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Engine basics => Engine set up tips => Topic started by: Motorman on August 13, 2014, 06:17:21 PM
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I want to switch from a full spray bar venturi to a smaller one with a spigot half way out. I've done the math to get the bore size difference. I can't imagine they both flow the same. I want to know from your experience, do you find that you get the same power out of the engine using the numbers this way or did you have to go slightly bigger or smaller? Also curious if the needle setting had to be richer or leaner to keep the fuel flow the same.
Thanks,
MM
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I want to switch from a full spray bar venturi to a smaller one with a spigot half way out. I've done the math to get the bore size difference. I can't imagine they both flow the same. I want to know from your experience, do you find that you get the same power out of the engine using the numbers this way or did you have to go slightly bigger or smaller? Also curious if the needle setting had to be richer or leaner to keep the fuel flow the same.
Thanks,
MM
You can go substantially larger in terms of cross-sectional area using a spigot over either a "true" venturi or a spraybar type venturi. But that doesn't necessarily mean you should.
Brett
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Bigger is better if your goal is maximum air speed with a suction system. Care to take an educated guess as to how much bigger?
Thanks,
MM
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Bigger is better if your goal is maximum air speed with a suction system. Care to take an educated guess as to how much bigger?
I would say experiment, and use lots of nitro. My guess is at least 25% but there's no easy way to figure out the maximum.
Brett
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I want to switch from a full spray bar venturi to a smaller one with a spigot half way out. I've done the math to get the bore size difference. I can't imagine they both flow the same. I want to know from your experience, do you find that you get the same power out of the engine using the numbers this way or did you have to go slightly bigger or smaller? Also curious if the needle setting had to be richer or leaner to keep the fuel flow the same.
Thanks,
MM
We did lots of tests on this back years ago, a true venturie, properly made yields the most power. note.. a drilled out venturie made to be a tube is not a true ventutie, and yes we made, used, and tested fuel post venturies also back in the 80s and 90s. I did read where Russian speed flyers found the most power using a true venturie with a hypodermic needle as a fuel post.. so it may behoove you to look for that info.
Randy
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Well, that's what this thread is about, switching to the F2A style spigot or fuel post as you say.
I think there are allot of terms floating around that can be confusing (to me at least). By True venturi I think you're talking about the shape of the hole?
The venturi I make has a parabolic mouth, a convergent section, a short straight section where the fuel enters then a divergent cone shape.
Anyway, to answer one of my own questions, todays bench test shows the numbers track very well. Engine put out the same power with the spigot venturi as the spraybar venturi of the same mathematical area.
In flight with the full spraybar, the needle had to be wide open (8 turns) and fuel was regulated mainly by the pressure head in the tank. This gave mixed results in the air. Next will be flight testing to see if the fuel signal is increased with the spigot set up and I can begin to regulate with the needle valve. Also a 25% larger venturi is in process.
MM
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George Aldrich called the spigot a "schnozzle".
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What is your goal with the engine? Maximum power? Better stunt runs? Solidifying your lead in endurance?
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" The venturi I make has a parabolic mouth, a convergent section, a short straight section where the fuel enters then a divergent cone shape.
That does not work quite as well as one made with NO or a very short straight section, the straight section should be no longer than the size of the feed hole
try an eleven degree intake section with a flared top, transitioning to a 22 degree exit (bottom section) , make the straight part no longer than the size of the feedhole , or no straight part at all , and that should be a very small hole.
The feedhole should be Right at the start or the downward angle, NOT below or above it, It is also oK to put the hole where the top of the feedhole just touches the edge of where the two angles meet.
Randy
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What is your goal with the engine? Maximum power? Better stunt runs? Solidifying your lead in endurance?
maximum power with a suction tank system. I find when the venturi gets too large the fuel signal drops off and fuel flow is greatly effected by things other than the needle valve. So, I'm attempting to increase fuel signal without losing power.
MM
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MM,
I did some tests back in "97 with my Profi F2A engines. The original venturi was basicly a true venturi as described by Randy with a bore of .282" but the jet hole was well below the minimum restriction. I didn't think this was anywhere near right so I had John Newton make me something better. He made a couple variations for me. One was a stock venturi as described but with the entry bell reshaped and opened up to bring the minimum diameter down to the top of the jet hole which brought the i d to .304". The next one was about .322" i d with a 1/16" spiggot extending about 1/3 into the opening.
On the test bench after break in with the original stock venturi without pipe and on suction the engine was leaned out to peak and rpm checked. I don't recall the rpm. The venturi was changed to the modified stock unit with .304" i d and the needle had to be turned in 2 full turns to peak. This despite having an i d increase of .024". The .322" venturi with spiggot needed a further 2 turns in to peak the engine. That's 4 turns leaner than the poorly shaped original venturi with an increase of .040" i d! My model was very easy to run and usually gave a clean run first try with only a ground run to check the setting. The spiggot feed was giving a strong signal compared to most everyone else and made things very consistent and fast enough to get a 9th place at the "98 W/C in Kiev at 285 kph.
The later Profi's came with similar venturis with a spiggot feed.
Jim Rhoades
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After flight testing today I can say that 25% bigger is way too big. Got allot of crazy flights in with various needle settings but there was no window to hit. When all the stars lined up for a few laps it was 1/2 sec faster in the half mile but fuel flow was all over the place most of the time. Basically the same problem I had with the spray bar venturi only worse. The spigot venturi with the same area worked very well but only had 1/4 turn between too rich and on song so I don't see where I could make it any larger.
MM
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After flight testing today I can say that 25% bigger is way too big.
25% in terms of area, right? Not diameter?
Brett
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Yes, area. 25% bigger diameter would probably just run off the prime and quit.
MM