There's a lot of engineering calculations that can explain air flow and sound waves,, but to keep it simple just view the tech forums here explaining the VT mufflers.  Also a cross over pipe has been designed in by ford for a reason.  They also have a tech manuel that explains why. Lot s of details.  

I also changed out tubo mufflers and went to a stock mufflers,,  The only thing that worked was these VT mufflers.  MSteve is putting them on his 427 plug in-hybrid diesel fuelie  ,, wait and see if they work,,   He's messing around with a brakeline now,, after that maybe he will start his car.   


see here,,  

http://www.dynomax.com/press_releases/dynomax%C2%AE-introduces-revolutionary-drone-free-muffler.html

Last edited by Don (8/17/2013 12:26 PM)

How is the exhaust system supported?  If it has those weld-on straps with the little rubber washers, they are probably transmitting direct metal to metal vibration to the car body.  If the exhaust system is able to move more freely, with lots of rubber allowing some movement, then that may help the problem.

Pay close attention to Don's answers on acoustics and vibration.  That is what he does for a living.  Same with Gary Patrick.  They know a whole lot more about this stuff than I ever care to learn!  But there is alot  more to it than one might think.

If you put some better mufflers on it, it will make a huge difference.  Not saying these are better, but I had almost the same car as you, and I ran flowmaster 50's on it.  You did not hear any engine noise or exhaust noise or drone going down the highway, but it turned on when you stepped on it.

I just bought a set od VT mufflers after seeing how they performed on Don's 66.  Whisper quiet on the road and the valve opens up as needed at full gallop.  I am hoping these make it quieter yet.

Magnaflows have that really cool hotrod sound, if you are standing 50 feet behind the car like the TV commercials.  Probelm is... You can't drive the car from back there.  And remember why Foose likes them so much, too.  They fund his show.  I'm sure he would like JBAs (or insert any brand here) if they paid him more money than Magnaflow.

There are a number of suspicous areas and one in particular is the "vibration" and wobble as this is low frequency vibration that coulld be an issue regardless of mufflers  Next is motor mounts.  Just like replacing tires every five years, the elastomers in motor mounts break down with time.  They may eventually break but in most cases, the elastomers harden which means more transmission of motor vibration into the frame and body turning every flat panel into a speaker.  Anything hard plumbed or wired to the engine like battery cables or fuel lines also create a sound/vibration path.  Any direct contact between the exhaust system and the body also creates a path.  Check header clearance everywhere and then as Steve mentioned, pay close attention to your hanger system all the way to the tailpipes.  Your symptom of changing mufflers with no improvement in interior noise points to a "vibration short circuit" somewhere.  Also, pay attention to transmission clearance.  If you are running an aftermarket transmission it may lack clearance as well.  The corrections you have made with mats are very effective in reducing air-borne vibration but not necessarily good for structure-borne vibration.  I'd start with your mounts and then work your way back and eliminate any direct connection between driveline and exhaust and body.  You could even experiment with removing all hangers and supporting the exhaust with jack stands.  Last, before my 24 hour hotel internet expires, I am concerned with your engine vibration.  Regardless of the noise, you may have a damaged crank or a compoent on the crank that is hurting other parts.  Check this out sooner than later as this is something that you have to eliminate at the source rather than trying to insulate it from the rest of the car.  The other hard part with this is that with age it seems to be harder to point to a source - a rattle in back can sound like it is coming from the dash.  Have someone else listen to what you are hearing to see if your impressions match up.

Food for thought - unbalance forces increase at a rate of the square in the change in speed.  The unbalance forces at 1000 RPM are 9 times higher at 3000 RPM.  So, an unbalanced pulley or crank creates significantly higher forces as engine speed increases.

Last edited by GPatrick (8/17/2013 3:30 PM)

Headers are not the issue, most likely.  Also, on the hangers, I forgot to mention...  Having some that are more stressed than others can cause problems.

As for the Magnaflows, they are pretty loud, but like I said, I had no problem with the flowmanster 50's.  They were very quiet at cruise and otherwise well-mannered.

Gary Patrick-

Much appreciated. I am not an engineer but I do a lot of technical reading and I write technical articles for the Studebaker Club.  I am a big fan of physics, if that does not sound too nerdy! So, I can appreciate everything you are saying.

One thing that struck me as very odd:  When I bought my remanufactured 351W engine from Jasper, I knew that my old engine had a crank (and therefore firing order) from a 289 Mustang.... it was that way when I bought it.  In buying the new engine, I decided I needed to buy a new harmonic balancer that was intended for a 351 engine, so I ordered one from Summit Racing. Well- when I bolted it on, the engine vibrated like mad-- even though I believed I had followed the directions and configured/ installed it the right way..and I switched back to the old balancer, deciding that metaphysics was at work here. 

I still have that other, new harmonic balancer sitting in its box. I never figured this one out. Maybe I erred putting it on, or, who knows.  Now, perhaps this is related, perhaps it is not. I just wanted to mention it. 

You can't balance a quarter on my air cleaner, but it is surely not "way off".

 

See one more question, contained in the below (in bold)

Steve-  oops. Duh-- I knew that about the cam controlling the firing order....in fact, as I read your answer I recall that what you said was what I had determined the problem to be in the first place.

So, to all who were kind enough to answer, I am going to do the follwoing based on your advice:

1) I am going to put the car on the lift and look at every sqaure inch for contact between engine, headers, transmission and exhaust with the floors, frame or shock towers.
2) I am going to check to see if the wobble in my crank pulley comes from the pulley, or if there is runout apparant in the harmonic balancer itself.
3) I will check the binding of my exhaust hangers and see if I can "loosen things up" a bit. There needs to be a compromise, I guess between the way it used to swing in the breeze and the rock solid way I have it be bolted up now. 
4) I am going to change my old motor mounts. What do you fellows think of these (see attached image) , available from Mustangs Plus, that claim to be adjustable in all directions to help fine-adjustment of engine and trans placement to eliminate any touching or interference? They are bushed with urethane, which is something I have tried to avoid as my impression was that urethane is harsher than rubber in communicating vibrations and noise. 

Many many thanks to our great community of people willing to assist others. I am glad to be among you. 
 

Ok  Now that we have gone into this and some have expressed a interest in "Physics"


here ya go,, a cheat sheet on everything you might want to know about sound

http://www.sengpielaudio.com/Calculations03.htm

Have fun and don't forget that I warned you.. 
 

This message is to any and all who are following this thread.  I have an update on my progress, something that as a follower of this board I always like to see to help complete the story.

I followed the advice offered by Don and checked the car from stem to stern looking for a vibration "short circuit" as he called it, one that would transmit sound waves through contact, and I did find one.  The early - production transmission mount for my Tremec 5 speed transmission had a poorly fabricated add-on bracket for the emergency brake lever. The position of the bracket allowed the brake equalizer lever assembly to press squarely both against the back of the transmission tailshaft, and the car's frame. This is surely transmitting unwanted vibration and I never noticed it before.

I have spent the past evening re-engineering this assembly, eliminating the direct contact between transmission and frame rail stub.I don't know yet if this alone will have an effect on my problem, but I will advise.

I am still planning on installing the H-pipe, and then the adjustable motor mounts, in that order.

Thanks to all for the  help and encouragement.
 

Another update:  Added welded H-Pipe today. Though I have not driven the car much since it was installed, I cannot report any noticeable difference in sound inside the car over not having it, both by "ear" and in using the db meter. It does however seem to improve acceleration power at mid-band, (it seems like it pulls harder) and the exhaust sounds smoother and more "nicely tuned" when I run it up to red line. I am glad I did it, but I can't say (so far) that I notice any change in the interior noise level/quality at speed. 

Next I am replaing my old BFG Radial TAs.... the old ones are 14 years old and look brand new, but the rubber is as hard as stone. We will see what change that brings. 

(for anyone reading this post and thinking to himself, "My, this guy works fast when something is bothering him", please do not alert my wife, who complains that when SHE asks for something to be done it takes me forever to get around to it)

If you bridged the rubber in the motor mount with that bolt, then yes, that could be transmitting sound to the frame. The motor mount through bolt is a racer's trick, not really recommended for street use. If your motor is "rearing up" when revved, you probably need new motor mounts. Rock Auto has stock pattern replacements cheap. Make sure you get the right style, there are several.