Engine Vibration - What is causing it?

[mikedsilva]

During my quest to rebuild a 3.0 motor, I have started trying various mufflers.
Here is the situation.. 3.0 single plug motor, using 10.2 CP pistons, webcam 464/465 cams, EFI from rasant, and throttle bodies from RHD Engineering. Headers are 1 5/8 from GT- Racing.

First muffler was the M&K GT3. The engine runs smooth everywhere with this muffler. It's loud as hell, but smooth. I soon found that I could realise a lot more power with open pipes so I tried a different muffler set up. This muffler is mounted in the traditional way, using large stainless straps to secure it to the back of the engine mount.

Second muffler was a Dynomax 17552. This one has an internal x-pipe. I had a muffler shop install it and mate it up to the headers. Sounded good, had a bit more flow, but the first drive exhibited a vibration between 2000 and 3000. A strange vibration that didnt seem to be dependent or gear.. just rpm. When I say vibration, it's like sitting on top of a washing machine that is severely unbalanced and trying to buck you off. The whole car vibrated and the windows and door trims rattled etc. Also, the first drive home from the muffler shop, resulted in a bunch of fibraglass packing hanging out one of the tips.

Quite a bit huh? And that amount of packing should not have been able to get inside a tube unless it was compromised.. I decided to open up the muffler and found that the cross over was not very 'complete' in the welding. So I had a local shop weld it up more completely and repack, close and re mount.


This muffler was also mounted in the traditional way, using large straps on the back of the engine cradle.

This actually seemed to help.. the vibration I am experiencing was limited to between approx 2300 and 2800.. but it was still there. Above 3k it was fine.. but any time you had to drive through that rpm range or slow down with the rpm falling through that range, it was quite shaky.

Next muffler is a Dynomax 17231.
This one is an oval with 2 straight through perforated tubes. Many people in the usa have used it, so I thought it should be straight forward.
I used a different muffler shop and they mounted it a little different. They made a little steel plate on the back of the muffler and formed a bracket so it could be bolted to the back of the engine cradle using the lower 2 studs. This negated the use of the stainless straps and was very neat. Unfortunately, the idiot that installed it, plumbed both headers into the same tube, so the exhausts were head butting each other.. and the gasses had to forced their way through the perforated tube, through the packing, into the other tube, and then out the tips. Needless to say, he had to fix it. You can see the plate and how it is secured to the back of the motor.
Here is a pic of how NOT to plumb a 17231

After it was fixed, I noticed it was a very raspy sound and still had a fair amount of vibration. At this point, I questioned if something else mechanical was amiss.. I swapped back to the M&K muffler.. dead smooth. So for whatever reason, these dynomax mufflers are vibrating. The Dynomax 17231 went back on, and the car went back to the tuner. Throttles were all balanced again and they modded the VE to suit. Also played with AFRs and timing trying to see if adding or removing made any difference and it made absolutely no difference. Still vibrating. I thought perhaps because of the initial blunder of how it was plumbed, that maybe the packing had been disturbed, so again I cut open this muffler to find this.
Pretty obvious what has happened here..


So I repacked it with fresh glass, and had it welded shut.
After starting the car, the vibration was WORSE.. really harsh.. and to be honest, undrivable. Wtf is going on?

At my wits end, I contact a different shop who recommend a Magnaflow muffler with internal x pipe. I get them to go ahead, and they also mount it with a metal bracket and bolt it onto the motor mount.

I don't have any pics of the magnaflow set up, but it is the same as for the Dynomax 17231 - yet another exhaust shop fitted it for me though.
After starting the car, it was a revelation - perfectly smooth..
But why?

Ony problem is, I am certain that the magnaflow setup does not flow quite as well as the dynomax 17231 (plus, I love the raspy sound of the dynomax_). So last night I bolted on the dynomax again. Took for a drive and as soon as I started it, I could feel the vibration.. but this time, not quite as intense.. the only thing I did differently, was I mounted a couple of thin fibre washers between the muffler mount, and the engine plate.. they would only be half a mm thick if that! Could this have helped?

I tried to record the vibration when it happens, but I only had my phone, sitting in a cradle suction cupped to the windscreen. When the vibration begins, the volume tends to be almost muted. I don't understand why.

At this stage, I know my engine is sound and that the magnaflow works well. But I want to get my dynomax setup working well too, because it should.
Can there be anything physical in the mounting causing this issue?
Do you think it is the tune?

Open to ideas and suggestions..
Mike
https://youtu.be/r3SUutjnzGw

[otto_kretschmer]

welcome to the world of acoustics

[sc]

I believe your problem are the inlets and outlets of the muffler.
The 17231 has both inlets on one side....so both layouts you show are different than the manufacturers drawings.
The vibration may be the internal exhaust forces working against each other in both the combustion chamber and muffler chamber.
Just a thought!

[otto_kretschmer]

Mike,

here is a video about flutter. It may help you understand whats going on inside your car's muffler or maybe you will find it entertaining like I did.



What you hear in that vibration at a certain RPM is a resonance at the natural frequency of the muffler.

here is another video of the same phenomenon



the shuttle's engines throttle thru the flutter and it stabilizes at a higher engine power

you can change the natural frequency, either make it higher or lower, by changing the internal geometry of the muffler. Making things stiffer (thicker walls or gussets) generally drives the natural frequency up.

There are many, many variables of your problem; header dia and length of the runners, locations and rigidity of the mounting points..

the only way your going to resolve this is experimentation and you're doing that already so keep going

[mikedsilva]

this is not encouraging!
The muffler has been used by William Knight in many applications.. I guess it is possible, that I have a very 'unique' situation happening with my car, my mounting, my headers, my tune, and my itbs, and my cams.. etc..

I am fairly familiar with flutter.. I used to fly radio control airplanes with my favorites being hotliners and pylon racers. If they went too fast, the ailerons would flutter and shake the plane to death.

[mikedsilva]

Maybe I need to try 2 separate chambers or smaller cylindrical mufflers. But this sure is getting expensive!

[mikedsilva]

Originally Posted by sc

I believe your problem are the inlets and outlets of the muffler.
The 17231 has both inlets on one side....so both layouts you show are different than the manufacturers drawings.
The vibration may be the internal exhaust forces working against each other in both the combustion chamber and muffler chamber.
Just a thought!

Hi, I appreciate the input. But the tubes are just perforated and there are no baffles. again, William Knight uses it and others have tried and used it too. I think I have to keep experimenting.

[jpnovak]

Remember each muffler has a resonance as already described. Acoustics and harmonics will dominate.

The frequency of that resonance is always based on a Helmholtz Resonator equation. In simplest terms, this frequency is a function of the chamber volume and the open area going into that volume. So you can look at the size of the can and the sum of the size of all the perforations in the internal piping. Looking at the Dynomax they have large punched holes in the cross-over tubes. The Magnaflow appears to have smaller, evenly spaced, cut/punched perforations in the cross-over tubes.

My guess is that the internal structure of the cross-over tubes is what puts the Dynomax muffler resonance directly in the rpm band. I also think the sonic interference between each bank is more pronounced with the larger, punched holes in the Dynomax. This explains why your packing is blown out as well. Just more constructive interference in the mid-2k rpm range. It is rpm dependent and not load dependent. Its just the fundamental frequency of the pumping pistons and closing valves.

It can also be the the length of your exhaust pipes is wrong. Meaning that the quarter wave node (or fundamental resonance) for an open end pipe happens to hit right at the center of the muffler.

Try to shorten the exhaust tips or inlet pipe coming off your header and this will most definitely shift the frequency. IMO a longer primary header would be beneficial here. You can also try a smaller diameter pipe to shift the frequency. You look to have a large step in diameter from your header collector termination. This step creates a sonic step/reflection point that can change your scavenging for the worse. You might try a smoother transition to remove the sonic reflection. Or a smaller pipe and shift the reflection point.

Have you run some basic calculations to determine header length, diameter, flow rates, etc for your engine build? There are many online calculators. You might be surprised what you get.

BTW, your cam spec should have a fairly wide lobe center (112 deg?) unless you had them ground special. The short headers typically favor a narrow lobe center. (98-102 deg).

[otto_kretschmer]

Mike,

I reread your first post and when I got to the part of the bad installation my first thought was "bogan"

Your car is unique, as you are finding out.

The glass packing is just a dampening agent. So the bigger internal volume and more glass you have, should result in a quieter muffler.

You may try a muffler with a bigger volume and more baffles. By the time you solve this you may end up learning how to weld and make your own.

[jpnovak]

BTW,

I want to add a few things. Take some measurements of the total exhaust path length for each of your different systems. Use a string through the pipe and then measure its length. Then note where the muffler is located along that length.

You said M&K was smooth. I can tell you that it has the shortest path and shortest distance to the muffler just based on the design where the pipes come into the font of the muffler.

What are the relative lengths of the Dynomax and Magnaflow? This will start to help you understand the acoustic tuning of each system.

[mikedsilva]

Originally Posted by otto_kretschmer

Mike,

I reread your first post and when I got to the part of the bad installation my first thought was "bogan"

Your car is unique, as you are finding out.

The glass packing is just a dampening agent. So the bigger internal volume and more glass you have, should result in a quieter muffler.

You may try a muffler with a bigger volume and more baffles. By the time you solve this you may end up learning how to weld and make your own.

YUP! Plenty of bogans here!

[mikedsilva]

Originally Posted by jpnovak

BTW,

I want to add a few things. Take some measurements of the total exhaust path length for each of your different systems. Use a string through the pipe and then measure its length. Then note where the muffler is located along that length.

You said M&K was smooth. I can tell you that it has the shortest path and shortest distance to the muffler just based on the design where the pipes come into the font of the muffler.

What are the relative lengths of the Dynomax and Magnaflow? This will start to help you understand the acoustic tuning of each system.

hmmm
Does the total length from the exhaust port in the head, to the exhaust tip of the muffler affect resonance?
Or is it the length of the pipe before the muffler?
Or the length of the pipe after the muffler?
or combinations of everything?

Here is some more information which only now, after reading your post, makes me think it is relevant.
The second muffler, Dynomax 17552 had some vibration but was a longer muffler. When I switched to the Dynomax 17231, we cut the muffler out and reused most of the original pipework. The diameter remaining the same at 2.25inch.

The Magnaflow had all new piping made, but still in the same size of 2.25inch.


So both dynomaxes vibrated, and utilised basically the same pipework, but the magnaflow didnt vibrate and has different pipework.

When I get a chance, I will measure as you suggest, and report back.

[mikedsilva]

https://www.youtube.com/watch?v=GqFBGPokAFI

Here is a video of the dynomax muffler compared to the magnaflow.. The magnaflow has a much shorter length of tubing.

https://youtu.be/hzqU4jK1i8E
And another showing difference in pipe length between the magnaflow and dynomax

[gomezoneill]

Put the M&K on and be done with it. What's the definition of insanity? You all know the rest.

[jpnovak]

@gomezoneill Its not about insanity. Its about the sensitivity of our engines with respect to resonance and power output from both intake and exhaust. Here is the rest of the story why M&K is not ideal.

Which Muffler? Dynomax, Magnaflow, M&K - Dyno Charts

[pampadori]

I run the Dynomax 17231. I don't have a bad vibration problem like mike does but I do have a different issue around the same RPM range - Drone.

Once I get over 3k rpm the drone goes away. I mounted my setup with two stainless straps.
Instead of 2.25" pipes, I used 2.5" pipes.
I also use regular OE style engine and trans mounts. Not super firm race mounts.

Mike, Have you tried the 17231 without having attached the welded-on bracket bolted to the engine? Also, is your muffler touching the car anywhere or is it clear of everything?

[mikedsilva]

Originally Posted by pampadori

I run the Dynomax 17231. I don't have a bad vibration problem like mike does but I do have a different issue around the same RPM range - Drone.

Once I get over 3k rpm the drone goes away. I mounted my setup with two stainless straps.
Instead of 2.25" pipes, I used 2.5" pipes.
I also use regular OE style engine and trans mounts. Not super firm race mounts.

Mike, Have you tried the 17231 without having attached the welded-on bracket bolted to the engine? Also, is your muffler touching the car anywhere or is it clear of everything?

Hi Shaun
I have not tried it without the bracket.. not yet. I am tempted to just remove the 2 nuts, which also means I would only have 2 nuts holding the engine to the mount! But, I am sure it would only take 30 seconds to see if it makes a difference.. It "shouldn't" make a difference, since the magnaflow is mounted the exact same way.

I have checked and the muffler is not touching any part of the car.

Interesting you mention drone; last night I drove it, and when above 2600 (my upper limit for the resonance) I have a really quiet system. It really doesn't drone at all. And it is actually the nicest and smoothest sounding muffler of all that I have tried so far.