Hello,
Looks as though I have got things wrong! I am using the OS venturi intended for the OS 46 LA. The one supplied with the 46 LAS. To be honest, I can't see much wrong with that venturi. I run the LA 46 in a wet two and it seems to be fine. Tell me, what improvement would I get with the one OS used for the FP20? The engine is completely stock and one OS headshim.
Nobody said that you should change anything. All I am saying is that the idea that the venturi size is some direct function of displacement *isn't right*. And then gave the examples that proves that it's not right.
What I get awfully tired of is the endless "rules of thumb" that just aren't right and are demonstrably not right but nonetheless passed around as some sort of gospel. For example, it predicts that you need .009 sq in of choke area for a .20, when in fact it runs dandy with over twice that. It predicts a that a PA75 needs .0334 square inches, when the engine that won the world championship has a little less than *half* that. If you use .0334 choke area on your PA75 you are going to need drop tanks because is manages to suck up 8+ oz of fuel a flight with the about 0.0175 square inches.
You have to choose the venturi based on the type of run you want, the type of airplane it is on, and the temperature and altitude, the fuel, and a bunch of other factors. Essentially, you need trial and error. If you need a starting point, I would suggest about .0184 square inches and adjust from there. That's not a hard-and-fast rule but it will tend to get you a lot closer than the magic formula for a huge range of engines.
If your engine comes with a venturi, you *already have a starting point* and thus need neither a formula nor a 20FP venturi.
Brett