ORIGINAL: rcjunkee
Cees,
Now we are getting somewhere. Very good info indeed.
My pipe HAS been cut for maximum RPM. I had been trying to get info on how pattern flyers cut their pipes but could find no info so I went for maximum. Please tell me what how I should cut my pipe e.g. percentage of max RPM / behavior or otherwise technique for cutting.
the pipe is a MACs quiet pipe 8.5 cc and the tuning procedure used is described here (although I stopped AT max rpm rather than going past as described)
http://www.macspro.com/tuning.asp
In the meantime I will perform the test you describe.
thanks again
Paul
Back in the day, we tried not to cut a pipe so short as to maximize rpm on the ground. There had to be some reserve such unloading in the air could occur yet the engine wouldn't go lean. We always tried to maximize performance in the air. It typically meant about a 1/2" longer pipe setting than the setting for max rpm for a given prop and fuel
I don't remember the problem you described earlier with my set-ups where the engine would go lean when tank was near spent. The settings apparently are just to close to max rpm on ground rather than max performance in the air....
Let me suggest that you do a couple things:
Stretch the pipe at least 1/2". It should drop max rpm on the 10x7 about a couple hundred. But it will increase tork a bit so you could get speed back up if it was lost to begin with by going up on pitch. Except the engine will handle better and run happier
Use pipe pressure to the tank. It's no substitute for a pump, but in a small plane how far back can you realy bring the tank anyway, to take full advantage of the weight directly on cg? And to me anyway, a pump is not needed in a small model.
The TN (twin needle) was a great carb on Webras. I never thought that throat diameter was excessive on these carbs (unlike some of the OS carbs). I wouldn't mess with throat changes on the TN
100+ mph was fairly normal for the classics.