Found this via Googling "OS61 tuned pipe".
ORIGINAL: dave_anderson
I may end up using my OS .61 FX for a ballistic ship. Anyone have a good tuned pipe setup they can suggest for that engine? Header length would be a help also.
Thanks,
Dave MACs has everything you need.... (don't get the pre-tuned system)
Header will depend on the aircraft (header reach length). Just buy the one sized for the OS61 with the reach you need for your aircraft installation.
http://www.macspro.com/headers_side.asp
Pipe ---- I prefer the regular MACS 1260 muffled pipe.
Performance is good, shorter, lighter, and it costs a bit less expensive than the quiet pipe 1060.
http://www.macspro.com/tunedpipes.asp
(And actually, the 8.5cc works well too)
If you have an concerns about sound/noise or some obnoxious 94db club sound limit, you will need the quiet pipe. Your choice.
Length: I would start off with 13.75" (measured from glow plug, along the center line of the header, to the weld/fat part of the pipe). This should be a bit long. With an APC 11x7 or 11x8, you will eventually get down closer to 13.5 - 13.2". Cut the header as needed to achieve the length. (of note, you want no more than 1/8" gap between the header and pipe where it joins at the coupler). And, its obvious .... it is easier to cut the header than to un-cut it. Start longer if in doubt.
Your target here is to get to 14,000 rpm on the 11x7 APC (to start with).
Important - do not even dream of testing/tuning the pipe install on the plane. Be sure to do any tuning on a test stand.