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[Rusty]

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[Steve Helmick]

I just emailed you Leonard Neumann's chart. I refer to it when I have to make a venturi for an offset spraybar, like TT's, ST's, and some others. In general, I always use a .157" spraybar and a .272" venturi will work fine from .25LA to .46 LA and beyond. The reason I always use a .157" spraybar is because I don't like OS NV Assy., and by using a .157", I can use a lot of different brands...but usually Randy Aero's.  Steve

[Bootlegger]

 Steve I sent ya' a P M

[Brett Buck]

I just emailed you Leonard Neumann's chart. I refer to it when I have to make a venturi for an offset spraybar, like TT's, ST's, and some others. In general, I always use a .157" spraybar and a .272" venturi will work fine from .25LA to .46 LA and beyond. The reason I always use a .157" spraybar is because I don't like OS NV Assy., and by using a .157", I can use a lot of different brands...but usually Randy Aero's.  Steve

   Unfortunately this chart is incorrect, and it's based on incorrect assumptions, specifically, that the choke area should be somehow a simple function of the displacement. It isn't. It will tend to give an abnormally small venturi size for small engines, and oversize venturis for large engines. You only need the small end, particularly, if you turn the engine abnormally slowly, which will cause you to use a very large prop with lots of pitch.

    On the other hand, a .272 venturi with a .157 (ST) spraybar is right in the normal range for venturi at .0178 square inches of choke area. The normal range is around .0175 to .0190 square inches, and it's not a straightforward relationship to displacement. I think Dave won the 2008 WC using something a little smaller than .0175 on a 75. The smaller engines tend to the high end - not that the 75 wouldn't work with .0190 or larger, it's that you can't figure out how to get rid of all the power that would provide.

     Brett

[Ward Van Duzer]

In all my years of working in Randy's shop (including turning hundreds of venturis) the one thing I did learn is there is no simple "Chart" to determine what venturi is "best". The diameter of the hole is only one dimension in the piece. The shape of the incoming funnel, the outgoing funnel and the overall diameter of the hole determines the speed at which the air passes through the venturi and therefore the relative mixture of air and fuel that feeds your engine. As such, the venturi determines how much power your engine makes and at what RPM. Also determined by this little piece of hardware is the point of the 2-4-2 break and the "hardness of the break. Re: hard, or soft break. We frequently made sets of venturis in 4-5 different sizes so the flyer could determine which one made him happiest in his weather conditions! Lets not forget the position of the spray bar, and it's size, etc, etc.!

My advice to any one needing venturi(s) for control line use is to call your Guru, tell him what you have, and what you want and let him make the decision(s).

Ward

[Brett Buck]

I have not ordered any venturis because I don't know what to order.  I wish I could figure it out.  I want a stunt venture for a KB 3.5, ST 60 small case and OS LA 25. 

   For the OS 25LA, the stock parts are ideal. They are available from Tower.

     Brett

[Brett Buck]

I thought everyone here is a 'guru' except me.  I want to buy my venturis from Jim Lee.   Can somebody give me the ID of what works for them.   I promise to be happy with it.   OS 25LA,

 You don't need gurus and gurus don't really exist. You *could* make use of the search function.


 No offense to Jim - but I very strongly encourage you to use the stock parts to start. Including the stock muffler.

   The stock parts are a 0.137" diameter spraybar and a .257 venturi. If you use an ST spraybar (not recommended), then enlarge it to get the same choke area (which is 0.0184 square inches).  For a .157 ST spraybar, use a .274 venturi.  This is the same for the 20FP, 25FP, 25LA.

http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXMU30&P=X
http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXCW99&P=X
http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXCF88&P=X

or

http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXCR49&P=M
http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXCW99&P=X


   While you are at it:

http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LX1532&P=7


   Brett

[Brett Buck]

Thank you Brett.   Since I have to buy several venturis, I wanted to by them from Jim Lee.  I will use the number you gave for the ID.  (the empire is out anyway) 

How about the ST60 and KB 3.5 you have any numbers for them?

Thanks

   I have never run the ST60, so I would refer you to the experts. The one bit of information I got was from Paul Walker a long time ago, and he used a .295 with an ST spraybar. That sounds about right so I probably remember correctly. But the exact version of the engine matters, and I don;t know enough about them to give specific advice.

    For the K&B 3.5, I don't think that it has ever been run by experienced stunt fliers in a full development program. And that's what it takes - you aren't going to just go out and run 5 engines in an afternoon and be able to give good advice. Although that's exactly what the self-styled "engine experts" do all the time. That's why they aren't really gurus. There's also a bunch of different versions of the 3.5. My guess, from very limited information, that this will not turn out to be a particularly good stunt engine, but that you can get it to be serviceable with a lot of experimentation. I would suggest the approach I used in the "small engine" tests back in the early 90's, and more-or-less described here:

http://stunthanger.com/smf/index.php/topic,17781.msg163868.html#msg163868

   except starting with a 9-4 APC. All that you need is a careful systematic approach and be able to tell when it's working or not. To be honest, I think your time (and this will take a fair bit of it) might be better spent on more promising engines and other activities, but if you want to try a development program, help is available.

     Brett
 

[Dane Martin]

Okay.  I will leave the KB 3.5 for RC.

Thanks for the advice. 

Does your k&b 3.5 look like this one?

[Steve Helmick]

IMO, the SSW chart will get you somewhere close, or at least running decently. Just keep in mind that it's a starting point. Remember that you can always drill out an insert that's too small, or put a layer or two of pantyhose mesh over one that's too big. Nobody claimed otherwise, including Leonard. Note that there is no column for your altitude, percentage of nitro you want to use, or how hot it is on any given day!  Steve

[Brett Buck]

IMO, the SSW chart will get you somewhere close, or at least running decently. Just keep in mind that it's a starting point. Remember that you can always drill out an insert that's too small, or put a layer or two of pantyhose mesh over one that's too big. Nobody claimed otherwise, including Leonard. Note that there is no column for your altitude, percentage of nitro you want to use, or how hot it is on any given day!  Steve

   The problem is that it is based on a faulty premise - that the choke area needs to be proportional to the displacement. That's just not right, and it works for a very limited set of conditions. If you use it, it will make you think a 20FP is really wimpy, for example.

     Brett

[Dane Martin]

Yes, it looks like that, but has the original muffler on the back. 

Yeah, those are awful. Just go ahead and sell it to me!

[GregArdill]

Here's a chart that was compiled by an academic flier in Egypt.

It's always worked for me. It gives a good range of choice.