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This is a discussion on DP help within the Engine Modifications forums, part of the Tech & Modifying & General Repairs category; I appologize if this material has been posted else where, I've looked through the stickies and tried working through the ...

  1. #1
    Registered User Jonney Knuckles's Avatar
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    DP help

    I appologize if this material has been posted else where, I've looked through the stickies and tried working through the pages for DP info, but I get a little confused and lost. This is my first Subaru, turbo'd car, and never have I had a "tuner" car. (sorry for that coined term) I've had and built many pre 70's cars, but those all seem to be a cake walk compared to what I am now interested in with my Subie. So please any information, tips, suggestions, or any FAQ's you can steer me to I greatly appreciate it. I have a stock '13 WRX, first mod was to get a Cobb AP and go from there.

    Ok to my question
    With a DP, are there any brands that stand out beyond others? I wanted to stay with a catted DP just for possible emissions testing. BUt from what I read its easy to just swap in the factory one for testing. As I dont live anywhere that has testing at this time, I am planning on moving back to Cali in a few years. If I go with a non catted DP, what happens to the O2 sensor? are they plugged off? how are they corrected? Does the Cobb AP has a reset code in the sensors are not plugged in or reading?
    Next whats is the difference/bennefit to a divorced wategate and non? I saw something about Bellmouth, but Im not sure if that aplies to a stock turbo or not.. Any help on information would be greatly appreciated. Im just trying to get as much knowledge as I can.
    Thank you
    Jonney

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    Registered User UEDan's Avatar
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    Quote Originally Posted by Jonney Knuckles View Post
    I appologize if this material has been posted else where, I've looked through the stickies and tried working through the pages for DP info, but I get a little confused and lost. This is my first Subaru, turbo'd car, and never have I had a "tuner" car. (sorry for that coined term) I've had and built many pre 70's cars, but those all seem to be a cake walk compared to what I am now interested in with my Subie. So please any information, tips, suggestions, or any FAQ's you can steer me to I greatly appreciate it. I have a stock '13 WRX, first mod was to get a Cobb AP and go from there.
    Have you checked out this thread:
    Exhaust FAQ
    As for an AP... ehhh I'm a cheap bastard. I went opensource. Look up "Tactrix".
    Quote Originally Posted by Jonney Knuckles View Post
    Ok to my question
    With a DP, are there any brands that stand out beyond others? I wanted to stay with a catted DP just for possible emissions testing. BUt from what I read its easy to just swap in the factory one for testing. As I dont live anywhere that has testing at this time, I am planning on moving back to Cali in a few years.
    There has been debate on brands, I myself am on the "its just a friggen 3" pipe" Cheaper the better.
    The newer WRX's have a tendency to boost creep when ran uncatted, but there are remedies to fix that IIRC.
    If you are mechanically inclined, which you are, a DP swap should take no more than an hour. Just remember to flash the stock/stage 1 map back.
    I'm in Cali strict-ass emissions, yet half the modded subies smell like a mobile gas station.

    Quote Originally Posted by Jonney Knuckles View Post
    If I go with a non catted DP, what happens to the O2 sensor? are they plugged off? how are they corrected? Does the Cobb AP has a reset code in the sensors are not plugged in or reading?
    A new downpipe should have an o2 bung. The AP, or just any tune for a downpipe will tell the ECU to expect different numbers from the O2 sensor.
    Quote Originally Posted by Jonney Knuckles View Post
    Next whats is the difference/bennefit to a divorced wategate and non? I saw something about Bellmouth, but Im not sure if that aplies to a stock turbo or not.. Any help on information would be greatly appreciated. Im just trying to get as much knowledge as I can.
    Thank you
    Jonney
    From the thread mentioned above:
    Downpipes

    Most aftermarket downpipes replace the section of exhaust between the catback and the turbo. Only '02-'05 MYs need to worry about the few "shorty" downpipes on the market (e.g., Bosal, Oakos) which only replace the section closest to the turbo (leaving the rear cat pipe in place).

    Modification of emission control devices (e.g., catalytic converters) is a violation of the Federal Clean Air Act (203(a)(3)(A)), and therefore is illegal in every U.S. state. Some downpipes include high-flow cat(s), while others are completely catless; a downpipe which utilizes HFC(s) is just as illegal as fully catless. When determining whether or not to run a catted/catless downpipe, it is up to the end-user to research their local/state emissions laws. On vehicles with stock or mildly upgraded turbos, a HFC will not impact the performance that much (minimal enough to be equated to "dyno error"). Some owners (mostly 2.5L) have had boost creep issues with catless downpipes, and will switch to a catted exhaust to provide some backpressure; some cars have boost creep, some cars don't.

    There are three styles of downpipes you can purchase:

    Bellmouth
    Divorced
    Flat-plate


    As for which is the "best" design, you want a downpipe to be as free-flowing as possible. The flat-plate design blocks the wastegate, and is generally considered an "inferior" design on an internal wastegate turbo (this is negated when an external wastegate comes into play). The difference between a flat-plate and bellmouth/divorced downpipe has a direct correlation with the flow of the turbo. When comparing bellmouth/divorced, in theory, separating the exhaust/wastegate gases with a divorced-style downpipe is ideal, but the difference is minimal enough to be equated to "dyno error". Pretty much any brand will perform the same (any difference you see by switching downpipes is minimal enough to be equated to "dyno error").

    The main thing to consider is whether or not the downpipe tapers or not. Most downpipes on the market are a full 3", but a few taper to 2.5" to mate with an OEM-style catback.

    You usually need to modify the OE heatshield to fit aftermarket downpipes. If the heatshield is missing, you can usually pick them up cheap from the classified section. If you also need the bracket to attach it, you want Subaru P/N 44021AA012.
    Good luck and have fun man!

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    Registered User Jonney Knuckles's Avatar
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    Thank you, I've been all over so many forums looking for information that I must have missed that one.. Thank you that did help a lot, but Im still on the fence as to go cat/non cat DP.. I've seen good reviews on the Invidia catted DP, so thats what I'll probably go with or until I can find some more research on the two.. As for the AP yea Im cheap too but I do like some of the features with the AP.. But still doing research on that as well.. I am too new to dealing with an ECU to try and tune myself.. I dont want to turn my car into an expensive paper weight... Again thank you for your response I greatly appreciate you steering me in the right direction...

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    Master Baiter EJ257's Avatar
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    Quote Originally Posted by Jonney Knuckles
    Thank you, I've been all over so many forums looking for information that I must have missed that one.. Thank you that did help a lot, but Im still on the fence as to go cat/non cat DP.. I've seen good reviews on the Invidia catted DP, so thats what I'll probably go with or until I can find some more research on the two.
    You talk about the possibility of buying an AccessPORT. Their notes require a catted unit to be used with their maps.

    Quote Originally Posted by Jonney Knuckles
    As for the AP yea Im cheap too but I do like some of the features with the AP.. But still doing research on that as well.. I am too new to dealing with an ECU to try and tune myself.. I dont want to turn my car into an expensive paper weight...
    Cobb AP vs OpenECU (Tactrix/Romraider/ECUFlash)
    2005 WRX STi (Mods | Virtual Dyno)

    Resident Tuner @ WTF Tuning

    "Never trust anything that bleeds for a week and lives ..."

    UNYSOC

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    Registered User Jonney Knuckles's Avatar
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    Quote Originally Posted by EJ257 View Post
    You talk about the possibility of buying an AccessPORT. Their notes require a catted unit to be used with their maps.
    Cobb AP vs OpenECU (Tactrix/Romraider/ECUFlash)
    Thank you EJ.. I had read that the AP required a catted DP, but also read some propaganda somewhere saying that when the '13 maps come out that might not be required?? Hence the reason I am still trying to find out as much info as I can before any purchases are made.. As for the AP vs Open source.. I am still intimidated by it, I am not that computer savvy. I don't plan on taking this car past a stage 2, I will leave the more indepth builds for a used WRX I find later down the road.. Anymore helpful tid bits or FAQ's you know of send them my way, I have a thirst for knowledge.. Thank you again..

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    Quote Originally Posted by Jonney Knuckles View Post
    Thank you EJ.. I had read that the AP required a catted DP, but also read some propaganda somewhere saying that when the '13 maps come out that might not be required?? Hence the reason I am still trying to find out as much info as I can before any purchases are made.. As for the AP vs Open source.. I am still intimidated by it, I am not that computer savvy. I don't plan on taking this car past a stage 2, I will leave the more indepth builds for a used WRX I find later down the road.. Anymore helpful tid bits or FAQ's you know of send them my way, I have a thirst for knowledge.. Thank you again..
    Something else to consider - not all DPs will match up to stock exhaust without adapters. As far as I know, only Cobb and Injen DPs match up to the stock exhaust by default. You will also need to purchase a new DP to Turbo gasket for every timee you are changing DP(back to stock for example will need another gasket).

    To answer bellmouth vs. divorced, it is just the form of the DP and for most intents and puposes most DPs bellmouth or divorced are referring to stock turbo. The difference is in the opening of the downpipe where it attaches to the turbo. Divorced separates the gases from the wastegate and bellmouth lets them all through in one big hole. There doesnt seem to be a general consensus on which is better. Seems to be more a matter of opinion.

  8. #7
    Registered User pauller's Avatar
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    I am catless and run regular ap maps on my 2013. I also recommend looking in the classifieds for a used AP. I got mine for only $400.
    pauller
    2013 Stage 2 SWP WRX Sedan
    DGR Coils-Invidia Q300 Ti Tip-Tomei 3in Catless Downpipe- Cobb AP w/ Stage 2+ 93 Map-Cobb SF Intake

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