Sorry in advance I know this has been beaten to death but I could not find what I was looking for. Let me know if I am off base here. On the fp 40 & LA 46 Use the large venturi that Tower sells with a ST nva. Or the smaller OS FP 25 venturi with the stock nva in the normal spot. Where I am confused is the 25 venturi OD is quite a bit smaller than the 40-46 venturi OD. Does the O ring, seal up the venturi and keep it from being sloppy? Thanks Rick

I have switched one .46LA to a .25LA venturi.  I have not seen a problem with the fit.

BIG Bear
RNMM/AMM

the  fix is to call Randy Smith or Jim Lee for the right venturi's

There's really no "one correct answer" to your question as the venturi size that's optimum will vary with: size and weight of airplane, altitude density in your area, propeller choice, style of run you prefer(4-2-4, 2-4, wet 2),  how much nitro you run, type of muffler, and head clearence or compression ratio. The 40 with the FP 25 (6mm) venturi & OS NV should be in the ballpark for starters assuming the variables above are reasonable. Test fly and maybe try a couple of props that you know folks in your area have good luck with ( most here in So-Cal are using something between 10.5x4.5 and 11.5 x 4 in a 4-2 with 10% nitro). Going to a larger venturi will: increase the RPM, cause an earlier and harder break, and consume more fuel.
      If you need to increase the venturi ID, steps of .010" will make a noticeable change so you can ream the small one in small steps or order custom ID venturis from Randy, Jim Lee, or Stuka, etc.
     Hope that helps 

First the question about OD.

The spigots on both factory venturis are the same OD. The flange is also the same so the venturis fit in crankcases from the FP 20 to the LA 46 and the O-ring seals. The OD of the part that sticks out above the crankcase is different and I suspect that this is what Tower refer to. The step from the flange to the body of the venturi above the crankcase is barely there—perhaps 0.5mm—on the large venturi but is at least 1mm on the small one.

This is a way to identify the different factory venturis.

As for the ID, experiences vary. Some have enjoyed success running FP 35s and FP 40s with the FP 20/25 venturi. My LA 46 is happiest with the large factory venturi but it's flying a 58oz model.

We have people at our club happy with the results they are getting using the small venturi that used to come with the FP 35 and FP 40 in the early days on the LA 46—particularly in Vector 40s. You can make one of these by running a 17/64" drill through an FP 20/25 factory venturi.

Otherwise, talk to one of the specialists.

This wasn't clear to me. What engine are you setting up? On what plane?

Try a 101/2x41/2 sport apc. It'll probably work on both planes. Pathfinders are larger and much more draggy than Banshees. Use a free flowing exhaust on both setups. A tongue muffler with a lot of holes. (I know that's not precise.) Make sure that the tongue muffler does not block the exhaust port. What fuel. Fuel should be 22%oil, 50/50, 5% or 10% nitro. Fuel needs to be of high quality, high quality components for the mix, mix needs to be made by volume not weight. We use Powermaster almost exclusively. If you have supplier, I recommend that brand. What plug. Need a hot plug. Enya #3 or Thunderbolt RC is standard for our club. An 11x5 sounds like too much prop. You want the engine to be run in a comfort zone. On the fast side. Over propping can lead to over heating. On Pathfinder sized plane I usually go to an LA46, which can loaf around pulling that weight and drag. But I have seen 40s work with this plane. Definitely a large venturi and free flowing exhaust for a Pathfinder. This will help cool the engine and give you more power. Use aluminum pads under the engine. Even steel pads, if vibration issues are encountered. On 40 sized profiles I use 1/8 five ply and run the maple engine mounts back towards the leading edge of the wing. Also I make them of unequal length. An extra head gasket or two will soften the break. It's possible to run these engines in a rich to mid 2-stroke, breaking into a leaner 2-stroke. Don't sweat it if your engine runs a constant 2-stroke and doesn't break much. If the engine run is even and predictable. The plane will work. Trying to get these engines to run a rich 4 stroke breaking into a 2 stroke causes frustration and delusion. Even mental illness.