Engine Venturi Question

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    Dean McGinnes

    I am rehabilitating several engines from rc (never capitalize) to Free Flight (Always Capitalize) use. As a consequence of their rehabilitation, I will be making venturi inserts in place of the throttles. These are OS MAX-III engines with only a short boss where the carburetor spigots into the case, and the part I am making replaces the whole intake stack.

    The Question: Is there some rule as to the length of such venturis (venturii, venturae?) or intakes? Is there any gain to tapering the bore? ๐Ÿ˜• ๐Ÿ˜•


    Length? Well yes & no, depends on tune of the engine, mainly crankshaft timing.
    lf it’s a stock engine then a shortish venturi should do the job, but if the crank has been ‘played’ with to increase the timing then a longer venturi will hold the charge in there and stop it blowing out into the wind.

    Re. taper. Larger hole at the start tapering to a smaller one can be ‘played’ with as when the air is sucked in it will speed up when the hole gets smaller, so increasing the charge rate—a nice bell mouth helps too, and looks good.

    Depends what you are looking for really—have a play, and see how it all goes.


    It all really depends on whether you will run pressure or suction. If you run suction, you should start small (about .218″ or so) and slowly increase the size until the motor won’t draw fuel. The shape should be an actual venturi shape. I ground a cutter for the lathe to do this shape. The location of the needle valve is somewhat critical with suction and it should be placed below the smallest diameter and the hole should be placed angled down, at about a 10-15 degree angle.

    If you are running pressure, then the hole can be much bigger and the actual shape is not really critical, for an engine like this. Start around 1/4″ and open it up, looking for a rpm increase. When it stops, you stop….

    I have tried many shapes and sizes, but most of it just doesn’t do much with a lower rpm engine….

    Good luck!!

    Dean McGinnes

    Thanks, Gos and John,

    To clarify, I always run a bladder tank. Simple and effective. ๐Ÿ˜€ ๐Ÿ˜€

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