Depends on plane. Same with extra head gaskets. Lightweight sport/stunt planes like the Midwest Magician and Twister should work well with the smaller .257 venturi in either the LA40 or FP35. In a Tower 40 (FP40 clone, but better materials and better machining) I had a .272 venturi (Lee Machine, he makes most in between sizes .257-.283) in a Light Weight Midwest Magician. 36 ounces. Walter Umland wing kit, fues made of wood as light and stiff as I had. Excellent flier. Very well matched engine run. 2 or 3 extra head gaskets, stock FP click needle valve, tongue muffler. This past season I pulled out a $10 flea market Prowler to work with. 46 ounce plane, much thicker airfoil. Power was a Tower 40, same set up as the Macigician. It would pull the Prowler but needed to be set to a leaner 2-stroke, the result was no break. Not ideal. Dan Banjock and Mike Palko insisted I go to a bigger venturi. The .283 improved the run. More power, more authority over the top, but still not much break, if any. Mike and Dan then got on me to remove head gaskets and up compression. OK. Did that. Now I had a break, good power, very nice match to this plane. Fuel, 10% Powermaster, 22%(50/50). Needle was not critical. Power delivery the same flight to flight. I did some further messing around. I had the actual engine from the Magician tuned as it was when flying that bird. I bolted it to the Prowler. It behaved the same as the other Tower 40. It worked best with the bigger venturi and one thin head gasket.