[pggto]

Originally Posted by 2002Z4CSS

Since the vibration got worse after the new engine was installed, how good is the engine and trans mounts? Sounds like one may be grounding out or the exhaust is grounding out.

The mounts are new, I put in the polyurethane motor mounts from Bulter and the trans mount was replaced a few years ago and is still in good shape. This vibration is definitely rotational due to the harmonics that I feel. I was an auto mechanic for 10 years and have fixed my share of vibrations, but this one has me stumped.

Maybe the reason its worse now with the new engine is because of the stiffer motor mounts, but that still doesn't explain the cause.

[Keith Seymore]

Originally Posted by pggto

Maybe the reason its worse now with the new engine is because of the stiffer motor mounts, but that still doesn't explain the cause.

Engine firing frequency!



When you say "rotational due to the harmonics you feel" does that mean it comes and goes?

If you are feeling it increase and decrease: that's a "boom/beat", which indicates you have two root causes. The increase and decrease is a result of the two out of phase amplitudes adding and subtracting. You would need to isolate and eliminate one of the root causes.

Does that sound like a good description of what you are experiencing?

K

[pggto]

Originally Posted by Keith Seymore

Engine firing frequency!



When you say "rotational due to the harmonics you feel" does that mean it comes and goes?

If you are feeling it increase and decrease: that's a "boom/beat", which indicates you have two root causes. The increase and decrease is a result of the two out of phase amplitudes adding and subtracting. You would need to isolate and eliminate one of the root causes.

Does that sound like a good description of what you are experiencing?

K

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

It starts out as a pulsating vibration and as speed and rpm increase so does the vibration, I guess at higher speeds and rpm the frequency of the pulsations increase making it feel like one steady vibration, this car at 90-100 vibrates a lot, just feel it through out the car. Because I have changed out all the drive train components over the years is why I was questioning the drive shaft angle or some thing related, It's a new drive shaft but who says its balanced correctly, I'll rotate 180 and see if any thing changes. this vibration has been with this car since I restored it back in the 80's, I was really hoping it would go away with the new drive train, and it has not. Maybe it's time to tear in to the rear end as well?? I bought it rebuilt from a guy that does rear ends, 12 bolt 3:55 gears. Thanks Keith

[2002Z4CSS]

Originally Posted by pggto

The mounts are new, I put in the polyurethane motor mounts from Bulter and the trans mount was replaced a few years ago and is still in good shape. This vibration is definitely rotational due to the harmonics that I feel. I was an auto mechanic for 10 years and have fixed my share of vibrations, but this one has me stumped.

Maybe the reason its worse now with the new engine is because of the stiffer motor mounts, but that still doesn't explain the cause.

Get a syrometer (sp) and find out what HZ you get at what RPM or vehicle speed. From there you should be able to calculate what it is.

[pggto]

Originally Posted by 2002Z4CSS

Get a syrometer (sp) and find out what HZ you get at what RPM or vehicle speed. From there you should be able to calculate what it is.

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?

[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