Eggenfellner Subie operation

Mornin' everybody.
A friend of mine's -7 has just been signed off and is equipped with a supercharged Subie. Others have told him to fly it with wide open throttle and turn the RPMs back. Apparently the wastegate is set to around 34 inches max. My contention (and I told him this) is that he's asking for extremely high cylinder temps and detonation coupled with short engine life, if not premature failure.
Running an engine with WOT and low RPMs flies in the face of everything I've ever learned about CS prop equipped engines. Since the widely accepted proper technique is to lower manifold pressure FIRST, then RPMs, where is the justification in holding high manifold pressure and lower RPMs with the Eggenfellner engine? One guy told my buddy that he's been running his Subie wide open and lowering the RPMs for the last 300 hours!! I've never flown or operated a Subie but I've owned and operated Cessna 310's, AT6's and several R1340 engined airplanes and many Ag airplanes with CS props and the procedure with all of these has been the same. What am I missing here with the Subie by Eggenfelner?
Pierre

Eggenfellner Operation

pierre smith said:

I've never flown or operated a Subie but I've owned and operated Cessna 310's, AT6's and several R1340 engined airplanes and many Ag airplanes with CS props and the procedure with all of these has been the same. What am I missing here with the Subie by Eggenfelner?
Pierre

Click to expand...

I think this answers some of the questions. This is not a Lycoming. It is a Subaru, and is designed much differently. I'm sure someone more well spoken will give you the technical details, but the ECM takes care of many of your concerns. It will advance/retard the timing to prevent the detonation you speak of. It automatically adjusts the mixture to give the proper power. I'm sure it does many things I can't even think of right now. I have flown mine for about 235 hours now, and rarely touch the power lever after advancing it for takeoff until I am back in the pattern. 25 inches and 3600 (engine) 1980 (prop) rpm is a typical MP/RPM combo for me at 5000 feet and cruise. I have the non-supercharged version of the engine, but 34 inches and the same prop setting would work just fine. You would go faster, and burn more fuel.

Hope this info helps.

Nathan Larson
N217JT RV9E

Pierre,
The engines you list are all large displacement, slow turning engines with combustion chambers designed for higher octane fuels.
Detonation is dependent on many factors but one important parameter is the dwell period at a highly compressed state.
Pistons in a slow turning engine spend more time at the top of the compression stroke than those in the smaller higher rpm engines. This increased time invites detonation when the throttle is wide open.

If you were inclined to spin those big puppies up to 3400 rpm or so I don't think you would be concerned with detonation at wot.

That is about the slowest the subaru engine will ever see plus the combustion chambers are much smaller and are designed for lower octane fuels. These are just some of the reasons why detonation is less of a problem in the subes. Also, it is hard to think of 3400 plus rpm as lugging the engine.

It's the variable ignition timing which makes the difference. If the rpm is too low for the fixed timing used on a lycoming engine then by the time the combustion is complete, the piston could still be near top-dead-centre. The hot gases in a small volume means high temperature and pressure, rapid conduction of heat into the head and possible detonation. Retarding the timing decreases the peak pressure in the cylinder. The electronic ignitions available for lycomings should retard the timing at low rpm just like the subie.

Conor
RV-9A 90990 wings

Alterative Engines

Please visit Traditional vs. Alternate Powerplants. By the way , Egenfellner does know what thier doing!

Subie Operating Technique

Pierre,

I have found that 1700 prop rpm, engine about 3000, full throttle, is the most efficient, best cooling configuration for the H6 Subaru.
As rpm comes back, the ECU reduces full flow to 6-7 gph. Yes, the engine is working but that's what it is designed to do. It does not knock.
Pulling the throttle back just a slight bit causes the ECU to go into closed loop and then the engine is at it best efficiency - like driving down the road at 55 mph.

dd
RV-7A N707DD
Subaru H6
MT Prop