Subaru is the automotive division of Fuji Heavy Industries Ltd. (FHI) of Japan and they produce automobiles with a horizontally opposed aka boxer type engine. And the engine is also used by aircraft builders and some small companies to convert this into an aircraft engine. Eggenfellner and Stratus, for example, have been doing this for years and they have created a very fine product.
Like Rotax, Lycoming and Continental Subaru realized that a horizontally opposed engine has less natural vibrations to compensate than an in-line or a V-engine. In cars this design results in an engine that has a lower center of gravity. For aircraft installations it means that the cowling has a lower profile and thus less drag, a very desirable property if you want to reduce fuel consumption by streamlining the cowling.
The low profile of the boxer engine is shown in the image to the right. As this is an automobile based gas engine the maximum power output is reached at around 5000 RPM. You will therefore need a gearbox to reduce this high engine RPM to a more manageable propeller speed between 2200 to 2500 RPM. Just like Rotax has done with their 912/4 line of engines.
The basic engine is designed with a single ignition system (as is quite common in cars), it runs on Mogas, is liquid cooled, and can be equipped with a turbocharger (EJ-22) and will deliver power up to 180 BHP. Which is more than enough for most small two or four place aircraft.
This is where Eggenfellner of Edgewater, Florida stepped in. They have created an installation package which includes the basic engine with a plate on the front where the two coolant and an oil radiator, the starter and gearbox (PSRU) are installed. The complete package includes (among other things) a low profile intake manifold, dual fuel pumps, dampened flywheel and an alternator of 40 amp (or 75 A optional) for your electrical instrumentation needs.
The E6/200 is a six cylinder, 200 HP at 2700 propeller RPM, fuel injected automobile converted engine. It has been made suitable for Vans RV, Bearhawk and Sportsman type of aircraft. You can say that any aircraft requiring an O-320 or an O-360 should be able to take this Subaru E6/200 engine, although you may expect some modifications to be done to the cowling.
Being an automobile derivative it uses Mogas but this engine is also able to run on AVGAS 100 LL too should that not be available while enroute cross country.
Stratus 2000, Inc.
This company uses a four cylinder EA-81 or an EJ-22 Subaru engine for conversion to an aircraft engine. The EA-81 is a 1.8 L, with two Bing carburetors, single ignition, liquid cooled, belt driven reduction for the propeller, equipped with a 45 amp alternator and its maximum power is 100 hp.
Zenith Aircraft Company offers a firewall forward package for the CH 601 Zodiac aircraft. The engine is not included, you must order it from Stratus 2000, Inc.
It is an ideal power plant for the new LSA type of aircraft, if you are not into the Rotax Bombardier line of engines.
Should you need more power, Stratus also offers the EJ-22. This 2.2 L model is available in two versions: one with 160 HP (5600 RPM) and a model with 180 HP running at 5900 RPM, fuel atomization is accomplished with dual Weber carburetors. And, no surprise here, both will need a reduction drive for the propeller.
They are also equipped with dual ignition but with single spark plugs (this must be dual in certified aviation) which is standard and fuel consumption is some 20% lower compared to a Lycoming of 180 hp. Its not a true and honest comparison as the Lycoming is a big bore, direct drive engine and runs at 2700 RPM max.
These engines run on Mogas and are liquid cooled, which is to be expected as they are based on a ordinary car engine. Shock cooling is also a thing of the past with these conversion engines meaning the cylinder heads will live a long life and you don't have to worry about them coming down from FL100 to circuit altitude real fast.