Venturi size

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  • #40347
    George Reinhart
    Participant

    Lost in the archives was a note regarding appropriate venturi sizes foe various engines. I don’t remember exactly how the discussion went but I could use the information if the respondents could answer again.
    Thanks!
    Cheers!

    #42483
    JLorbiecki
    Participant

    What are the engines you are interested in? Maybe I can help a bit…It does vary from engine to engine on what it will take. However, there is a way you can start. Run the engine with a stock venturi. Then start opening it up, looking at both rpm and how it needles. Most engines on pressure (I would assume you are running some type of pressure system) will take a pretty big hole and still run, but it may not needle as well as a smaller one. Plus, there will be no rpm increase after a certain size…

    #42484
    George Reinhart
    Participant

    @John Lorbiecki wrote:

    What are the engines you are interested in? Maybe I can help a bit…It does vary from engine to engine on what it will take. However, there is a way you can start. Run the engine with a stock venturi. Then start opening it up, looking at both rpm and how it needles. Most engines on pressure (I would assume you are running some type of pressure system) will take a pretty big hole and still run, but it may not needle as well as a smaller one. Plus, there will be no rpm increase after a certain size…

    John,
    I have four engines currently that are not top of the line, so are not where I plan to end up in a year but I have them as gifts from friends and family who are helping me get back into Free Flight after a 30 year lay off.
    First three recent product tio Fox engines. A .40 with an r/c throttle assy that, when removed leaves a rectangular ventiri abou a half inch high and maesures approx. .42″ by .38″ the throttle wide open with no spray bar restriction is .28″ (that is there is no bar across the hole). The other two are a Fox .25 BB Schneurle with a u/c venturi and there is a Fox .15 BB Schneurle also with the u/c venturi All three, instock form are meant to run on suction and have small intake diameters. the last two have a regular spray bar spanning the venturi..
    The .40 icould put a needle valve in the set screw holes for the throttle body and sleeve it down if necessary.
    The .25 & .15 I’ll have to bore out the venturi (whch is detachable from the case) to an appropriate size orget a machinist friend to make up smething for me.

    Tyhe last engine is a .61 Torpedo of uncetain age with an r/c throttle. Also it is non-Schneurle ported. I will likely not use it for anything.
    I was originally going to go with larger AMA gas and use these engines as a starting point but I am now aiming mre and more at 1/2 A and F1J. I think that’s where my major interest is going to be.
    Thanks very much for your response.
    Cheers!. Pete
    Pete Reinhart, Austin, Tevas

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