Obviously the LA46 makes a fair bit more power than the FP 20. I am then amazed to read that people run the LA46 with the stock FP 20 venturi set up! I tried this on one LA46 and found I was way down on power, even leaning out the mixture didn't restore all the missing power. The run quality of the LA46, choked down, was pretty abysmal. So why do folk make this apparently retrograde step and how do they get the lost power back?

   Lots of reasons, some of the sound, many of them not. There has been a school of thought for many years that "RC Motors" are not "real stunt motors" because they won't run reliably at 8000 RPM on a 6" pitch prop in a 4-stroke.  A "Real Stunt Motor" has to be as gutless as possible. By extension everyone running piped engines with 3.75 inches of pitch is Not Doing It Right, hobbling themselves, but nonetheless winning contest after contest for 25 years by masterful piloting skills, or more likely, cheating. I have letter after letter to that effect from the mid-90s.
 
    If you are going to run around at 8000 rpm, you had darn well better have a small venturi. Many people have not yet gotten the relationship between power output (and by this I mean power in the engineering sense, not "power" to a stunt flier) and fuel draw/air flow.

   In other cases, they may be responding to the fact that if you put a 46LA in a Nobler, you still only need .3 hp or so, and needling the engine down doesn't work. So a smaller venturi is intended to reduce the power to something useable.

    Brett

Andrew,

One thing to always consider is the inside diameter of os Venturi are not all the same . I have seen stock os Venturi on la 46  and fp 40 range from .285 to over .300. , the fp 20  and 25 seem to be more consistent, andi will matter what size and brand nva you are using ,

Sometimes when  people tell you a specific Venturi  type or size, they do not mention needle valve assembly or the actual Venturi size,

I just changed a la 46  from .288 to a 265 Venturi, with supertigre nva , ran great on a little less than 4 ounces , 12 / 4 APC prop, just like Ray did,  and his do run great, main reason for change was limited fuel tank capacity.

Thanks for the responses. I run the usual APC 12.5 x3.75 or a Thunder Tiger 12x4.5. I am using off the shelf OS venturis together with the appropriate standard OS NVA, in conventional front mounted style. I appreciate that venturi IDs have changed over the years, but not enough to make a large difference. The planes are suited to the power output of a 0.46 engine, that is around 55 ozs mark. Using the FP20 venturi just simply kills the performance. The standard FP40 venturi is just streets ahead in terms of power delivery. I can't really say how the performance compares with a Fox 35. I don't much like 4-2-4 engines and much prefer a low pitch high rpm style engine. So Fox 35s are pretty much off my radar. The only plus point for the small venturi is it doesn't use so much fuel, but then my planes are big enough to accommodate a larger fuel tank.
From what has been said, the LA46 with the small venturi is suited to the smaller style of plane that could be flown by a Fox 35. If I have got that right, it seems to be an odd way to use a modern 0.46, as someone said, it would be more sensible to use an LA25 in such cases, or have I got this all wrong?

Regards,

Andrew.