The muscle pipe (MP), designed and distributed by Curtis Youngblood, has proven to be a great performing and highly customizable exhaust system for my OS61sx WC. This is a summary of my experiences.
- OS61sx WC, stock head shim (1)
- OS8 glow plug
- CP 15% fuel
- Fuel/Air mixtures (*using muffler/pipe pressure*):
Low ~ 2 turns open
Med. ~ 1/5 - 1/4 open
High. ~ 1 - 1 1/4 open
Heli Settings (Vigor CS)
- hover at just under 5 deg. for normal mode
- +/- 10 deg. collective for idle up
The parameter that I am considering in this brief review is the overall length of the pipe, from the engine exaust flange, through the header, and straight back to the end of the bulb on the MP. I have varied the total length over the range recommended by different sources (11" - 11.75") and here is a summary of what I've found...
- Shorter lengths make it lowder, and more peaky (more power, tighter
power band). Longer lengths make it behave more like a muffler: quite,
and less peaky with a wider power band.
- Shorter lengths develop power at higher RPM - but note that if you make it too short, you may not be able to get onto the pipe at all.
- When 'on the pipe' shorter lengths will generate more power.
- If you 'hang on the pipe' at the start of autos, try richening the idle mixture AND shorten the pipe length. The richer idle mixture will help slow/cool the engine down to idle, and the shorter pipe will push the power band higher and make it more narrow. The latter is important since, if the power band is too broad, then the engine will hang on the pipe to much lower RPMS.
- I found that with the correct idle mixture, the engine RPMS remain constant for about 2-3 seconds after pinching the fuel line at the carb inlet, and then engine accellerates. If it takes too long for the engine RPM to change, then you're too rich. The converse is true as as well.
- 11.75" total length: very quiet, smooth operation. Unloaded head speed of about 1800 RPM, but the full power climb out drops to about 1540 RPM (too low). Engine hangs on the pipe entering autos, so idle mixture needs to be richened.
- 11.0 to about 11.25" total length: lowder, more raspy sound, but also smooth once on the pipe. Unloaded head speed of about 1850, and full power climb outs of about 1700 RPM. No hanging on the pipe.
What's good for you... you decide. I'm going for more power, and no hanging on the pipe. There is no doubt that the optimal settings will differ for different fuels. I suspect that since a majority of performance fliers use 30% nitro, the longer lengths are tailored for them. As for me, I'm getting the same performance on my MP using 15% fuel as I did using the (more expensive) KSJ muffler and 30% fuel. The muffler was certainly smoother running, but at a 50% cost saving per gallon of fuel, I can wear my earplugs!