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Tailpipe Resonator

I like my car to be quiet.  I installed a Hooker Aerochamber Cat-back exhaust primarily because it was known to be the quietest after market good flowing exhaust system for the LS1 in an F-Body.  It was a bit louder than stock at idle but not objectionable at all.  At WOT it was quite a bit louder than stock and probably well within most people's tolerance level.  I wanted quieter.  A lot of people have added a resonator in the mid-pipe jammed in next to the drive shaft.  To install it this way you have to clearance the floor (ie. bash it out of the way).    

Resonator in Mid-Pipe - Photo Credit Unknown

After looking for a better solution I noticed that factory exhausts that use resonators typically place them after the muffler and very near the end of the tail pipe.  So I searched around for a resonator that might fit into the straight portion of the twin tail pipes.  The problem was nothing was short enough.  I eventually bought a cheap Cherry Bomb muffler and cut it apart.  My original intent was to splice in a portion of the muffler into each tail pipe.  Unfortunately, I discovered that the cheap muffler also choked down the cross-section of the tail pipe from 2 1/2" down to 1 3/4", despite being sold with end fittings for 2 1/2" pipes.  You can see this in the next photo.  Two 1 3/4" tail pipes have less total area than the 3" mid-pipe, so I decided against using it.  Clearly, part of the way the cheap muffler would have worked was by restricting the flow, not good for flow but one of the better ways of quieting a motor.  So, what to do?

 

Comparison of cheap muffler core versus tail pipe.

 

As you can see I decided to make the actual tail pipe the new core of the resonator.   I used a metal chop saw to slice the pipe as shown.  I had to spend a fair bit of time cleaning out the slag created by the saw at each cut.  You can also see that this resonator will have more open cross section than the cheap muffler did.  The function of the resonator depends on having sound pulses in the exhaust radiate outward into the fiberglass matting in the outer chamber of the muffler.  If the acoustic path is blocked off then no sound reduction will occur.  Typically this is measured and described as the insertion loss of the muffler (the reduction the muffler provides versus the straight pipe it replaces).  

Absorption Muffler Function

An unmuffled engine produces a wide spectrum of frequencies.  At large throttle openings at high revs most engines produce a very harsh white-like noise often referred to as "rasp".  This style of muffler typically does nearly nothing to attenuate the lower frequency content of the engine exhaust.  These resonators are 12" in length so they will deal with the only the "rasp" aspects of the exhaust noise.  If you are selecting a resonator, maximizing the shell diameter relative to the core makes it perform best.  The shell in this case is only 3.5" versus the 2.5" tail pipe, not optimal but its what I have available.

 

Components of the resonator - Section of muffler shell, sliced tail pipe, custom ring

For each resonator, I wind up using roughly half of the original shell (red), the piece of insulation that originally came in the shell.  The capping ring I made from scratch out of 1/8" mild steel plate.

 

 

Assembled with original fiberglass stuffed back into shell.

 

The insulation fit in just fine which means the original cheap muffler had barely anything in it.  The insulation was probably able to move around.

 

All welded up - Passenger side pipe ready for prep and painting

 

Driver side tail pipe hanger modified

The original driver side hanger as built by Hooker was 3/4" too long and the driver side of the muffler was too low.  The hanger arm was also in the way of where I wanted to put the resonator case.  In order to fit the resonator, the hanger was sliced off and flipped around to make room for the shell.  In the photo you can see the grinding marks where the hanger was originally.  Since the hanger arm simply inserts into the rubber hanger on the car, you can flip it to point to the rear of the car as seen above which gave me at least 2" more length along the pipe for the resonator.  I also took the opportunity to shorten the hanger height which pulls the driver side of the muffler up, to make it level.   

 

Driver side resonator components

 

 

Insulation partially stuffed in place

 

 

Ready for welding

 

 

Both tail pipe assemblies were painted with POR15

 

 

Tailpipes mounted back in place

 

Initial Results

This is hard to describe.  When I fired the car up for the first time with the new exhaust tailpipe resonators, it was deceptive trying to figure out what had changed.  At idle the sound did not seem quieter, but did seem somehow smoother.  Overall it seems less hollow than before.  The initial bark when it first fires seems more distant now, still there but damped out.  Listening at the tail pipes the car also seemed quieter than before.  Driving the car, the "smoothness" effect is still there, the car seems quieter but the engine still lets you know its there.  The remaining exhaust note is deeper.  Its what you would expect since the higher frequencies are being muffled leaving the deeper bass frequencies behind.  Overall the sound is deeper, smoother sounding.  This is pretty much what I was looking for and I am happy so far with the results.  Its too cold to drop the top so I will have to wait to see what that sounds like.   Top up convertibles are pretty boomy to begin with so I am happy with top up results, the engine just seems less intrusive.  Only more driving time will tell me what its really like.