Gutted hatch OEM muffler re-design ideas

[bdearnley]

I managed to get an OEM hatch muffler for free and decided to have some fun gutting it and making my own ‘sports’ muffler for want of a better term.

I have a 2012 wrx hatch that currently has mods including a 3” catted and resonated exhaust from the turbo back to the axle. The muffler is stock, firstly because I read it doesn’t restrict flow that much, and secondly because I did have an aftermarket muffler but it was so loud and boomy inside the cabin that I couldn’t stand it and had to get rid of it. This was supposed to be one of the quietest on the market according to the forums.

I should say that I don’t like loud exhausts, and I have welded an extra resonator (Vibrant) into the downpipe just after the cat to keep the volume raspiness of my Killer B headers to a minimum. There was an annoying loud ‘waaa’ sound at somewhere between 2K and 3K rpm, which is where you spend a lot of time on the street. It works like a charm and my exhaust now sounds very modest. The goal for this muffler project is mainly to have fun, but also to increase flow slightly with a minimal sound increase over stock.

So I am asking for help with the design ideas. I would love to see inside all the aftermarket hatch mufflers out there and see how they are designed, but it is very hard to get any info or cutaway pics.

I know some mufflers have the entry halfway along the body and presumably split internally with a Y-pipe, probably drilled with holes all the way along and packed with fibreglass wool. But the ones that enter from the factory location, how are they designed? With a central chamber with one entry and two exits, into wool-stuffed chambers? One exit pipe is on the same side as the entry, so the gasses have to make a u-turn to exit out this side.

As a side note, I have seen cutaways of the factory muffler before, but what I was shocked to see when I cut open my own was the capped end of the entry pipe (second pic below). I assumed this was open, but it is closed off completely, with all the pressure having to escape from the holes drilled into the side of this pipe, at right angles. I can’t help wondering how much extra flow and sound would be achieved by cutting off this end cap alone, then welding the muffler closed again.

My proposed idea is below, in a rough diagram. Basically gasses entering into a central chamber which has exits going through drilled pipe out to the tailpipes through wool-packed chambers. I’m hoping this will give me a bit of noise cancelling from the bounce in the central chamber without restricting flow too much. I know some sort of straight through design would be best for flow but it might be as loud as the aftermarket muffler I bought before.

Any suggestions on other ways to build this with basic tig and mig welding skills would be appreciated.

Ben

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[Hyper]

while you are at it, why not modify it 3" all the way?

centuries ago, my friend did a similar thing on my OEM STI muffler, modifying it to be a true 3" with ultimate stealth look
it lasted for 5 years and worked great, but then started to rust like hell from abuse and ignition cut launch control

[bdearnley]

Originally Posted by Hyper

while you are at it, why not modify it 3" all the way?

Yeah I thought about that, but the tips have 2.5" entries and keeping it 2.5" will probably keep the volume down. It's just easier.

[bdearnley]

So just an update on this.

I think I have managed to make a muffler that is quieter than the stock one

I ended up using perforated 2.5" tube for every section, and also ended up cutting off the oem outer 'skin' of the muffler and welding a new one on. Much easier than trying to weld the thin oem skin, which was actually two layers of paper-thin steel.

This muffler is deeper in tone but doesn't drone or throb too much, probably because of the chamber I built into the central section. I packed it quite tightly with 3 or 4 kgs of fibreglass wool, after wrapping and securing each perforated tube with stainless steel wool and metal cable ties.

I assume it flows about the same as the stock muffler because I haven't seen any extra knock activity or spool difference, and I did try to increase timing marginally but that resulted in some FBKC so I went back to my previous tune. In hindsight I probably could have made it 3" all the way in instead of welding in the reducer, and it probably wouldn't have added much more volume.

So the result is a quieter car, which I'm happy with, and a fun project completed.

The only downside was the awful sweet smell that came out of the exhaust for a good month or two after it was installed. It was almost like the cat converter had failed, but my cat was quite new. Must have been all the glass packing material I put in there which was 'going off' or giving off fumes whenever it got hot or something. It has largely stopped now.

Below is a little comparison video for anyone who is interested. These were recorded with exactly the same settings, manual volume control etc and you can tell the modified version is so much quieter, deeper and less 'raspy'.

The internals, ready to be packed:



New skin: 0.9mm stainless sheet tacked on ready for welding:



Video comparison:

https://youtu.be/8U0C-zLxf6M

https://youtu.be/8U0C-zLxf6M

[danscomp]

That is awesome! Glad to see people do a neat DIY and bring actual feedback.

Where'd you get the perforated tubes? I'm trying to find aluminum ones with minimal success....

[bdearnley]

Thanks

I got the tubes on eBay. Same with the fibreglass wool etc.

[Turn in Concepts]

I REALLY like how that sounds! Good tone, nice and mellow. Totally something I would daily, and the wife would not complain.

[bdearnley]

Originally Posted by Turn in Concepts

I REALLY like how that sounds! Good tone, nice and mellow. Totally something I would daily, and the wife would not complain.

Yeah, it's so quiet it's crazy. Quieter than stock maybe.

I should flow test them to compare, but don't know how to do a proper DIY flow test.

[Charlie-III]

First off, great idea, decent execution.
Pat on the back to you for doing it as well as documenting it.

Me, maybe I would have continued the 3" into the muffler with solid pipe.
Then a 45* angle plate to force flow to the 2 outlets so flow is a bit more balanced.

Would it make a difference in sound or flow? Not likely, I am just being picky.

Great job.
If you're happy, that is ALL that is important.

Thanks again for sharing.

[bdearnley]

Originally Posted by Charlie-III

First off, great idea, decent execution.
Pat on the back to you for doing it as well as documenting it.

Me, maybe I would have continued the 3" into the muffler with solid pipe.
Then a 45* angle plate to force flow to the 2 outlets so flow is a bit more balanced.

Would it make a difference in sound or flow? Not likely, I am just being picky.

Great job.
If you're happy, that is ALL that is important.

Thanks again for sharing.

Great tips, I wish I had this before I did it. Of course I could do it again, I have my stock muffler that is still factory spec.

[Smurftastic]

That sounds super good, reminds me of a friend with an e92 M3 that did the same thing.. sounded epic. If I ever do get a hatch, I shall do this

[Speed Assault]

i know this is old, but i have read quite a bit on engine back pressure, tube size. so this really comes down to how happy is the engine, how much torque was lost or gained, any Mpg lost or gained?

Like to try it, but really what are the real results ?

[bdearnley]

Unfortunately I didn't measure torque or HP after the swap, nor MPG. Basically went by seat of the pants feel and if there was any more knock logged. Felt just as quick and no more knock was recorded, so it was a success in my opinion. And a fun project.

[biohazard69]

It sounds incredible