My guess is interference is at least part of it.   It sounds similar to my symptoms and I keep finding areas of interference as the engine gains hours.   I just removed another two areas of contact this past weekend, making about 20 spots fixed over the last few years.   The engine does move quite a bit when you roll the RPM down and the cowl changes shape as airspeed increases.    If you had high-res shots inside I bet I could spot an area or two for some detailed examination and application of witness paint or torque-seal where there may be concern about contact.

Paul
Legacy RG
On 2013-11-25, at 8:08 AM, Dan Ballin <dballin@gmail.com> wrote:

I am trying to figure out the source of a vibration in a fixed gear
Legacy with an IO-550.  The plane had a recent condition inspection
and raced at Reno- vibration present before Reno but maybe worse now.
It is RPM related with the worst being about 24-2500 RPM better at 26
to 2700 RPM.  Somewhat speed related - worse with higher speed, but
mostly it is RPM that determines the severity.  The prop has been
balanced dynamically and I actually hooked it up to another instrument
just to see if the equipment was OK.

We have looked for anything rubbing or loose including engine mounts
and isolators, baffling.  All EGT and CHT's are fine and close
together - haven't done an in flight mag check, but will soon.
Compressions are fine on all cylinders.  All the wheel pants and
struts look good and nothing was seen vibrating on close inspection in
the air.

So one question is can the vibration be engine related if the prop
balance is good or does that pretty much rule it out.

Any thoughts or suggestions would be appreciated.

Dan Ballin

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