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How Far Back Can Tank Be From Spray Bar?

Old 09-15-2023, 03:06 PM
Crazy Legs
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Default How Far Back Can Tank Be From Spray Bar?

I am working on a Phoenix Model PH011 Corsair Kit and it's my opinion that the manufacture designed the Fuselage so that the Fuel Tank is way too high for a partially Inverted Engine (it's rotated counter-clockwise about 120 Degrees). The center of the Tank is about an inch above the height of the Carburetor's Spray Bar. I modified the Tank Floor (by lowering it) so that I could lower the Tank about an inch. However, this required me to also move the Tank rearward about an inch and a half because of an interference with the Muffler Clearance Tunnel that they designed right under the existing Fuel Tank Floor. The Tunnel is tapered downward toward the rear, so I will need to push the Tank rearward slightly since the new Fuel Tank Floor will be resting on the top of that tapered Muffler Clearance Tunnel. So in a reply, please let me know what the limitations are for how long the Fuel Feed Tubing can be that comes from the Fuel Tank and goes to the Carburetor. I read the instructions that come with the .55AX Engine, but they don't mention what that maximum distance is; they only point out that the Center of the Fuel Tank should be at the same level as the Center of the Carburetor.
Thank you for your help.
Michael D'Amico
3319 Sepia Street
West Melbourne, Florida 32904
Old 03-01-2024, 09:07 PM
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You should be fine with what you described if you plan to use the muffler pressure tap for fuel feed. 6-8 inches of fuel line wouldn't be a problem and the HS/LS needles will handle fuel metering regardless of carb orientation
Old 03-02-2024, 08:52 AM
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I am facing a similar dilemma. Soon I will be building an old Morris Top Cap from a set of plans. With the fuel tank inside the wing as designed, the fuel tank centerline will be about 1.25" below the carb. With the tank mounted outside on the plank fuselage, it would be about 1/2" above the carb. I'm not sure which condition is worse. Moving the tank back far enough for an optimum level would put it way too far behind the C.G., so that isn't an option.

I have to pick the least worse of the 2 height options. Kinda like pretty much any election as far as I can remember.
Old 03-10-2024, 05:32 AM
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Some engines are more tolerant of tank position than others. For the most part it's related to the carb throat area or venturi area. The more open the less fuel draw. Sadly, in the quest for more and more power, manufacturers over the years increased the venturi area which then required some sort of pressure, usually muffler, to assist with fuel delivery.

I once bought a pail of old used engines and ran them one by one. Surprisingly, they all ran very well and most did not have, and were not designed for mufflers or muffler pressure ,instead they had a flat plate on the exhaust that rotated with the throttle. Full throttle, plate/exhaust open, full idle plate/exhaust covered or closed. Even more surprising is they all idled extremely well, and idled reliably with the tank located over a foot below the spray bar. One Merco .60 with dual glow plugs even ran with the tank almost 2 ft below the spray bar. Airplanes of the day were designed so that the top of the tank often sat even with spray bar and sometimes even lower. I have a more modern K&B .28 Sportster that runs BETTER without muffler pressure and draws fuel just fine.

So the answer to the OPs question is: It depends on the engine. A little running on your engine test stand should reveal the correct answer for your engine.

Last edited by 049flyer; 03-10-2024 at 06:56 PM.

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