Past U.S. Navy and Air Force flight students will warmly recall their first trainer, the T-34 Mentor.

Most of these two-place, bubble-canopied, prop-driven Beechcrafts were powered by a 225-hp Continental flat-six engine.

Smooth, sonorous and (thankfully) reliable, this compact opposed-piston beauty struck some as a little like an airborn Porsche “boxer” mill.

Today, Porsche and Fuji Heavy Industries Ltd. (maker of Subaru vehicles) are the only major auto makers using horizontally opposed-piston boxer engines. (Honda Motor Co. Ltd. and BMW AG do use boxer engines for some of their motorcycles.)

Given the inherent advantages of the horizontally opposed cylinder configuration, including packaging, dynamic balance and a low center of gravity, why don't more auto makers use them?

“It can be more expensive to design an engine that way,” says David Sullivan, product line manager for Subaru of America's Legacy, Outback and Tribeca models.

“And it drives us into using unique transmissions. Our rear-facing block does not meet up with most common transmissions, so we manufacture all of our own. That could be one disadvantage,” Sullivan says.

Unlike the lovely old Continental flat-six aircraft engine, Subaru flat fours of the not-too-distant past were criticized for being less than smooth. Some critics called them “agricultural.”

They also were not especially fuel efficient, partly due to the standard all-wheel-drive system they were compelled to power. But this latest iteration of the engine pleads “not guilty” on both counts.

“The all-new Subaru Legacy GT's 2.5L turbocharged H-4 boxer engine makes this year's 10 Best Engines list because it provides the ideal combination of restrained power and frugal sensibility,” says Ward's AutoWorld Executive Editor Tom Murphy.

As tested in the '10 Legacy GT sedan, the 2.5L H-4 generates a healthy 265 hp and 258 lb.-ft. (350 Nm) of torque, is rated at 18/25 mpg city/highway (13-9.4 L/100 km) and is much smoother and more efficient than the previous 2.5L turbo.

Sullivan concedes the '10 2.5L turbo's new-found creaminess is more the result of improvements to the vehicle rather than the engine.

“It's more of a vehicle architecture story than an engine story,” he says. “(The engineers) spent a lot of time optimizing the 4-cyl. (applications), because that is where our volume is.

“There has been a lot of structural work, including an all-new front subframe structure and a new engine mounting system that was developed to enhance NVH. Those structural improvements and a lot of vehicle-side tuning can make a huge difference in the way you perceive the feel of a vehicle's powertrain.”

Not so the fuel economy. Sullivan points out the primary objectives of the '10 H-4 upgrade program was achieving more power, torque and fuel efficiency from this long-running EJ25 engine architecture.

“The way it was achieved primarily was by redesigning the turbocharger location and plumbing,” he says. “And it also was fitted with a 25% larger intercooler, which allows a more aggressive charge.”

Most important was relocating the turbocharger from the top of the engine, near the intercooler, to the bottom, next to the exhaust outlets. This heats up the catalyst faster and improves turbocharger response time 30%.

“It allows us to maintain our LEV 2 rating for emissions while generating more power and better fuel economy,” Sullivan says.

“The torque curve goes nearly straight up to its peak at 2,000 rpm and holds that to 5,200 rpm, which means the car pulls from mid-range extremely well. This engine is really all about mid-range performance, so when you get on the gas in third or fourth gear, it goes ‘right now.’

“There is no turbo lag whatsoever. You feel like you have a constant stream of power, which really makes it a great engine,” Sullivan says.

One reason there are few bottom-mounted turbos is that locating them there is a tough packaging challenge. “That's not a very package-friendly location,” Sullivan says. “There's a lot of stuff down there within the subframe that you have to get around.”

The other key challenge was achieving the optimum attribute balance. “Without a basic architecture change, maintaining compliant emissions while getting more power, torque and fuel economy is not an easy balancing act. We ended up with 9% more peak horsepower, 7% more torque and 1-mpg improvements in both city and highway EPA ratings.”

Beyond the turbo relocation, the most significant change to the engine was an upgrade from variable valve timing only on the intake side to dual VVT, which Subaru calls “dual AVCS (Automatic Valve Control System)” for improved breathing. Other improvements include new iridium spark plugs that are said to enhance fuel economy and a redesign of the piston's top surface to improve combustion efficiency.

A number of other technologies, including direct injection, are being studied for future improvements that would enhance both performance and fuel economy, Sullivan says.

“But we always keep in mind that we're considered a value in the marketplace. Subarus need to be affordable. This car has been tested at 5.6 seconds 0-60 (97 km/h), making it one of the fastest midsize sedans around, but at a very reasonable price. So when we talk about future technologies, we have to be careful, because DI is not inexpensive.”

Sullivan admits, however, that DI and other technologies likely will be required to meet fast-accelerating future fuel-economy standards.

“Customers always are going to want more performance and better fuel economy for the same or lower price. That's the challenge that all auto makers are balancing right now.

“One way we were able to get a big fuel-economy benefit in the '10 Legacy and Outback was by going to our new Lineartronic CVT transmission, so you may see us look to improve fuel economy in other models via transmissions as well. There are a lot of tools in the tool kit that you can use to achieve those goals. It's a system target, not just an engine target.”

Like everyone, Fuji is studying ways to reduce vehicle mass without adding much cost, and Sullivan says there is room for improvement to the engine as well.

“You may see more developments in terms of head design. We haven't gone so far as to institute (variable) valve lift on this engine. Right now we're dual VVT but not dual VVT/VVL, though we've had that on 4-cyl. engines before. You also may see us further tweak turbo designs and locations.”

But Sullivan doesn't see this EJ25 H-4 — naturally aspirated variations of which provide standard power for nearly everything Subaru sells in North America save the 6-cyl.Tribeca — going away any time soon.

“I think you'll see this family of engines survive for a while,” he says. “It's a very good architecture. We've gotten a lot of volume from it and a lot of accolades for it, so you can expect us to continue to develop this engine for the foreseeable future.”

THE STORY BEHIND<br />10 BEST ENGINES

The Ward's 10 Best Engines competition has recognized outstanding powertrain development for 16 years. This month, Ward's examines the design philosophy behind Subaru's 2.5L H-4 boxer engine.