[Keith Seymore]

Originally Posted by pggto

Engine frequency? I have had 3 different engines in this car over the years and the vibration is the same.

That's the joke: all cars get really quiet when you shut the engine off. A muffler engineer taught me that one.



But, in all seriousness, engine firing frequency is a 4th order disturbance of engine rpm (there are four firing pulses per revolution for a V8 engine). If it gets close to another distubance, like 2nd order driveline, it can result in the "boom/beat" phenomenon I described.

Originally Posted by pggto

I had to Google that! Sirometer is a vibration tach generally used on small engines. Not sure exactly how that would help? What would the frequency tell me?

All of these components are rotating at a specific speed during the disturbance (in rev/min, which can be converted to cycles/sec = frequency, in Hz).

The hope is that if you can isolate the frequency of the disturbance you can back calculate which components are spinning at that same rate (either the base frequency or some harmonic of it), thereby pinpointing your suspect component. It virtually eliminates all this guesswork.

It simulates on a smaller scale how we quickly diagnose and repair vibration problems at the GM Proving Ground, except we use power spectral density plots and/or order tracking on one of our Noise & Vibration dynomometers, analyzing several strategically placed accelerometers at the same time.

Disclaimer: You do, however, have to be careful where you place the vibration tach. If you set it on the steering column, for example, and the column is going off on it's own resonant frequency rather than the forced frequency of the actual disturbance, it can cause some confusion.

K