Neil A. Kruiswyk wrote:

Jeff,

    Thanks for the input.  All is welcome as we try and sort this one out.

    The problem isn't resonance (i.e., a vibration being induced to a fixed
object at a sympathetic frequency) but rather a beat frequency issue.  (2
vibrations of slightly different frequencies adding together or subtracting
depending on the cycle.)

    Resonance goes away when you move away from the sympathetic frequency.
This strange beat frequency vibration doesn't go away.  It goes up in freq.
with engine RPM and is noticeable from 3000 right to MAX which is about 5900
rpm.

    To Ed A.  Thanks for your input Ed.  There is no mistaking the sound and
there is no way you'd miss it if you had it in your setup.  It is VERY
pronounced.

    If weather permits, I'll try and get an audio sample over the weekend.

Neil

I'm guessing but I think the theory of the prop pulse and the exhaust
pulse has some merit. Think it through with the PSRU ratio of 2.17.

If it was exactly 2:1 those pulses would remain in sync. As it is now
they
go in and out of sync at some rate. I could be wrong as this rate should
vary with RPM I would guess. The individual pulses with the three blade
are
weaker. What does the muffler look like?

Paul Lamar
 
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