Can you sample your accelerometer data with your 20G sensor and then look
at it with
a sound editor so you can scroll through the data set easily (Audigy is
free OSS).
You could then also do a FFT on the signal at a constant motor RPM and see
if the
cylinder firing rate is the dominant noise source or something else.
You really need to look at your signal to see why it isn't filtering as you
expect.

3M makes a noise damping tape that you might be able to fit on one side of
your mounting plate. It is 0.15 mm thick but works amazingly well to damp
HF ringing
of metal.  A 2 x 6" steel plate going from 'ping' with long ring tone, to
'thunk' with just
a 1" square of tape applied.

I think you actually want the LEAST stiff mounting plate so that you damp
out
the high frequency components coupled in from the mounting holes.

I think you will find that if you want good frequency response to tilt,
you'll need
to go with a composite (accel + gyro) solution.

R
--=20
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