Rotorcraft Power – Subaru Conversions

subaru ej22 engine gyro conversion
Courtesy Homebuilt Rotorcraft AUGUST 2000

In this article we want to delve into the Subaru auto engine since that is where most of our experience lies. The Subaru engine is one of the most reliable, smooth, compact, and light weight engines available to convert for aircraft or rotorcraft applications.

EDITOR: Subaru engines a well known for their HIGH torque, great for towing – but their low horsepower is where they need a “tickle” to bring them up to aircraft power ratings.

Many late model auto engines are good candidates for aircraft and rotorcraft applications, but few can compete in the power to weight department with the Subaru. One exception is the 13B Mazda rotary engine that is capable of developing up to 200 Hp at around 5500rpm depending on it’s set-up and porting.

The reason is the horizontally opposed configuration of the Subaru engines. The engine is shorter by a third to a half as compared to inline four cylinder or V6 engines. Width and height are also reasonable — more narrow than even a VW engine and shorter in overall height than most 4 and 6 cylinder engines.

A horizontally opposed configuration provides a more balanced and smooth running engine and the Subaru line of engines don’t require large counter weights on the crankshaft for smooth operation which is another weight saving advantage. All Subaru engines have forged crankshafts instead of cast units which are found in most auto engines.

One can make a simple prop adapter and mate the prop to the crankshaft and have an aircraft engine without really doing anything else. We have built over 300 direct drive systems and sold as many conversion manuals for the EA81 engines and have never received a report of a failed engine.

Of course, you can’t extract the maximum power out of a direct drive configuration, but you can certainly put together an inexpensive engine for a gyroplane or airplane that is still competitive as far as power to weight is concerned.

For engines in the 50-250 Hp range the Subaru is the best choice — most compact and best power/weight except maybe the VW Bug engine in the 50-80 Hp range as far as power/weight is concerned.

But, the Subaru is definitely more reliable and fuel efficient than the aircooled VW and converting to direct drive is expensive with the Bug engine. The EA81 push rod engine has been very popular in the 50-100 Hp range and a number of companies are making either conversion accessories or complete engine packages for the EA81.

For EA81 or EA82 direct drive applications I’m Fly’n, RFI, and Rotary Air Force manufacture prop hubs and starter systems. For reduction drive systems there are a number of companies — RFI & RAF both offer cog belt drives. Planetary drives are available from Knight Hawk, New Horizons, and Ross Aero.

Joe Souza EA81 gyroplane

Joe Souza Gyros offers adaptor kits for installing a Hirth gearbox on the EA81. Sub4 Performance parts offers a number of accessories for the EA81 including performance heads, cams, custom intake manifolds, and a gear box reduction drive.

Sub4 is distributed by Rotor Flight Dynamics, the company that produces the Dominator line of gyros. Sub4 even has a 120 Hp version of the EA81 which incorporates their 4-port heads, custom intake manifold and electronic EFI.

If you need 100-200 Hp the EJ22 or EJ25 with a direct drive or reduction drive is a good choice. And for 230-260+ Hp the EG33 six cylinder is the ideal candidate. Anything over 260 we suggest you look at some of the V6 and V8 engines.

One can obtain 100-125 Hp direct drive with the EJ22 and EJ25 respectfully and be competitive with an a certified aircraft engine of equivalent power. Add a reduction drive to the EJ22 and you have 130-160 Hp at an all-up weight of around 300 lbs.

The EJ25 provides 165 to near 200 Hp using the same configuration and will weigh an additional 25-30 lbs. All of the EJ and EG series engines have 4 valves per cylinder, are multiport fuel injected, and utilize crank fired ignition systems.

What this means to the pilot is automatic compensation of mixture and ignition timing at any altitude up to 14,000 ft. ASL. Reliability is excellent. Out of over 80 stock Subaru EFT systems modified for aircraft and rotorcraft use, no failures have been reported to date.

There are a number of companies making accessories for the EJ/EG line of Subaru engines. RFI and RAF both produce direct drive and cog belt reduction drive systems for the EJ22/EJ25. RFI also offers cog belt reduction drives for the EG33.

Knight Hawk, New Horizons, and Ross Aero also offer planetary drives for both the EJ22/25 and the EG33. So, now that we have some basic information on engines, let’s look at some applications for rotorcraft.

In the case of a helicopter application one needs to look strictly at required horsepower and Rpm and the weight of the engine since a propeller speed reduction unit (PSRU) is normally not used in a gyroplane application one needs to look very closely at all the variables in order to make a wise decision as to the type of engine, direct or reduction drive, cost trade offs, etc.

Subaru EA81 four stroke engine

A single place, light weight gyro with a light weight pilot (140-165 lb.) can probably make do with a direct drive EA81 or EA82 engine. Everything flying is a compromise and the various Subaru engine models vary considerably in weight, so one should take into consideration the gross weight of the ship and available power.

For instance, I have seen two place gyros flying with an EA81 engine equipped with a PSRU. Without some hop-up work the ship will probably be a little short in climb performance.

If you want outstanding performance you might consider one of two options—add some performance parts like the Sub4 or I’m Flyn’ for instance, or go with an EJ22 which will add 60-70 lbs. empty weight to the ship but it will produce a solid 130 Hp stock or 150+ with a $200 cam grind.

Just keep in mind that when you add weight you may have to increase the diameter of the rotor disk and possibly beef up the airframe to take the additional load.

The typical weights and Hp of various Subaru long blocks (that’s engine case, oil pan, and valve covers, with no accessories) are as follows:

  • EA81 135 lbs. 72-120 Hp at 4800 – 5400 Rpm

  • EA82 155 lbs. 85 -120 Hp at 5200-5600 Rpm

  • EJ18 170 lbs. 110 – 125 Hp at 5400 – 5600 Rpm

  • EJ22 185 lbs. 130 – 160 Hp at 5400 – 5600 Rpm

  • EJ25 200 lbs. 165 -195 Hp at 5600 -5800 Rpm

  • ER27 250 lbs. 145 -175 Hp at 5200 -5600 Rpm

  • EG33 300 lbs. 230 -260 Hp at 5400 -5600 Rpm

Accessories such as intake and exhaust manifolds, carburetors or fuel injection system, alternator, starter, and reduction drive will add at least another 50 pounds for the EA81 and EA82 and a good 75 -100 pounds for the Subaru EJ and EG series engines. That may seem like a lot of weight but compare the EA81 to a Continental engine.

You are looking at an all-up weight with a PSRU and accessories of around 200 lbs. As compared to around 240 for the Continental 0-200 (100 Hp) engine. And the EJ22 will come in at 300 or so pounds for 130—160 Hp as compared to the 0-320 Lycoming at about 350 pounds with all accessories.

two seat gyro ej22 subaru

The EJ25 competes well with the 0-360 Lycoming at several pounds less weight and considerably less fuel consumption—and that’s auto gas instead of 100LL. If you need 230+ Hp the SVX is the engine of choice. It weighs 100 lbs. Less than the Continental 0-470 of the same Hp and sips fuel as compared to the Continental.

Another good feature of the EG33 SVX is that it has tremendous redundancy—like six individual coils, two throttle-bodies, two crank angle sensor, two oxygen sensors, and two knock sensors. The new single overhead cam version of the EG33 is coming out this fall. It should be considerably lighter and more compact (only 3.5 inches longer).

Okay, now we have a bunch of information on the Subaru engines, so how does one go about getting one converted for aircraft/rotorcraft use? Conversion is very simple and straightforward. Nothing has to be done to the crankshaft- the forged crank is bulletproof and there is plenty of thrust bearing on any of the Subarus to handle direct drive applications.

For a direct drive application, just add a prop hub and maybe a starter system and you have an aircraft engine ready to install. Unlike some auto engines that require reaming out the cylinder walls to keep the pistons from seizing, adding heavy thrust bearings, etc. the Subaru can be run just as it comes out of the car.

The stock fuel injection and ignition control system can also be run in its original configuration except for some minor modification of the harness in order to “fool” the Electronic Control Unit (ECU) into thinking it is still in the automobile.

Some people are intimidated by the electronics. When they see around 100 wires running between the engine and the ECU they get concerned when I tear down a carburetor I get intimidated by all the tiny parts required to make the carburetor work properly. There are almost as many parts in a two-barrel carburetor as there are wires in the Subaru EFI system.

Of the 80+ harnesses that we have modified for customers, we have not had a reported failure other than one fellow who tried to make it home after losing the alternator without a backup battery and one or two pilots who lost fuel pumps and didn’t have a backup unit installed.

Everything is going electronic fuel injection—even two-stroke recreational vehicles and they are especially more efficient and reliable with EFI instead of the archaic carburetor. The Subaru EFI system is one of the most efficient and foolproof systems that I have come across.

There is a lot of redundancy built in and the quality of the ECU connectors and printed circuit boards is as good as Mil Spec equipment. If auto electronics wasn’t reliable the automobile manufacturers would have some serious problems and a lot of recalls.

You seldom if ever hear of a recall for electrical problems — it is backup electrical system. The optimum two cycle engine will utilize direct injection like the diesel, but DI is still a ways off. We’ll have to be content with EFI on two strokes for a while.

For optimum reliability the four cycle engine is still number one, but one pays a weight penalty for that comfortable feeling of security when flying over unfriendly terrain. Four stroke powerplants can not compete in the power-to-weight department with two stroke engines, but they can compete very well with certified aircraft engines.

My expertise and experience is with Subaru engine conversions and any of the Subaru engines are competitive, weight and fuel efficiency wise with aircraft powerplants. To cite some examples, a 100 Hp EA81 with reduction drive will weigh in at around 200 lbs. while a Continental 0-200 weighs 240-260 with full accessories.

subaru rotorcraft power

An EJ22 equipped with reduction drive and a mild cam grind will develop 150+ Hp and weigh in at around 320 pounds while its certified counterpart (Lycoming 0-320) weighs around 350 with full electrical system, muffler, and cooling baffles.

The EG33 SVX engine weighs 400 pounds with redrive and accessories and develops 230 Hp in stock form while a Continental 0-470 (same power rating) weighs 500 pounds.

Fuel consumption of the Subaru engines (and any liquid cooled auto engine ) will be typically 35% less than an air-cooled aircraft engine. How about reliability and TBO? We have had two reports of engine failure with our systems out of over 500 in airplanes and rotorcraft.

As far as TBO is concerned, there hasn’t been enough time put on engines yet to gel a real accurate estimate, but we do know of a number of EJ22 and EJ25 engines on gyroplanes that are used for flight training that have accumulated over 2500 hours.

subaru auto conversions for gyrocopters

Auto engines should be at the very least as long lasting as any certified aircraft engine and TBO of a well maintained auto conversion is probably a good 50 percent more. One thing we know for sure; the cost of a typical auto conversion is much less than an aircraft engine.

A factory new Lycoming aircraft engine in the 150-180 Hp range will run around $25,000. An EJ22 or 25 even with a factory new long block and all accessories will not exceed $5,000. So, how about turbo-charging or super-charging for more power and better power to weight ratio.

Well, like I said earlier, everything flying is a compromise and boosting the intake system of an engine has its good and bad points too. In other words, “all that glitters is not gold,” When you boost the intake manifold pressure you certainly do increase the power output of the engine.

An engine is basically an air pump — the more air (and fuel) you can push or pull into a cylinder, the more power the engine will make. But one pays a penalty for that extra power—it is not absolutely free as one might think.

First and foremost, a super-charger or turbo-charger has finite weight. A typical installation will add 30-40 lbs. of weight. And, on top of the extra weight, the engine will need a richer mixture to help cool the engine, especially in the case of the turbo-charger.

And…. You also have to consider an intercooler. When you compress air it becomes heated which reduced the density which reduces combustion efficiency. An intercooler adds still more weight and complexity, so there just ain’t no free lunch.

If you want to fly at high density altitudes of a mile or more above sea level, than a turbo or super-charger is worthwhile. I would lean toward a supercharger because there is less overall weight and less heat buildup in the exhaust system.

EA81 subaru gyroplane

If one sticks to very mild boost perhaps he could get by without an intercooler. If all you want to do is normalize the engine to sea level conditions at altitude, then a super-charger or turbo-charger is worth the extra cost, complexity, weight, etc. One application where boosting will pay off is in a direct drive application.

The weight of a reduction drive over a direct drive system is at least 30 pounds. A supercharger or turbocharger installation on a direct drive engine will provide similar performance to a normally aspirated engine with reduction drive.

I have been very negative with respect to boosted performance in the past, but after talking to a number of people in the racing circuit, the Subaru EJ22 looks promising in a supercharged configuration, provided it is done properly and professionally.

We are investigating the possibility of adding a centrifugal type blower to an EJ22 (with an intercooler) and getting up to 200 reliable horsepower. The racing people are doing it on a routine basis and there are aftermarket products available for installation in Subaru automobiles that are not really compatible with aircraft configurations.

We have been assured that the EJ22 is perfectly capable of producing 300 Hp and still retain a good measure of reliability. At around 200 Hp output the engine should last exceptionally well in an aircraft or rotorcraft application.

We were looking at the SVX engine for applications in the 200 Hp range but the SVX engine is in very limited supply and the weight is up there. A supercharged EJ22 will come in at about 50 pounds less weight than the EG33 and it is a more compact engine and very plentiful.

A 200 Hp engine that weighs in at around 350 pounds is approaching the power-to-weight ratio of a typical two cycle engine. You will pay the price for such performance, but look at what one pays for some of the low power two stroke engines. In the long run a four stroke engine is the best choice for now.

Rotorcraft Power - Subaru Conversions
Article Name
Rotorcraft Power - Subaru Conversions
An educated insight into rotorcraft power. Don Parham discusses the merits of the EA, EJ, EG & SVX Subaru auto engine conversions for gyroplanes.

12 Comments on "Rotorcraft Power – Subaru Conversions"

  1. Hello, and what is the situation with the Subaru EJ16 or EJ20 engines, I want to build a light gyroplane and the higher engines are at a good price.

    • Hi Lukasv, these Subaru engines are extremely popular for gyros – but more-so from a DIY perspective as many of the Subaru engine conversion companies seem to come and go quickly. Do some more searching online for further information.

  2. Where can we get in contact to purchase a psru for an EJ22?

  3. I need to purchase some EA 81 parts – I have $$$$. Call me: Steve Beatty at Airscrew Performance (623) 842-3902

  4. Hi,
    I want to use EJ22 to diy a 2 seater helicopter,
    What gearbox I must use in please

    • There are plenty of options, but a Speedway Quick Change style seems to be popular. Note, these are not approved or rated for helicopter use! Other option is get one from a Robinson R22.

  5. Need source for reduction for Subaru EJ25 engines.

  6. Dear community,
    I have an EA81 original from a car (good condition). All the years always stored carefully. Now i am not sure to do with this great engine. I need a deduction gear to build it up (Ellison injector is present) for an experimental aircraft (puller). The other way is to give up the project and bring it to the scrap iron. 🙁
    Are there ideas where I get a cheep reduction gear for continue the project?
    Greetings Heinz

    • Can only suggest searching Google and looking through You sometimes see them for sale on USA forums and Aerotrader. There are a few people still making them but they are expensive. If you could get the original drawings for AMAX Engineering reduction drive, it was the simplest and lightest.

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