Gentlemen,

     I've been flying a profile with a G21-46.  The engine doesn't have nearly as much power as some of
my more modern engines, but I love the sound of it.

     Using a venturi with the spray bar going through the body of the venturi, how large can I open up the
inside diameter of the venturi without having to use muffler pressure?

  I don't care much for the "through the middle" venturi - if it's not already drilled through, i would certainly suggest using the stock arrangement.

   Assuming that ship has sailed, I think about .280 is about all I would have much enthusiasm for. And that will likely be marginal, particularly on 5% fuel.

    If you want more power, I would look into fuel rather than venturis. The last gasp of the ST46 was David's at the 89 TT, running 15% or 20% Cool Power with some castor added. You definitely need some synthetic to keep everything cleaned out and the ring moving. And don't be afraid to throw nitro at it. 15% for sure, more if necessary. If it starts doing funny ignition things (and ONLY if it starts running funny) you can add shims until it stops. I think this is far better solution and far more reliable way to get power out of it than pushing the venturi to the limits.

      And in any case, to get any sort of power out of it, *you need a good ring*. That's about the only thing that matters as far as the engine itself goes. Engines with weak rings will run nice and smooth but be gutless. If you have one of the early really good stock rings, then good, but if not several people still make rings and I would suggest looking into that. I have one of the strongest ST46s that my buddies and I have run across, and it's absolutely bone-stock internally - with an exceptional factory ring. Never even had the head or backplate off.

   We all thought that my engine was the so good that we couldn't imagine what else you would want - until we got a 40VF!  A really good ST46 is a nice stunt motor but no matter what you do to it, don't expect miracles. And AeroTiger will eat it's lunch as far as stunt performance goes.

   See *many* previous posts on the topic of ST46s. I was one of the last holdouts and trying to keep up with the ST60 crowd and then,briefly, with the 40 TP motors, we pretty much had all the tricks figured out. And it boiled down to having a good ring and the right (pretty ordinary) fuel. Without that it didn't matter what you did. Most of the bizarre theories at the time (porkchop cranks, "0065" cranks, hemi-heads, shaving the head fins, etc) were either pointless or detrimental unless for some strange reason you thought you needed to "detune" it.

    Brett

Spigot type venturi  7mm small , 8mm large , 11-11 - 10 to 15 fuel and I'll think you would be happy , engine 11/6 to 10 1/2 /6 P. for happy engine.   
                                                          Jimmy Jacobs

 8 mm= .315 in real units. WAY, WAY, too big for stunt unless you are running 25% nitro and a 9" prop at about 15000 rpm.

   Brett

If you can run a venturie with the NVA thru the case on the back side of the venturie you will use a .160 to .170 diameter True venturie or sprinker
as Brett stated use nitro to control power output, we used a Rev up or BYO 11x5W, The carbon 12 x 5.5 cut to 11 3/4 work very well on them too

    To expand on that, using a conventional arrangement (stock sprinkler-type venturi), I eventually settled on a .173 as the best compromise between fuel suction and power. It pretty well demanded that you run at least 10% nitro for that, on 5% it was just too touchy on the needle. I think Ted ran as much as a .186 at times (maybe in Lake Charles?) but I don't think there's any way to use that big a venturi on a regular basis.

    At the end I used either the 12-6 Bolly (2-blade) depitched to about 5.8 for most of the blade and going up to 6.3 over the last three stations for normal conditions. If there was any consequential wind I would switch to a Rev-Up 13-5 cut to 12 with 4.5 going up to about 5 at the tips and tremendously thinned out and reshaped so the airfoil looked like  a HLG wing (or a Y&O, *Bull* BY&O). I also had some luck with the Bolly 11.25-6.5 3-blade in "vacuum air" condition. But it's really hard to say what was going to work unless you experiment a lot. A strong engine will turn a 13-6 or even a 13-5 (which is much tougher since in the air the drag coefficient is about the same as the -6 but it has to be 7-800 RPM faster - a huge increase in the power required) without too much drama. At the other end of the scale I have seen as little as 10-6W, but I expect that was pretty wimpy. A lot of the prop magic we learned was trying to get the best performance out of ST46s and there's no way I know how to optimize it without trial and error (hopefully guided by insight you gain along the way)

      Every time this topic comes up the so-called "good old days" and all the memories of endless fiddling comes flooding back. A lot of guys who didn't fly a lot in that era don't realize how frustrating it could be always struggling to get more power without screwing up the run completely. Then along came the 40VF, and everybody dropped ST46s (and 60s) like hot potatoes. It's just *so much easier* now.

     Brett

Trying to "squeeze more power of a perfectly good ST46 is really not the way to go. Just buy a more powerful engine,(Saito72 CL). While everything Brett said is course, perfectly true, the one thing he didn't mention was NICENESS!!!

That's right, I consider the ST 46 to be THE SWEETEST stunt motor ever developed-period. Walter Hicks described the best as having a sweetness that hard to match. I personally think no one has ever done it.

ST46's are best, bone stock with a good ring. A bone stock ST46 with say, a Bowman ring with a perfectly matched and balanced prop, is a thing of pure joy. Needless to say, like the Fox and the McCoy, it's time is long past.  But if you want to know what all the fuss was all about, by all means rock and roll, I don't think you will be disappointed.

Merry Christmas!

  Merry Christmas to you, too, big guy!  Aaron, and your family, too!

    Brett

The ST 46 I have is stock and one of the best engines I own. She's not a power horse just a sweet running baby. Mr. Frank Bowman fixed her up for me several years ago. Starts very easy and is a lot better engine than the plane she lives in. If you're looking for power go to the OS Engines on pipe not as exspentive as the high dollar engine but they're good power horses. Some fliers shorten their flying lines to help with the power issue??? Just a few feet not a great amount but it seems to help?

   It helps, but there's just no way to make it competitive with any of the engines we currently use. An LA46 is far more effective, for instance, and it's dirt cheap.

    Don't misunderstand me - I have many fond memories of ST46s and it brought me home a whole lot of hardware. Ted Fancher and I were among the very last holdouts with it, even up against the 40/45 FSRs and ST60's. It took every sort of trick we new at the time. But as soon as people got OPS and OS 40s on pipes - and learned the advantages of low-pitched props - there was just no way to match it with an ST46.

   For me it was the 87 or 88 Golden State meet - Paul showed up with a 40VF and it was immediately obvious that if you didn't have one you didn't have a chance of beating him. And in fact, essentially no one did until 95 when he decided to fly the B-17.  There's still, even 20+ years later, nothing that is a whole lot better.

    That doesn't mean that ST46s don't still run well and you can fly airplanes with them just like we did in the mid-80's. In absolutely ideal conditions with the fortunate day it ran perfectly, you might have a chance. But in the long run trying to overcome the disadvantages up against people of equal skill running something far better is really a hopeless proposition.

      Brett