Brett,
Hope you don't mind if I piggy-back off this thread to ask an FP .25 question or two.
I jumped to buy an FP .25 at a good price and a day later remembered your BBTU rules say "DO NOT, repeat DO NOT, replace the spraybar and venturi with the "better ST needle" and/or a venturi from the "How I came in 4th at the 1956 East Podunk Stunt Exhibition and Chili Cook-off" venturi chart." Well the FP .25 I bought has a Delrin venturi and ST NVA. I haven't received it yet so I don't know the diameter of the venturi.
I found a source on ebay (shttrman) who notes there are no more OS NVAs for the OS FP .20 - .46m, and says he has a newly made direct replacement for the original OS FP .20 - .46 Needle valve assembly. He also says, "No more drilling." [He also sells Enya NVAs for OS FP .20 -.46.] He also has original OS venturis for the OS FPs .20 - .46 in both 6 mm and 7 mm.
Question #1: Is the ST spray bar a larger diameter than the original OS spray bar? And if the ST is larger diameter, is there a way to go back to a smaller diameter OS spray bar?
Question #2: Which OS venturi is the correct one, the 6 mm or the 7 mm? I'm guessing the 6 mm, but I'ld like to buy the correct one the first time.
Thanks,
Joe Ed Pederson
The correct *stock* OS venturi is 6.5mm/.257". The stock OS spraybar is 3.5mm/.137", and the Supertigre spraybar is 4.0 mm/.157". Changing to an ST spraybar and a stock venturi results in about a 25% loss of power. An equivalent, hopefully, venturi bore diameter for the Supertigre to give the same choke area (0.0184 square inches) is .274. The closest equivalent US drill size is a letter I drill. A metric drill that is kind of close, but on the wrong side (larger) is 7.0mm.
If anything, on the 25FP, I would err on the side of making it smaller - a letter I drill is nominally .272, it will make a hole about .274 under most circumstances. An adjustable reamer appears to be required to get it smaller than that, but bigger than the next smaller size (letter H at .266 and 17/64 at .2656).
For these tiny airplanes we are talking here, maybe a letter H drill, which will make a hole about .268 or so. It will reduce the power slightly (~9%) which may be helpful to keep them subsonic. Note that even 0.008" (8 thousandths, two thicknesses of paper) changes it by 10%.
For reference, a typical "adjustment" for a full-size stunt engine is .005", and this is a very noticeable effect. I have seen 0.005" change improve the engine so much that it probably raised the pilot's score by 40-ish points. So these changes need to be *subtle* - changing it from , effectively, .257 to ~.240 by stuffing a much larger spraybar in it is a HUGE change that has very drastic effects on the power and characteristics of an engine run.
The way to restore the original configuration, assuming you have a proper 3.5mm spraybar, is to sleeve each end of the spraybar to make the diameter larger where it passes through the case and venturi. It looks like a "dogbone" when you are done, because you want the middle 6.5mm of it to stay .137, but about .155 or so for about 1/8" on each end. This is not terribly tricky as long as you are comfortable cutting small bits of tubing. You can cut a kerf along the axis of a piece of 7/32 OD tubing with a razor saw, compress it to close the kerf back up to fit tightly around the spraybar, then cut it into short lengths and J-B Weld them to the spraybar so it will line up with the case/venturi. It doesn't have to fit all that well, since this is just there to take up the space created by drilling the hole in the crankcase, but if necessary, file the kerf wider so that you can make it a spring fit on the spraybar. Keep the glue out of the obvious spots, or you will probably have to toss the entire thing and try again with a new one.
I might post some dimensions and pictures (but not with glue, because my engines don't actually have the holes drilled out) later or tomorrow.
Brett