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Exhaust Resonance

ak diesel driver

ak diesel driver

6.5 driver
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Location
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So I notice that when it gets colder than 5* above zero I have alot of resonance. The colder it gets the louder it gets. It got loud enough I thought the pipe was hitting hard on the frame. It is worst at about 45 mph and about 1500 rpm. I put it up on a drive on lift so it would have the suspension compressed and checked out my clearance on the pipes. Lots of clearance everywhere. Started it and tried to get it to do it on the hoist but no go. Only does it when it cold outside. Thoughts?
 
Probably a different air mass going through due to turbo change and density (temp/humidity) that just crosses a harmonic frequency more irritating. Loudness and frequency irritation are different.
Stiffness of ruber mounts might very well have some compounding issue. Sorta like you can’t ring soft metal. The softer things are the slower vibration. Changing air mass will change sound “quality”.

Is this the truck with electric fan clutch? I did some sound testing once and it’s almost a black art. dB levels could be the same number and the sound irritation could be way different due to frequency. Air noise is irritating and the radiator fan is one of the noisiest components of an engine. Changing radiator fans did make a difference in sound quality.
 
That's just your teeth chattering from the cold! :D

I would put another glasspack style resonator in the system as close to the engine as you can get. Some aftermarket exhausts had a setup like this. AKA 2 mufflers instead of just one.
 
Les, when I put my ATT on, the 90* off the turbo didn't rotate far enough to the pass. side. The down pipe just touched the frame and would resonate at certain rpm's. I had to take the 90 off and file the 3 slots to give more rotation.
 
I put it up on a drive on hoist and I have plenty of clearance. You'd think if it was hitting the frame it would get worse under load but it totally goes away when I give it some throttle.
 
Resinator.
Adding the ATT has allowed a lot more flow through the pipes at a slower speed because of the larger diameter. This changes the frequency to a lower “note” and drones more. Tiny choked exhaust doesn’t have it and why resinators never used to exist. Now everyone has learned large bore works better but produces lower tones and buys resinators to lessen the issue. When you step on the throttle, air flow increases and leghtens the sine wave eliminating the problem.
If you download a tuning app on your phone or have a microphone and an oscilloscope, you can know the frequent being created and get a resinator tuned to that frequency to eliminate it. That works perfect but is pricey. Most people buy the $180 versions that are “close enough” and knock out most of the sound.

Many modern cars have “sound cancellation” where a microphone hears the sound, an tone generator matched that frequency, and it is played back out of phase at the source so the sound waves cancel each other out. Elaborate & $$, but amazing how well it works.

If you can put a belt strap or something around a couple of the offending areas -usually after bends it creates a dissonance or in the middle of a long straight run is a twanged deep note- that will show you where you can weld on a piece of linear metal (like cooling fins of a heat sink sticking out the side) and that will lessen the vibration of the pipe to around half. That wont help people standing at the end of the exhaust pipe, but you won’t hear it in the cab newrly as much.
 
While I had it on the lift we started it and put it in gear to try and recreate it. Nada no movement and no noise. I've never thought resonance could be so loud and metallic.
 
Did you put the parking brake on, or 4x4 high and stand on the brakes, and load the drivetrain while on the lift? Brake stand while on the pavement recreate it? Of course vary the throttle during the brake stand to see if the noise shows up. Careful with max 2 min of this so you don't overheat the auto trans.

Metallic noise to me is something hitting, loose converter bolts, broken flexplate, locked up u-joints, bad front hub bearings, center bearing or rubber around it bad... 45MPH is where the TCC locks up and increases engine load at any throttle. The increased load can move the engine on the mounts.

I would look close at the fan and fan shroud for rubbing as well as the trans cooler lines etc. being too close to the fan. I had an engine mount get coated in biodiesel from a injector body leak and the mount has sagged. It rubbed the fan only under harder throttle. Made a lot of noise when I was on it just right and took a significant amount off the shroud on it's own. Fan shroud clearance from the small OEM fan diameter takes into account complete engine mount failure. We are running bigger fans that have trouble with less than total failure and some require shroud trimming from the start.

Back to the exhaust hitting - these engines move a lot even with good mounts. Frame is not the only place the exhaust can hit as it can hit the cab with the downpipe. Think beyond the exhaust: look at the hood for the CDR pipes hitting it although that hitting and going clean through the hood insulation never created noise for me.
 
while on the lift we put it in gear and brought the rpm up to 1500 . I stood underneath and watched/listened and nothing. no movement no sound. I t does it almost continuously when it's cold and the speed is around 45 and the rpm around 2500 and just loafing along. Increase or decrease in speed or throttle makes it go away. I have to drive at that speed for about 15 minutes one way every day so it can get rather annoying
 
You seem real good at finding deals on tools... look for a chassis ears kit. Expensive as all get out, but you can find issue quick with them.

Mess with tcc lockup at that speed to change rpm to see If it is just an exhaust note.
 
Could be your bed rattling on the frame. Check torque and see if any of the little disc cushions are missing. May even check spot welds on cab and bed bracing as well.

For the exhaust, try a slash cut tip that fits tight on the tailpipe. The straight cut can cause unwanted resonance sometimes..
 
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