Nomenclature.

[Perry Rose]

What is the correct nomenclature for a venturi that has the spray bar going through the engine block and lower portion of the venturi? What is the system called when the spray bar goes through the upper portion of the venturi only? And what is it called when there is the spray bar beside the venturi and not going through it?

[Randy Cuberly]

What is the correct nomenclature for a venturi that has the spray bar going through the engine block and lower portion of the venturi? What is the system called when the spray bar goes through the upper portion of the venturi only? And what is it called when there is the spray bar beside the venturi and not going through it?

Webster's definition of a venturi is:
 a short tube with a tapering constriction in the middle that causes an increase in the velocity of flow of a fluid and a corresponding decrease in fluid pressure and that is used especially in measuring fluid flow or for creating a suction (as for driving aircraft instruments or drawing fuel into the flow stream of a carburetor).

A very loose interpretation of this definition could call just about any intake in a model engine a "venturi".  However to achieve an actual useful venturi effect rather more restrictive constraints would have to be used.

Without getting too technical a true venturi will have a specific angle and length to the inlet taper, a specific  size to the restriction and a specific taper and length to the outlet section.  The only restriction will be the size of the center restriction.  any other tubes such as a needle valve protruding through that situation would certainly spoil the venturi
effect.  Therefore I believe that only an intake such as the type found on PA engines or early Super Tigre engines with the needle valve assembly in an area behind the venturi and a small passage opening into the venturi could actually be called venturis.

Those intakes where the needle valve assembly actually passes through the center of the air flow should more correctly be called restrictors.

My opinion but most certainly based on engineering facts.

Randy Cuberly

[Dan McEntee]

  As I know them, there is the true venturi, like Randy Smith sets his engines up and. The spray bar is behind the venturi and fuel is drawn through a hole that is perpendicular to the vertical center line of the venturi. Another version of this is known and the "sprinkler" and is found on stock OEM older SuperTigre engines. The spray bar is in the same place and the fuel is drawn in around a groove in line with the spray bar and enters a series of holes in the venturi. The venturi size is normally a lot smaller than most are used to seeing.
   Another is the restricter venturi has the spray bar going through the center line of the venturi horizontally, with the hole in the spray bar pointing directly down or slightly to the rear of the venturi bore. It is called this because the spray bar causes a slight restriction in the intake and the bore hole is sized accordingly.
   Another is the "spigot" type of fuel entry to the venturi. It is just a small tube that protrudes through the side of the venturi body to the center of the venturi bore. It can be fed by a remote needle valve, or by a needle valve assembly that the spigot is part of. Old OS carbs have a needle valve assembly that us useful in this manor.
   That is all that I have any experience with. and may not be using proper "nomenclature."
   Type at you later,
    Dan McEntee

[Brett Buck]

It really is semantics, and there are no agreed-to moenclature. There are three basic types (and a bunch of variations)"

"through the middle" - a spraybar crosses the venturi bore

"flush inlet"  - a round bore of some shape where the fuel enters via hole or holes in the wall. Randy calls these "true" venturis when they are a smooth interior curve, however, the venturi effect (pressure drop due to a reduction in cross-section in a closed channel) applies equally to any suction system. Any venturi with a flush inlet, I call "dribble hole" venturis because that's what tends to happen to the fuel, it comes out the hole/holes, then dribbles down the interior surface of the venturi. The original Jett venturi is like that for stunt since the dribble hole is way too big to stay filled across it, reducing the hole side from .100 to .0625 made a big difference. Sticking a "fuel post" in the smaller hole made a lot more difference.

"spigot" (a coin I termed, I think) or "fuel post" (which is what the inventor/discoverer Frank Williams called it) - round bore of some shape with the fuel entering through a protruding inlet.

     It's spelled venturi (not "venturie" or "ventury" or any of the other variants)

     Venturis were an area of intense experimentation about 20 years ago due to the tendency of many "schneurle" engines to "go lean" at odd times, particularly in hard outside corners. The solution was to get rid of the "dribble hole" inlet and use a spigot/fuel post. One particular line of engines drove the experiments, including the only engine from which I personally never got what I would consider an acceptable run.

      The type with multiple inlets (like the ST or the Cox Tee Dee) have even more potential variations/operating modes. The ST usually worked well (mostly because it had so much fuel suction you would more or less get fuel in the general area of the hole and it would wind up in the right spot). And it had many very beneficial sharp edges to cause the flow to separate and become turbulent at a fixed position.  People who should really know tell me the Tee Dee had a tendency to feed fuel through one dribble hole, and suck air *in* the other two if you ran it on suction, which all sorts of wild inconsistency. That's why it works so much better on pressure, more than just plain low fuel suction. I have seen in real life a remarkable improvement made at the field where two of the holes were blocked, and then it ran like an electric motor on suction, dead-steady in the air.  I tried the same thing with the ST, but it didn't make any noticeable difference.

     Of course there's a huge amount of nonsense about what is going on in venturis, along with some observations that are generally true but miss the point that are just as problematic. Most of the theoretical work is about pressure recovery and optimizing the power, which is generally correct (and was important at the time), but mostly irrelevant today. You don't need to get more power when you can put a 75 in an old ST46 airplane and control it.

   What *does* matter is consistent flow and consistent atomization, not how much power you can get. The venturi issue is essentially a solved problem, and at least David and I have long since moved on to the other half of the system - the fuel delivery side.

      Brett