Catless DP w/ stock exhaust?
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This is a discussion on Catless DP w/ stock exhaust? within the STi Technical Forum forums, part of the Tech & Modifying & General Repairs category; Thinking of getting a catless DP and hooking that up to my stock exhaust. What size is the stock STi ...

  1. #1
    ST!
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    Catless DP w/ stock exhaust?

    Thinking of getting a catless DP and hooking that up to my stock exhaust.
    What size is the stock STi exhaust?

    For those of you who have done this, how does it sound? Much louder than stock?

    If I have just a DP can I run cobb stage 2 without getting a cat-back?
    Last edited by ST!; 02-09-2006 at 05:52 PM.

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    Moderator YBNormal07's Avatar
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    Size is 2.5"

    Cobb Stage 2 can be run with just a catless DP.

    Might want to do a search on boost creep though. With a stock exhaust the chances of having boost creep are lower, but there is still a chance you'll get it once you take out the cat. Maybe consider a HF cat?

    If you have the stock exhaust on, it will sound deeper and meaner, but by no means will it be loud.
    tEh PriUs=teH faSTah!@@!
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    SOLD: 04 MPS/SW WRX Sti. 12.971 @ 103.97 (stock)
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    Registered User AlxSTI's Avatar
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    doesn't the cobb ap usually take care of boost creep issues?

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    Moderator YBNormal07's Avatar
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    Quote Originally Posted by AlxSTI
    doesn't the cobb ap usually take care of boost creep issues?
    No EMS can take care of boost creep. It is a physical restriction of the wastegate size.

    Exhaust flow powers the turbine side of the turbo, spooling up the impeller and compressing your intake air to your manifold.

    To control boost, you regulate the flow of this exhaust by bypassing exhaust around the turbo once it achieves set pressure. The wastegate is the exhaust bypass around the turbo. As you close the wastegate, this directs more flow through the turbo, increasing boost. As boost rises to set pressure, the wastegate opens to allow exhaust to bypass around the turbo, slowing the turbo down and lowering/maintaining peak boost pressure. In reality, the wastegate is merely a bypass valve though, not a true pressure contol system.

    On the stock system, you have backpressure in your exhaust line from the cat, resonator, muffler, and piping sizes. This works to keep the exhaust flow throught the turbo to a rate that can be managed in conjunction with the wastegate.

    Once you remove these items and free up exhaust flow (reduce backpressure), your turbo now has more exhaust flowing through it at peak throttle than it did before. The wastegate is sized with the stock backpressure in mind. It now can't flow enough exhaust at peak pressure to slow the turbo down, and thus you get boost creep. This ultimately leads to an overboost condition, usually exhibited by a CEL and fuel cut (your car suddenly lurches and dies or cuts out and back in at full throttle.....a horrible experience believe me!!!!)

    The only way around this is to enlarge the wastegate hole (porting the wastegate), increase the exhaust backpressure (high flow cat or keep the stock catback with a catless DP), or use an external wastegate. Of the 3, the external WG is probably the best as you can buy one that controls boost in both directions, unlike the internal wastegate the stock turbos come with. Plus the sound it makes is awesome!

    Personally, I've always erred on the side of caution and used a DP with a HF cat. No boost creep to date with this setup (on my STi and Legacy). Prior to the HF cat though, I did experience boost creep at a track day once. It is not something I enjoyed as I thought my STi had blown it's engine. Luckily for me, it was just overboost.

    I have not personally verfied using a catless DP with the stock catback exhaust to ensure creep does not occur, but it makes sense so I suspect it will work the same as a catless DP. I prefer the HF cat route though, as it is still emissions friendly and precludes having to swap the stock DP back in to pass emissions.
    Last edited by YBNormal07; 02-14-2006 at 05:00 AM.
    tEh PriUs=teH faSTah!@@!
    SOLD: 05 SWP Legacy GT Limited 5MT, 13.3ish@105 (stage 2) crappy stock clutch
    SOLD: 04 MPS/SW WRX Sti. 12.971 @ 103.97 (stock)
    278 AWHP/283 AWT, Catback exhaust
    306 AWHP/341 AWT, Stage 2
    (Scott) Moderator-STi, Drag Racing, Car Purchasing, and Tutorial Forums

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    Registered User AlxSTI's Avatar
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    good info.. makes sense to me now. i wish i knew how to port it myself or actually trusted one of my mechanic friends to do it for me. it would suck to pay 645 for the AP and ~1000 for a TBE just to get boost creep issues.

    to mod or not to mod.. that is the question.

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    TurboXS Stealthback ftw.
    sounds like a stock car besides the nice sound of the turbo spool. I love my stealthback so much and I would reccomend it to any1.

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    Moderator YBNormal07's Avatar
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    Quote Originally Posted by AlxSTI
    good info.. makes sense to me now. i wish i knew how to port it myself or actually trusted one of my mechanic friends to do it for me. it would suck to pay 645 for the AP and ~1000 for a TBE just to get boost creep issues.

    to mod or not to mod.. that is the question.
    Just get one with a HF cat. The power loss is very negligable (5-10 hp at the most). Balanced against the gain (50-60 hp to the wheels) it's a pretty decent sollution.
    tEh PriUs=teH faSTah!@@!
    SOLD: 05 SWP Legacy GT Limited 5MT, 13.3ish@105 (stage 2) crappy stock clutch
    SOLD: 04 MPS/SW WRX Sti. 12.971 @ 103.97 (stock)
    278 AWHP/283 AWT, Catback exhaust
    306 AWHP/341 AWT, Stage 2
    (Scott) Moderator-STi, Drag Racing, Car Purchasing, and Tutorial Forums

  9. #8
    ST!
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    Quote Originally Posted by antinnios
    TurboXS Stealthback ftw.
    sounds like a stock car besides the nice sound of the turbo spool. I love my stealthback so much and I would reccomend it to any1.
    Do you have boost creep problems with your stealthback? Also, did you install the entire turbo-back (catted or catless?) or just the muffler?

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