DIY gutted STi axleback

Discussion in 'Tech, Tips, Newb Info' started by idget, Feb 28, 2009.

  1. idget
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    idget Want to pokéman? PM ShortytheFirefighter Staff Member

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    So Ryan was looking to get a little more tone out of his daily driven WRX wagon while avoiding the drone of an aftermarket muffler. While there are some solid aftermarket options, they are all $$$. Being broke and bored, we decided to cut open a stock STi muffler I had laying around to see if there is anything we could do to alter the tone.

    Here is our process including our findings and conclusions.

    Tools needed:

    1) cutting tools to cut open the muffler and it’s contents (drill helps)
    2) pry bar (hammer helps)
    3) vice grips/pliers
    4) metal sheers (90 degree angled ones help)
    5) protective gear (gloves, eye protection, respirator for fiberglass)
    6) A way to button it back up

    [​IMG]

    Step one: cut an access hole

    Here is the stock axleback:

    [​IMG]

    Cut along the lines, and drill out the rivets outlined in red here. You can follow the general outline I’ve posted, but I’d suggest making longer cuts for better access. I also left an inch or so of room in case of error… which turned out to be a very good idea lol:

    [​IMG]

    The inside of the STi muffler looks like this:

    [​IMG]

    Exhaust flows from the inlet side (chamber 1) to the outlet side (chamber 3), back to the center section (chamber 2) through a bunch of small baffles in the chamber wall, then into chamber 1 via 3 trumpets and a number of small baffles in the chamber and finally out the 2.5” exhaust outlet that goes from chamber 1 to the exhaust tip. Here’s a quick diagram:

    [​IMG]

    [Step 2: cut away the center pipe]

    We decided to cut the tips off of the 3 pipes leading to the inlet-side chamber (what I called trumpets from earlier) so that we could access more of the center pipe.

    The idea is to allow some of the exhaust gasses move straight from the inlet to the outlet. Given the high and low pressure areas that still exist in the gutted version, we thought some of the gasses would still bounce around from chamber to chamber, and ultimately (hopefully) produce a slightly lower tone with slightly higher noise output and slightly better flow.

    The center pipe is actually 2 pipes with fiberglass packed in between. The inner pipe is baffled. This can be seen here:

    [​IMG]

    We decided to cut the entire center pipe out, rather than trying to cut the outer center pipe away from the inner one. It’s strongly advised to wear a respirator, as inhaling fiberglass is baaaaad juju.

    [​IMG]

    Step 3: Clean up and weld-up the muffler

    Take a grinding wheel and smooth out the jagged edges of the trumpets and center pipe. We also took some JBWeld around the leading edges of the center pipe that we cut away, as we didn’t want the inner and outer pipes vibrating against on another.

    Re-weld the axleback, or find a way to stitch it back together. In our case, it was more like find someone to make a mess out of the muffler, then have Boomer cut out the original welds and weld in a new plate. Thanks Dan! Very nice work, sir!

    Re-welding proved to be the most difficult part of the process. Partially due to the thin steel, and partially due to the debacle described above. I'm exploring other options (not involving welding) for the future. If you have any suggestions, lmk!

    [​IMG]

    Results

    The car sounds very slightly louder at all rpms with a distinctive lower tone. I would put the tone and volume somewhere in between a stock sti catback and an spt catback (leaning slightly closer to the sti in terms of output, and slightly closer to the spt in terms of tone).

    So we were able to accomplish at least 2/3 of our goals. Slightly louder output and slightly lower tone. I think it's safe to assume that the flow has freed up as well, but we don't have any data to back this up.

    The data that we do have is with a sound meter and in cabin as well as out of car impressions from 3 different people.

    Sound metering:

    These numbers were collected on the same car with no other changes in between testing except for the axleback. The test car is Ryan’s blob-eye WRX wagon running a stromung shorty (divorced wastegate) downpipe, stock last cat, and STi resonator pipe.

    The test equipment is a hobbyist digital sound meter (+- 2db at 114db. In my use, I've found it very precise and have not found anything to suggest it's not accurate as well). All data was collected from a distance of 6ft with the meter set to a-weighting and slow response.

    Sound metering on Ryan's wagon

    STi axleback
    76db at cold start
    67db at warm idle
    74db at 3500rpm

    JDM STi Genome
    70db at warm idle
    78db at 3500rpm

    Gutted STi muffler:
    72db at warm idle
    77db at 3500rpm

    Sound metering on other cars

    Cobb catback on an 07 STI (Maddad header/uppipe, stromung shorty, MD test pipe, Cobb catback with 2.5" flat flange on the DP side)
    92db at cold start
    81db at warm idle
    89db at 3500rpm


    Eli’s exhaust
    72db at warm idle
    94db at 3500rpm


    Stay tuned for driving impressions from Ryan and Eli.
     
  2. Paul Revere
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    Paul Revere BANNED

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    Fun fact

    Perception of Loudness (20dB = 4x)

    Interestingly, our perception of loudness is not the same as sound pressure level. Although the actual formula
    is somewhat complex, as a rough rule of thumb, an increase of 10db SPL is perceived to be approximately twice as loud.

    • Thus a 20 Db gain would seem to be about 4 times as loud.
    • And a 40 Db gain would seem to be about 16 times as loud.
     
  3. idget
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    idget Want to pokéman? PM ShortytheFirefighter Staff Member

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    Yes, a decibel is a logarithmic unit that describes a ratio... 10 log (P2/P1) dB where P2 is the power of the louder sounding version of the same sound as compared to P1 (taken from one of my physics books from way back).

    Ask Tony if my hella horns are 4x louder than stock...

    Stock horns (07wrx) at 30ft - 87db
    4x (2 pair) hella horns at 30ft - 106db
    Tony drunk at 2 ft hearing the 4x hella horns - :eek4::crying: :laugh:
     
  4. blackozone
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    blackozone Well-Known Member

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    Wow, very nice writeup!
     
  5. curly2k3
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    curly2k3 Well-Known Member

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    initiation ftw
     
  6. Shibbs
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    Shibbs The Daywalker

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    I personally like the gutted, resonator-less wagon. No drone in the cabin. Idle sounds very very unique, think a V8 chevy, missing a spark plug, idling thru headers and a flowmaster 40-series muffler. lol

    And the turbo-whistle is definitely more audible from the tailpipe as well.
     
  7. devildevil
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    devildevil Well-Known Member

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    wow, i need to get a few sets of hellas :laugh:
     
  8. modsuby
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    modsuby New Member

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    nice write-up i was thinking about doing this for a while. Do you have a sound clip?
     
  9. Bullwinkle
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    Bullwinkle Well-Known Member

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    Sheen/Ryan, let me know when you want to do some baseline and after install pulls. We could get some firm "power" numbers. Good writeup, aside from missing that :).
     
  10. Shibbs
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    Shibbs The Daywalker

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    We didn't get a sound clip of just the axleback, but if you ask ryan nicely he may put one up of his current setup - single high-flow cat very far back, and resonator-less midpipe.
     
  11. carl
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    carl Well-Known Member

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    post pics of it welded back up ahaha
     
  12. Shibbs
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    Shibbs The Daywalker

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    Whose welds? Boomer's or Dave's? :laugh::laugh::laugh::laugh::laugh:
     
  13. carl
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    carl Well-Known Member

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    both!!
     
  14. tonyM
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    tonyM Well-Known Member

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    yes........the horns are loud. lol
     
  15. TSTRBOY2004
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    TSTRBOY2004 Well-Known Member

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    reminds me of the Hella Horn install night at my old shop... who was that again.. Dan or Fong that I told to look closely at the install... yep..:biggrin:
     
  16. idget
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    idget Want to pokéman? PM ShortytheFirefighter Staff Member

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    We'll post up pics of Dan's handywork soon. I tried to upload pics of the original welds, but they were so hideous they broke my screen.

    We will also have more dB data soon since Ryan is now running a maddad high-flow cat and non-resonated midpipe.
     
  17. TSTRBOY2004
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    TSTRBOY2004 Well-Known Member

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    let me know next time you fooooools wanna play... I wanna play too...
     
  18. idget
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    idget Want to pokéman? PM ShortytheFirefighter Staff Member

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    Sounds good. LMK if you get Boss done this weekend. If not, we should try to meet up during the week or maybe next Saturday.

    Definitely! I have part2 lined up. I'll pm you when we start on that.
     
  19. Dynapar
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    Dynapar Well-Known Member

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    haha! I did this to my L muffler about 3 or 4 years ago, except I removed everything from it. The sound is better than a stock muffler but IMO not as good as an aftermarket.

    Just and FYI: If you decide to do this to your muffler BECAREFUL those baffles are hard to get out and are VERY sharp on the edges. I know I have done this before.
     
  20. TSTRBOY2004
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    TSTRBOY2004 Well-Known Member

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    if you guys need a lift and garage to play in... ;) or if I am free I will for sure take a drive..
     
  21. carl
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    carl Well-Known Member

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    nate needs to do some minor work to the SlowTI this sunday too :biggrin:
     
  22. Shibbs
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    Shibbs The Daywalker

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    BullTinkleToonz FTW.
     
  23. TSTRBOY2004
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    TSTRBOY2004 Well-Known Member

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    any hackings happening tonight??
     
  24. carl
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    carl Well-Known Member

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    if I had my car back I'd have people over working on that. We have a fire system to install, the intercom to wire, new mudflaps to cut and mount, a fuel tank guard to fabricate and some other misc. wiring to finish
     
  25. bobafett04wrx
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    bobafett04wrx New Member

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    I heard this exhuast today, I thought it sounded alot like a V8 or something. It wasn't too loud but has a nice deep rumble to it. I would like to hear it drive by sometime.
     
  26. Soupboy
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    Soupboy Well-Known Member

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    Glad my donor muffler was put to use. (emo)
     
  27. boomer
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    boomer Well-Known Member

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    I'm no expert welder but I think it turned out pretty good. I had to cut out 1/2 the shell of the muffler and replace it with new sheet metal.
    Not to shabby considering there are no leaks or rattles.
    I wouldn't mind doing another one as long as I don't have to fix someone's ugly welds again.:ugh:

    Dave is not allowed to touch a welder ever again:laugh:
     
  28. idget
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    idget Want to pokéman? PM ShortytheFirefighter Staff Member

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    I got 2 more I'm cutting up as soon as I find some time!



    Got some new dB readings on a car with catless bellmouth and gutted STi resonator:

    with stock STi muffler:
    69dB at warm idle
    82dB at 3500rpm

    with gutted STi muffler:
    71dB
    83dB at 3500rpm

    sound was noticeably deeper but not louder. Turbo spool sounded like a diesel truck :laugh: I'll let Thomas post up his impressions after he drives around with it for a while.
     

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