I know that O S engines from the 15 size engines to the L A 46's have different size venturi's .  Does anyone know where the information is with the different size venturi's?

   In other word's the actual venturi size for each engine.
 Thanks a lot,

  Thanks Randy, on most of the engines I have been using the Tiger knock offs.  I think that they are about .159 in Dis.
  Again thanks...

I can't help with the 15—I don't have one.

However, here's what I've measured/found—

FP 20/25, 6.5mm

FP 35/40, 6.75mm (small from when OS supplied 2 venturis with the FP 35/40). You can drill out the FP 20/25 venturis from Tower with a 17/64" drill.

FP 35/40, LA 40/46, 7.1mm (large from when OS supplied 2 venturis with the FP 35/40)

The sizes quoted for the FP 20/25 and the large FP 35/40 are what I measured from the items supplied by Tower, not the sizes they quote on their web site

Luiz, I'm no expert flyer but what you hear is correct. However, from my experience, it is not that simple. I believe that there is a range of variables that all interact.

Wing area and model weight have an effect on engine load, as does the propeller. I have generally heard more about people having success with the FP 20/25 venturi to tame an FP 40. I have not tried it but I suspect that the FP 20/25 venturi might be a bit too restrictive for the LA 46.

I have been flying a model that is ~560 sq in but rather porky at 58 oz. I flew it first with an FP 40 with a hemi head and, initially with the FP 20/25 venturi and a Bolly Clubman 11.5 x 4.5 prop but the engine struggled. I then tried it with the 6.75mm venturi and the Bolly Clubman 11.5 x 4.5 prop. It flew OK but the engine still struggled a bit. I switched to an LA 46, 6.75mm venturi and the Bolly prop. It was better but the model really shone when I fitted the large, i.e. stock, LA 46 venturi with, again, the Bolly prop.

Different model, different prop and a former Oz champ was happy with his LA 46 with the 6.75mm venturi. The same person is very impressed with the capabilities of my model using the final setup listed.

I think that as starting points, FP 35 and FP 40, try the 6.5mm and 6.75mm venturis and various props. LA 46, try the 6.75mm and the 7.1mm venturis.

All this assumes the standard 3.5mm OD OS NVA or a Super Tigre with the same 3.5mm OD. We have the latter in Oz but I don't know whether anyone in the US does them.

I'm reluctant to drill out the crankcase and venturi for the 4mm NVAs and since I don't need to, I won't.