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Old 05-28-2022, 09:46 AM   #31
mikep
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Join Date: May 2003
Location: 38° 27' N 75° 29' W
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Quote:
Originally Posted by TheLoganZilla View Post
Would you be able to elaborate on this a little bit so I can understand better? When you say testing the frequency, how would I go about that? Are there other symptoms I could see that would indicate misfire? And when you guys say 2nd order, are you talking about the order of a modeling ODE or something else?
Even order harmonics (half, double, etc) would indicate the vibration is directly related to the engine rotating, but sympathetic (something shaking, tapping, droning in a separate part) or geared up or down at double or quadruple, or half (etc) speed. Even frequencies mathematically.
I worked in a few shops way back thad had a reed tachometer, kind of expensive but will show the frequency. If the engine was at 3000 and the shake was 50hz, that is 1:1, or first order. It?s probably a flywheel or balancer on the front of the crank in a car sittind still, or also possibly the driveshaft if in a 1:1 gear while moving. (3000/60sec=50) If it was half that, or 25hz, in a car moving in a 1:2 gear ratio, it is the driveshaft. Half speed could also be a misfire in a 4cyl, but it feels different and gets harsher under load.
You can also get even order vibrations in a broken motor mount or even a bolt or nut jammed in between 2 parts that should be isolated.
Random (unrelated mathematically) vibrations can be an odd gear ratio. Real slow mathematically related vibrations are usually wheel/axle, due to the final drive reduction.
Odd order vibrations that change order with rpm usually fall into the unattached random parts waving around due to rom or road shake or whatever.
Higher freq is usually a lightweight parts (cat shroud), lower freq is usually heavy (long exhaust pipe) when it comes to rando parts.

When picoscopes started getting popular i took some classes and we messed around with transducers. You can do up to 4 channels (i think?), and plug in a speed sensor, crank, pressure, shake, whatever. It?s pretty cool, but i never pulled the trigger on buying one.
https://www.picoauto.com/products/no...g/nvh-overview
Is the link to a vid about it.
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