fuel efficient .40

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My local club hosts an endurance contest. The rules are pretty relaxed, basically the engine must be a .40 and you're limited to 8 ounces of fuel. My question is, what make and/or model of engine would be most efficient? I'm thinking in general a 4 stroke would burn less than a 2 stroke. And a glow driver on the plug to ensure a reliable idle for extended periods. Is there any other modifications that could be made to make it more efficient? Any specific manufacturers or engine models known to be more efficient than others?
Thanks,
Terry

[Dave Robelen]

Hello Terry,
You are correct about the 4-stroke. Also, nitro in the fuel increases power (and consumption). The fuel labeled FAI is strictly oil and alcohol, and will give the longest run time for a given fuel. Reducing the bore of the carburetor will cut the power a bit, but also reduce fuel consumption. That glow driver will become important as you strech the runs out.
Regards, Dave

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Thanks, Dave
Wouldn't reducing the carburetor bore be the same as closing the throttle to idle?
I think the club provides the fuel for the contest, so I have no control over that. Thanks for you input.
Terry

[Dave Robelen]

Hi Terry,
What I was reffering to would be the equivalent of using a smaller carburetor. The throat bore would be smaller all the way through. The benefit is increased fuel draw at low RPM's, and a broader needle valve setting. Assuming that you use a larger than normal prop to reduce RPM and fuel consumption, the smaller carburetor bore will restore the fuel draw without hurting the economy. I have had very good results with a .19 carburetor mounted on a .56 when I was running an oversize prop. Good fuel draw, and excellent economy. The more you can increase the air velocity in the carburetor, the better control you will have over the mixture.
Regards, Dave

[bigedmustafa]

You should be able to sustain reliable ignition at idle simply by using a glow plug with an idle bar on it. That should save weight versus wiring a glow driver for in-flight voltage.

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Good point, bigedmustafa. I have a tendency to over-complicate things. Some experimentation will be needed to determine if I'll need the glow driver. If not, it would certainly save a lot of weight. And we all know that lighter flies better.
Thanks for explaining the improved fuel draw with the smaller carb, Dave. It makes perfect sense. Also, using a larger diameter, lower pitch prop would help the idle and hopefully maintain enough thrust to maintain altitude....or would I want a higher pitch prop?.
But nobody has answered my initial question; is there a make/model that is more efficient by design than the others?
Thanks, guys!

[Dave Robelen]

Hi Terry,
Your question about engine efficiency is a tough one. Many of us measure various aspects of power output, etc., but absolute fuel efficiency is rarely if ever mentioned. I would tend to go with a Saito 1st, with an OS as second choice. Both are quite decent. In the case of a 4-cycle, the idle bar plug is not really an option because of the shape of the plug chamber. For long, slow runs, a seperate battery on the plug would help insure full combustion at low revs when it is just sipping fuel.
As far as the pitch/diameter, that will depend on the model, but I would not rule out going to a higher pitch to keep the revs dow. Meanwhile, the wingspan will be a large factor in efficiency, much like a powered glider. Speaking of which, how about adapting one of the small 4-stroke mills to a powerd glider airframe? Kind of a headstart.
Regards, Dave

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Thanks, Dave
Unfortunately, one of the "rules" for this informal contests is 'no glider-type airframes'. This is my first year at this club, but I understand that last years winner was a trainer (a Goldberg Eagle, if I remember right) that flew for 50 some minutes. I don't know what engine he used, and I've been meaning to investigate this further...
I've been thinking of modifying a Sig Kadet Senior, but now we're getting off the original threads topic!
Thanks again,
Terry

[Dennis Flora]

I agree, Saito all the way !!! I have both a 72 and a 56, and they are fantastic !!! They both have the same glow plugs in them that I installed 3 years ago when I bought them. Scarey but true... Dennis Flora (good luck in the contest)