Andrew,  i started using that setup a while back and the advantages i have found are no loss in power and fuel savings up to an ounce per pattern. Most of my planes using the la46 were originally designed for 35 to 40 engines anyway so any slight loss of power is not noticed. One other advantage is the venturi will still work on a 20 or 25 size engine if you need it. The reason i originally tried it was i needed a venturi for a 25 la for speed limit  combat and used a .46 venturi, that resulted in an easier needle setting and still did not exceed the speed limit, so i have switched the 46 venturi's to my speed limit 25's and use the 25 venturi's on my 46's. Try it,  you'll like it!

Thanks guys,
   Why didn't I think of fuel consumption! I am very happy with the needling using the larger venturi, but if it gets better with the 20 FP venturi then I think I should give it a try.
  I am not mean or tight fisted, but I am averse to spending money unnecessarily! The saving of an ounce per pattern sounds very attractive. I have quite a few BBTU FP20s, so I shall steal some venturis from them and give it a try.
 
Thank you all for your comments,

Andrew.

Steve,
on the surface you are correct, however one of the anomolies we found with the "old" setup, ie larger venturi, 11.5x4 prop, is that it was actually MORE critical on fuel burn with relation to needle setting. a small variation in needle setting made a HUGE impact on flight times with the old setup. with the smaller venturi, even considering the smaller fuel load, it was much easier to regulate the fuel needed since the setup was more stable in different conditions. Keep in mind I go from 2500 foot altitude( and much higher density altitude during the summer) to virtually sea level when I attend contests. I found that I had much less trouble gauging fuel load with the smaller setup .
HOwever you are correct in your observation that a 1/4 ounce is a much higher percentage on 3.5 ounces than on 5 ounces, its just this particular setup makes it less critical in my experience

I have not tried this, or had this experience, but some have reported that a small venturi made their engine run as though it had low oil content in the fuel. Theory is that increasing the venturi size increased the amount of fuel and oil going through the engine, and was supposed to fix this. I'm not so sure that just adding oil to the fuel wouldn't do the same thing. I can see that if the engine is overheating (due to too much prop load), adding nitro might make it run cooler, since you would have to run more fuel and thus more oil through the engine during the flight. And of course, the engine would produce more power, so swing that prop easier...and  a richer setting for the same rpm at launch.   Steve

Edit: I did have to reduce the venturi bore to .272" (.156" spraybar) on the .46LA in my ex-McClave Skylark, just to get enough run time, even at the altitude in Tucson. I'd thought it would be ok with the .281", but wasn't. I don't really know how big the tank is, but I think Don told me it was 4.5 oz. I was running the 11.5 x 4 APC. If I'd run the 12.25 x 3.75, I would have needed a smaller venturi, because the fuel consumption would probably be higher.  

@ Steve,
this is a new one on me, I have not experienced this

@ daniel
thats what we are talking about, the FP/LA venturi

I would venture to suggest that the runs would be similar to the LA46, although I have not used an LA40 myself. All the above advantages with maybe even more fuel economy?

Regards,

Andrew.

Usual problem with the .40LA is too much prop load. The TT Cyclone 11 x 4.5 works well here, on 10-22. With less nitro, maybe too much prop. At higher altitudes, a 10.5 x 4.5 APC would be a good bet, and the 11-4 APC might work. Adding a head shim might be in order. I'd stick with the stock venturi and put a .156" Randy Aero NV Assy. in it, but would eventually try the .25LA venturi.

FWIW, I have my .25LA running on a venturi drilled to .272" with a .156" spraybar. I can restrict it down with a few layers of pantyhose stock, or a Bruline air filter, but it runs alright, and without muffler pressure.   Steve

Edit: This reminds me...why not use the stock .40LA-S or .46LA-S venturi with any spraybar you like, then add some air filtration to make it think it's smaller than it really is? It does work, and you don't have to remove the venturi to test changes. I will say that I see no way of getting any consistent adjustments out of pantyhose material, so I do prefer to use either the coarse or fine Bruline filters.