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12iS Exhaust Upgrade And Dynamic Balance

Bob Y

Well Known Member
Installed the SB 00013 exhaust upgrade during my condition inspection. And just as I suspected, it did change my dynamic prop balance. I’m usually in the other camp, but this time I lucked out and it IMPROVED from 0.12 to 0.03 ips at WOT. And I ran three averages to be sure it wasn’t an anomaly. Anyway, if you hadn’t thought about it, you might want to verify after installing. Not sure if it’s the different pipes or the removal and reinstalling of the muffler that made the impact. Don’t know if anyone else experienced the same thing, but just wanted to share my experience.
 
And just as I suspected, it did change my dynamic prop balance. I’m usually in the other camp, but this time I lucked out and it IMPROVED from 0.12 to 0.03 ips at WOT. ... Not sure if it’s the different pipes or the removal and reinstalling of the muffler that made the impact.

Very interesting, thanks for sharing. Is dynamic prop balancing always done at WOT? Is there an "official" procedure somewhere I could check out?

Here's my take on what was observed...
If no rotating components were touched, then the rotating imbalance of the system did not change.
I suspect the original exhaust system had a natural resonance (likely the first-bending mode) within the operating range of the engine, which is what led to the fatigue cracking.
Based on your observation, that mode was likely near WOT, or at least it was when you made your 'before' measurement.
The exhaust tubes are like tuning forks hanging off the engine. If the engine is run at the right 'tuned' frequency, the tube will vibrate like crazy and that will be picked up in any vibration measurement.
The new tubes have likely been beefed up to move the resonant mode above the operating range of the engine - no more crazy vibration leading to fatigue cracks.

And yup, it's possible to move resonant modes around a bit just by removing and reinstalling the same components.
 
Curious what type of balancing machine your are using? I'm not really sure how an exhaust system can create a vibration at the exact same freq as the blade rate/first order vibration unless the balance machine is not filtering the input.
 
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Curious what type of balancing machine your are using? I'm not really sure how an exhaust system can create a vibration at the exact same freq as the blade rate/first order vibration unless the balance machine is not filtering the input.

Sorry in advance if this is too much nerd detail... :eek:

Let's say the exhaust system has a normal mode, aka resonant frequency, at 97 Hz. (Actually, it has many many such modes at all sorts of different frequencies)
Any excitation source in the system with 97 Hz content will set the exhaust ringing like a tuning fork - resonance.
The source could be anything from a rotating imbalance, combustion forces, valvetrain forces, flicking the the pipe with your fingernail, or who knows what else, the exhaust system doesn't care.
When excited at a resonant frequency, the response of a vibrating system is amplified.
The exhaust is not creating it's own excitation source, it is simply amplifying the excitation it receives.
So if the engine is run at 5800 rpm, the imbalance will be generating a 1st order excitation at 97Hz, which will excite the 97Hz exhaust system resonance and increase the 1st-order vibration measurement.
In order to electronically filter out the effect of any resonance, you would have to increase or decrease the engine speed such that the entire width of the band-pass filter was sufficiently separated from the resonant frequency. That distance depends on how much damping there is in the mode of interest and the shape of the filter used.
Although, if there is a stable resonance at the balancing speed, it would just boost your signal to noise ratio. Have to be careful though because modes move around with temperature and such.
 
I see several questions and comments to my initial posts will try to address in one response…

I’m using a DynaVibe. I tested at 2k rpm (remained at 0.05 ips), 3k (improved from 0.35 to 0.29 ips), 4K (improved from 0.25 to 0.11 ips), 5k (improved from 0.15 to 0.03 ips) and WOT of 5100-5200 rpm (improved from 0.12 to 0.03 ips). As to test rpm question, see page 13 of the DynaVibe user manual indicating test at cruise rpm or peak static rpm.

http://www.speedcomfly.com/sito-ecommerce/file_info/0504532 dynavibe_classic_user_manual_v1-09.pdf

I’m an electrical engineer, not mechanical, but to me, anything one does to the components which hang from the engine could potentially change its resonant frequency. Face it, it’s hanging from rubber vibration dampers. At least, right or wrong, that was the belief I had which prompted me to revalidate the balance.

This was my experience. Would be nice to hear from anyone that might have made the same upgrade and revalidated their balance to see if I’m alone in this observation.
 
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