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

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

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

George Aldrich called the spigot a "schnozzle".

 " 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

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

After flight testing today I can say that 25% bigger is way too big.

   25% in terms of area, right?  Not diameter?

    Brett