Thought I'd pass this along, from Schu.....thoughts on Auto Conversions>>>
CldLk-Tim I looked at the subaru pretty hard and did a lot of research that I'll try and summarize for you: The biggest problem with auto conversions isn't the auto engine itself, it's everything else. I would bolt the subaru engine in my airplane in a second because I think it's a far better engine than a lycoming because of the very short stroke (piston speeds aren't much higher even at 5k rpm), a main bearing per rod, very good tolerances, and liquid cooling to make winter flying as simple as plugging in a freeze plug heater, and heat in the cockpit. But it's not just the engine, it's the alternative fuel system, the PSRU, the ignition system, and cooling system. Let me explain: The PSRU is a really big deal. Engines don't turn over smoothly, they accelerate and decelerate as each piston is doing it's work. This combined with the large 2-blade tuning fork on the front of the engine can cause harmonic issues that can destroy the PSRU or the engine in minutes if the engine is running at the resonance frequency of the prop. It's kinda like tapping a tuning fork at it's resonance frequency, the harmonics can cause the forces on the gears and crank to go to theoretical infinite. In order to get around this cars have large harmonic balancers or the ultimate harmonic balancer: the drive-train and tires. The engine won't have as much torsional vibration if the harmonic balancer/flywheel/drive-train keep it at a steady speed, but these things don't exist on an airplane, in face most auto conversion vendors get rid of the flywheel or make it as light as possible to offset the weight. Other problems are the fuel system. You must use electric pumps which can fail, so you need two of those, then you need to have a way to detect alternator failure since you will be running off of the battery if it fails. Some people add a second battery or a second alternator, but that is even more weight. All of that said, most of the recent accidents have come from high pressure fuel lines failing in flight. A fuel line problem caused Bud Warren's wheeler to go down, then another fuel line problem in a customers airplane caused another crash, but he didn't survive the second one. A fellow at vansairforce had an RV10 that had a chevy v8 from Versa that had a fuel line leak and an explosion in the cockpit after landing. He and his passenger got out and are now fine, but his airplane burned to the ground. All these things said, there are people flying auto conversions and they do just fine. Dan Shilling has a subaru 3.3L SVX engine in his with an autoflite PSRU. If I was to build a auto conversion this is what I would be building for a few reasons: 1. The autoflite PSRU has a rubber coupler to deal with the harmonic issues and it's a commercial unit designed for high vibration PTO applications. 2. Dan has solved the cooling issues, so just copy his design. 3. The 3.0L engine is more complex than what I would want in an airplane and the 4 cyl engines aren't powerful enough, especially given the weight. I think the 3.3L engine is the right engine for this application. 4. We know that his airplane weighs the same as many io-540 so you aren't going to have CG problems. 5. We know it fits under the cowling and the engine mount has been worked out. This engine combo isn't without some drawbacks. The prop turns the opposite direction so you would need to mount the vertical stab on the opposite side of center to combat p-factor, which would make it hard to go back to a lycoming if you ever wanted to. Another option is to mount the stab in the center and use a trim tab like a Cessna. I wouldn't buy an eggenfellerner simply because I wouldn't trust my life to his PSRU, and I don't think his engine options will work that well. I wouldn't run a turbo because the late model engines are already more complex than I would want. In the end I don't think the auto conversions are a good option unless you like that kind of fabrication and enjoy the challenge. If your one of those types then my hat is off to you!!! If not, I think that a o-360 is the best engine, unless you must go fast or have floats, then the bigger the better. schu
