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Engine basics => Engine set up tips => Topic started by: Terry Bolin on September 06, 2007, 06:52:55 AM
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What effect does the height of the venturi have on a glow engine? I saw a picture of a engine in a 2006 Stunt News that was VERY short but yet the Brodak engines have a tall venturi. Any thoughts? Trial and error?
I know on a car engine, that kills the low RPM Torque.
Thanks, Terry B. Neosho, MO.
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FWIW, there is reported to be some "tuning" effect by the venturi lenght, however if I recall from another post it is very minimal. My engine "guru" told me the main premise behind longer venturis was that some engines "spit" fuel back out of the venturi in certain situations and having a longer throat helps minimize that. something about the timing on certain crank ports? Thats what I know. (sorta know? ???)
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I remember that some studies were done about tuned intakes at about the time tuned exhausts became the rage in speed events. The length required to get any tuning effect would be close to the same as that for an exhaust. Pretty hard to do.
I believe that some use longer intakes just for the stated reason, to try to stop some of the backflow from the venturi. There's an awful lot of that happening to every engine. It's the nature of the beast. There is some flow in both directions due to the cyclic nature of the heat pump we call an engine. With some it's more obvious from raw fuel in and around the engine after a run.
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And, Terry,
There's also a thing called choking the engine to draw fuel for starting...
Some engines have short front ends, and very short stock venturiis. When mounted inverted in a full-body model, adding a longer extension makes choking a bit more possible... I've used exhaust header (hi-temp neoprene?) tubing to hold such an extension onto a Fox 35 on at least one such model. About 3/4" length of 1/2" OD aluminum tube; fit the neoprene tube over the bell at the top of the venturii, and lashed it on with fine copper wire.
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I have used long venturi up to 1 1/2" and they make no diff. The reson I use them is so I could choke the engine,some cowl make it really hard to do. y1 A buddy thought it made my engine run better with the longer venturi ,so I made him some . Did not help him, so get the engine set up right long or short they will run good. y1
Jimmy
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Way back in the days of AMA slow combat, many people tried to run the Fox Mk III on suction with generally poor results. For the Mark IV, Fox made the venturi taller and provided a location for the needle valve higher up in the venturi. There were actually 2 needle valve holes, one low in the venturi for pressure and one higher for suction. The Mark IV with the needle valve in the top hole ran very well on suction so there is some effect of venturi length.
Steve
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On the Fox combat model the top holes in the venturi put the needle valve in a much better position relative to the fuel tank than the lower holes. Fuel system geometry probably had much more to do with the motors run than the longer venturi.