ORIGINAL: cmoulder
Fruitful afternoon overall.
My afternoon was fruitful yesterday, although I got zero prop data.
What I
did learn is that a tuned pipe ''reacts'' very strongly to being 3/8'' shorter on the baffle distance!
RPM figures were
waaaaayyy down and the needles were not happy. I thought it was caused by my static line modification, so I took the line off and it made no difference. Thought maybe the H needle was perhaps moving around on its own. Nope. Only later did I notice that my Velcro pipe strap had slipped off the back of the silicone insulator on the back of the pipe, then I put 2 and 2 together and realized the coupler distance was incorrect. Gotta re-do that to the magic number:
27-1/4'' plug to baffle.
Good news is, the coupler held tight and didn't leak one molecule, and the header held firm and was not bumping anything in the forward pipe tunnel. Ran very quietly, but then the RPM with the 18.1x12 was ony 6300 because of the pipe issue.
Bob,
Are you saying that somehow your pipe was shortened 3/8", plug to baffle?
When the load is too large for the engine, what you describe is how the engine will react, all else being equal. When I was playing around with the 19x13 stock prop, I could only get around 6K, but the 18,1x10 would turn its normal rpm of around 7800 to 8000 on the ground. The 18.1 x 12 may be too much load. Blade shape has a lot to do with how the engine reacts
You are right, we don't want to run shorter than 27 1/8" plug to baffle. Longer is okay (don't know yet how much longer is tolerable).
I need to figure out what the slight jumpiness at 1/3 throttle is and how to modify it, to smoothen it out. I suspect that running the pipe a little longer will eliminate that, but will probably drop the high end rpm slightly also. A slight drop, say 100 rpm or so, is not a problem. Ed Skorepa may also shed some light on this.