Subaru 2.5 liter — totally stock

Installation details….. Subaru engine into aircraft.

General:

 

    False rumors:

There is a huge amount of misinformation about auto conversions. 

"Auto conversions are expensive". I have $6200 into mine. My install could be duplicated for less than $5000. 

"Auto conversions greatly increase aircraft build time". I believe this is often true, but this 2.5 liter engine required absolutely no airframe mods. Even the dip stick and oil fill locations are identical to Lycoming. I strongly believe in using the stock wiring, which greatly reduces install time. It took me six years to build my aircraft, which is slightly less than average. 

"Auto engines weren't designed for high rpm, high horsepower use". It's true that continuous high power use is unusual for the engine. However, there are hundreds of these engines used in such applications. I've never been able to find a single incidence of failure related to workload. What sounds logical and significant is not supported by  empirical data. No workload related failures.

   Scope:

This page describes the changes needed to install any 1995 thru 1998 Subaru auto engine into an aircraft. Specifically a 2.5 liter engine. The goal is to use stock components and minimize the changes. Thus taking advantage of the reliability of the new auto systems. Exceptional auto reliability in recent history is partially due to the engine control module (ECM) programming. It monitors the accuracy of many of the sensors. For example, the ECM recognizes if a critical component like the crank sensor fails. It then uses the cam angle sensor as a replacement. This is not an easy programming task. If you research it, you will find that all non-stock engine controllers don't have this important safety feature. 

When I started installing my engine I was unable to find accurate info on how to modify the engine for aircraft use.  These newer engines have OBDII firmware which is more sophisticated than previous engine control systems. I was fortunate to have access to an oscilloscope, a Subaru expert, and a number of vehicles. I was able to fly without error conditions early in the development stage, it's just that a fault code would pop up during taxiing. I would read the code, verify it was insignificant fault, then reset the ECM.  The ECM thinks it is driving down the road at 37 mph. I've detailed the few changes needed to accomplish this. The mods are easy to put in place. 

Special thanks to Brian Oakes. There isn't a single design characteristic I didn't run by him. He always was able to provide alternatives and feedback. He has an immense knowledge of Subaru engines.

Performance:

I burn 7.4 gph auto fuel at 4200 engine rpm at 175 mph cruise. Gross weight 1854#, 2.0 sq. ft. drag,1145# empty weight. My Cozy IV departs the runway at exactly 2000 ft with 2 heavy occupants at sea level. Climb is 900 fpm at 100 mph (1000 fpm if I fly alone). It looks like my performance numbers are very similar to an 0320 Lycoming.

I have gained another very substantial benefit from the auto conversion. The slower turning prop and greater spark control yields fantastic descent rate when needed. This is very important safety advantage in a canard aircraft. I can do short base and final similar to 30 degree flaps on my Cessna 150.

You will not find a quieter, smoother operating, more fuel efficient combination.

ECM (Engine Control Module) modifications:

The ECM expects certain inputs that aircraft builders don't have available. Inputs such as the transmission status, EGR solenoid, and vehicle speed aren't applicable to the aircraft. If you just disconnect the wires from these devices, the ECM recognizes these as faults. It then turns on the "check engine" light and stores the fault code in ECM memory. You don't want to be flying around with a check engine light on, as it might be on for a truly significant reason. So I had to find ways to fake out certain inputs. These changes described below.

It appears that auto manufacturers tend to add more and more input requirements for each model year. Newer ones now look at fuel temperature, power steering fluid level, etc. This can make our conversion process more difficult.

 

Changes needed for 1995 thru 1998 year ECM:

1) Tell the ECM you have a manual transmission. Since your engine most likely came with automatic tranny, you need to remove wire #2 from the mass air flow sensor (this connects to pin 47 of ECM). You also need to supply a ground wire to pin 81 of the ECM. Auto tranny wiring harnesses don't have a pin at that location, so I just stole one from an unused wire. While I was at it, I also snipped the wires from pins 61, 79, and 80 since manual transmission vehicles have no connection at those pins. Note, the transmission code still faults occasionally. I have yet to completely eliminate it.

2) Pin 83 of the ECM needs to see a square wave form that goes from less than 0vdc to more than 5vdc. This circuit is really the only significant change you need make. It prevents the ECM from going into "limp mode" 50 seconds after you apply full throttle. (Rumor has it that this limp mode is not an issue with newer engines). One way to accomplish this is to use a 555 timer, a couple resistors and a couple of capacitors. You can look up instructions on how to wire this on the net. I suspect there are even simpler solutions, and the mod in step 1 above may negate the need. 

3) You need to duplicate the ignition switch wiring. It needs to see at least 5v at pin 82 of ECM in order to start the engine. I just wired the 12v from the ignition "accessory" terminal to that pin. This tells the ECM that the tranny is safely in "park" position.

Engine modifications:

The EGR valve interferes with my engine cowl. So I just removed it along with the supply tube. I then mounted an aluminum plate over the opening to seal off the intake manifold. I placed a large ball bearing behind the tube fitting to seal the hole.  I left the controlling solenoid in place and wired, as the ECM checks for the voltage drop. Optionally I could have removed solenoid and installed a high watt resistor. See pic below.

The throttle body was the only other component which interfered with the engine cowl, so I just built a triangle shaped spacer (Yellow item in pic) that rotates it to a horizontal position. I added an extra gasket, as it is essential to have absolutely no intake leaks.

My propeller speed reduction unit (psru) requires a return line to the oil pan, so I brazed half of a 1/4npt pipe coupling to the pan near the top. Make sure you fill pan with water and verify your weld will not leak.

I also cut the oil fill tube, rotated it to a better position, then glassed it back together. I drilled part way into the tube at the joint to make sure the glue had something to grab onto. Don't be tempted to glue this with pvc glue, it will fail eventually.

Vacuum hose routing:

Two of the error codes I saw initially were related to the vacuum hose plumbing. The hose going from the evaporative canister to the "Air pressure solenoid" (above pic) is necessary. It lets the ECM know that the fuel cap is installed and the tank is under pressure. So I just routed that hose to my fuel tank vent, since it sees pressure during flight.

 

Exhaust:

I elected to make use of the oxygen sensors supplied with engine. I rolled my own muffler out of cold roll steel (crs) and inserted a ss baffle which separates the inlet from the outlet. This baffle must be ss as it gets extremely hot and rapidly degrades if not. The neat thing is that this muffler results in the two oxygen sensors having different output values, a requirement of the OBDII system. Ideally, the whole muffler should be ss. I can provide more details on muffler construction, including cad files.

Psru modifications:

I'm using a Ross planetary gear reduction unit having a 1.85:1 ratio. This allows the prop to spin relatively slow at high engine rpm. There were a few reports of psru failures. All of the failures were related to the input shaft moving fore and aft. This causes a noticeable rattle at idle rpm. The movement can fatigue the clutch plate sheet material. It also causes the flywheel pilot bearing to degrade after only 40 hours of operation.

I never had any failures, but I did observe pilot bearing wear and loss of bearing lubrication. While investigating input shaft end clearances, I realized that differential heat expansion of the steel and aluminum components could allow the clearances to change substantially during normal operation. I changed the design to eliminate this variation and to reduce the input shaft end clearance. The input shaft can no longer impact the face of the pilot bearing.

Note that the thrust bearing does not see prop thrust loads, only input shaft thrust loads. Thickness I describe were ideal for my unit, your unit may be different. I used 2 thrust washers .092" thick, and one custom alum washer .02" thick. The aluminum one mates with the face of the sun gear. Sorry, none of these are shown in the picture.