was just ahead of its time.

Hidden Workhorse – The Subaru Crankshaft

At the center of any internal combustion engine is the crankshaft, which converts the motion of the piston into rotational motion through a connecting rod. Again, keep in mind that on Subaru vehicles, the pistons move side to side. The crankshaft can revolve 5,000 times per minute or more, and can shoulder a force of nearly 10 tons at any one time. This crankshaft design is an integral part of the boxer engine and contributes to the outstanding driving performance you have come to expect from your Subaru All-Wheel Drive vehicle.

Another Subaru Secret Unveiled

The crankshaft on a Subaru boxer engine is shorter and lighter, resulting in improved performance.

The crankshaft in a Subaru engine is short because a 4- or 6-cylinder horizontally opposed engine has two banks of two or three cylinders rather than a long single row of four or six cylinders, as with in-line engines. Second, because the horizontally opposed engine is arranged symmetrically, the weight is more balanced than in in-line engines and the crankshaft needs no extra weight for counterbalance. These two features make the crankshaft both short and light, a significant advantage for any engine part that revolves at such high speeds.

At Subaru, the crankshaft is forged from an extra-hard chrome molybdenum steel, and the engine is designed with five main bearings to support the crankshaft. This combination produces a strong, durable engine. With regular maintenance, it is not uncommon for these engines to go hundreds of thousands of miles without major repairs.

Crank It Up!

This hard-working crankshaft is incorporated in all Subaru models. This crankshaft has done its job with power and precision, even in engines with turbochargers and in engines built for the World Rally Championship series vehicles. This shows how strong, durable and flexible the basic crankshaft is. Even today, the Subaru crankshaft has not yet reached its limits. Its untapped strength is a testimony to the insight and skill of the engineers who designed it.

The Subaru pistons and crankshaft are designed for power and driving performance. They are the result of top engineers insisting on excellence, even in the smallest details. Together, these two engine parts help keep your boxer engine performing at peak efficiency for an extraordinary driving experience.