Post Number: 170
|Posted on Saturday, 20 April, 2019 - 21:24: |
Is there any way to check that the crank shaft damper is working correctly without stripping the whole thing down? Are there any tell tail sign of a seized damper?
Just wondering as I have no history of the engine and therefore, I do not know if it has ever been overhauled.
Post Number: 73
|Posted on Tuesday, 23 April, 2019 - 08:29: |
Dampers needing help make noise and or cause vibrations.
Broken parts make knocking sounds. A damper that is slipping excessively or is frozen solid will sing. This sound is the crankshaft vibrating in torsional mode at half its harmonic frequency. (The actual harmonic frequency is very near 120 miles per hour, and would cause severe damage, but the car is designed to make that speed occur outside the possible driving range.) With 4.25 liter engines, the sound occurs about 55-60 miles per hour in fourth speed gear, and is NOT a changing pitch. You hear it at full throttle, and as the car accelerates into the harmonic speed, the sound will fade into awareness and become louder as you near 60 mph, and then become quiet as you pass 60 mph. Note, it is CONSTANT PITCH and does not increase pitch as you increase speed, only become louder, and finally become quiet as the engine rpm exceeds the harmonic frequency of the crankshaft. The volume is also loudest at full throttle, an silent at very light throttle. This is the defining characteristic of a vibrating crankshaft in torsional mode at the harmonic frequency. The pitch is about b flat, just below middle c on your piano.
You can also feel the vibration but not everywhere. Rest the side of your foot lightly against the edge of the clutch pedal, and you will feel a high frequency vibration, same occurrence parameters as the sound.
The 4.5 engine has a stiffer crankshaft than the 4.25 due to narrower main bearing journals. This will cause the sound to be higher pitch at higher engine rpm, which moves the danger rpm further away from usual driving range. This is the reason the crankshaft was redesigned
Post Number: 171
|Posted on Wednesday, 24 April, 2019 - 10:45: |
Thank you for the very comprehensive explaination. As my car has none of those symptoms ,I have to assume it is working as per design
Post Number: 76
|Posted on Thursday, 25 April, 2019 - 12:51: |
I agree, probably OK. Likely it is not perfect, as most of them are in pretty dreadful condition when disassembled during engine overhaul, but without symptoms, it does not need to be inspected.
When you have an old car, you always need to pay close attention to anything unusual .