Exhaust FAQ
+ Reply to Thread
Page 1 of 9 123456789 LastLast
Results 1 to 15 of 126

This is a discussion on Exhaust FAQ within the Engine Modifications forums, part of the Tech & Modifying & General Repairs category; One of the most common questions in the "Engine Modifications" section is about what parts to purchase. Below is a ...

  1. #1
    Master Baiter EJ257's Avatar
    Join Date
    Aug 2007
    Location
    On a stream bank somewhere
    Posts
    19,852
    I Support ClubWRX

    Exhaust FAQ

    One of the most common questions in the "Engine Modifications" section is about what parts to purchase.

    Below is a diagram that gives a graphical representation of the intake/exhaust components:


    There are four main types of aftermarket exhaust components you can purchase:
    • Catbacks
    • Downpipes
    • Headers
    • Uppipes


    Catback Exhaust

    Like the name suggests, this will replace the exhaust after the catalytic converters. This section of the exhaust is the largest determining factor of the exhaust note/tone.

    Things to consider when getting ready to purchase a catback:
    • Style (straight / angled)
    • Volume/tone
    • Taper (some exhausts taper to 2.5" to mate with an OEM downpipe)

    Choosing an exhaust is something that needs to be done in-person. While a YouTube video can give you an idea of the various exhausts, there is no quality control to really be able to differentiate (e.g., camera quality, distance between exhaust/microphone, etc.). Where I find YouTube to be useful in deciding on a catback is to narrow down your list of choices. Once you've got a few exhausts to choose from, it's time to hit up Subaru meets; here will give you the best chance to hear how different exhausts sound. At one of our local meets, you'll usually have at least 6 different exhausts you can hear. Hearing the exhaust outside the car is well and good, but you also need to hear it in-cabin. Ask the owner to take you for a drive; on the drive, try to hit every style of driving you can (city, highway, etc.). Once you've had a chance to hear all the exhausts on your list, pick whichever one you like best.

    Because something like a catback is so subjective, no one can really answer a question like "What's the best catback?" Personally, I prefer a more "stealth" approach both in looks and volume. Pretty much any brand will perform the same (any difference you see by switching catbacks is minimal enough to be equated to "dyno error").


    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.


    Headers

    Headers replace the exhaust manifold, which connects the engine to the uppipe.

    There are two styles of aftermarket headers:
    • Equal-length (EL)
    • Unequal-length (UEL)

    Turbo Subarus come with UEL exhaust manifolds from the factory, which is what gives it the signature "Boxer Rumble". An equal length header will make a Subaru sound more like a Porsché. Strictly from an efficiency standpoint, aftermarket UEL headers will usually make smaller gains than EL headers.

    The power difference between brands is minimal enough to be equated to "dyno error". There are, however, differences in the way an aftermarket header impacts the powerband (e.g., some have great top-end at the cost of low-end, some offer a huge bump in the mid-range, etc.). Aftermarket headers are sometimes prone to cracking. An alternative to aftermarket headers is porting/polishing (PnP) the OEM exhaust manifold.


    Uppipes

    This exhaust component connects the exhaust manifold to the turbo.

    Any 2.0L WRX has a catalytic converter in the uppipe, and should be replaced. The following thread goes into more detail:
    Attn '02-'05 WRX Owners: The Importance of the Uppipe Mod

    There are two types of uppipes:
    • Flex-joint
    • Solid

    There is no difference in performance between a solid uppipe, and one that has a flex section. Personally, I prefer uppipes with a flex section (the OEM 2.5L uppipes have a flex section). The power difference between brands is minimal enough to be equated to "dyno error".

    This following only applies to 2.0L WRXs: Subaru included an EGT sensor in the OEM 2.0L uppipes as a precautionary measure to check the healthiness of the catalytic converter. Since you're going to be removing the cat, it is no longer necessary to run this (EGT should be measured in the exhaust manifold for accurate readings anyway). Reusing the stock EGT sensor is only going to run the risk of that breaking off and getting sucked into the turbo, causing damage. If the uppipe you're installing has an EGT bung, then use a bolt (M12 x 1.25) to plug the hole. There is a CEL associated with this modification that needs to be taken care of: P0546 Exhaust Gas Temperature Sensor Circuit Malfunction (High Input). This is often disabled with "StageII" maps, but if you're running the stock map, you'll need to do the "2.2 KOhm resistor mod". You can buy these at any electronics store (e.g., RadioShack), and should cost you ~$1. This resistor gets placed in the bottom connector just in front of the passenger strut tower.


    Miscellaneous

    Heat-wrapping or ceramic coating exhaust components is a popular modification. It serves two purposes: retaining exhaust gas temperature (EGT) for better flow, and reducing heat soak. The one downside to heat retention is that it may accelerate material fatigue/wear. Most people believe the pros outweigh the con. Before doing any type of heat retention to your exhaust component, make sure you understand the effects it may have on that parts warranty. Personally, I feel it's worthwhile to do this on any exhaust component in the engine bay (exhaust manifold / headers, uppipe), and the first half of the downpipe. You can wrap the entire downpipe if you'd like, but maintaining EGT isn't as important post-turbo (as it is for the other exhaust parts), and there no longer is the benefit of added heat soak prevention.

    When possible, consider purchasing used exhaust components. You can save quite a bit of money when purchasing parts from the classified sections (and you help another Subaru owner get rid of unused parts). Exhaust parts may be shiny when purchased new, but it's not long before they've begun to discolor, so why not let someone else take the hit in depreciation? While my example is a bit extreme, it just shows how much shopping used can save: my downpipe and catback retail for ~$1000 total new; I paid $200 used.

    On the subject of "eBay parts": the reason they are less expensive than other brand name parts is because the companies use inferior materials, utilize what can be equated to slave labor, and have no R&D overhead because they copy an already proven design. While the cheaper cost of parts may be good for the consumer, it does nothing but harm for the Subaru aftermarket community. Supporting this business practice leaves the companies that actually do the R&D and develop quality parts little incentive to continue if some other company can just purchase their part and copy it, and be able to sell their "own" part for a lot less. Greddy had quite a bit of financial trouble in 2009 and filed for bankruptcy; which popular, well-respected company is next? Some vendors have realized the popularity of these parts, and have begun selling and stocking them, which is also a complete disservice to the aftermarket community, since it gives the parts a more legitimate reputation. If cost is really your only consideration, then seriously consider purchasing used parts instead.
    Last edited by EJ257; 01-15-2012 at 07:09 PM.
    2005 WRX STi (Mods | Virtual Dyno)

    Resident Tuner @ WTF Tuning

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

    UNYSOC

  2. Remove Advertisements
    ClubWRX.net
    Advertisements
     

  3. #2
    Registered User sccoby823's Avatar
    Join Date
    Dec 2009
    Location
    New Bedford, Ma
    Posts
    341
    great write up!
    ***Stink Eye Mob 169***
    Stage 2 WRX Tuned by Brendan at Brentuning
    296 awhp
    317 awtq

  4. #3
    Registered User
    Join Date
    Dec 2009
    Posts
    102
    maybe merge this with exhaust clips?

  5. #4
    Master Baiter EJ257's Avatar
    Join Date
    Aug 2007
    Location
    On a stream bank somewhere
    Posts
    19,852
    I Support ClubWRX
    Quote Originally Posted by 2litrerex
    maybe merge this with exhaust clips?
    Quote Originally Posted by EJ257
    While a YouTube video can give you an idea of the various exhausts, there is no quality control to really be able to differentiate (e.g., camera quality, distance between exhaust/microphone, etc.).
    2005 WRX STi (Mods | Virtual Dyno)

    Resident Tuner @ WTF Tuning

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

    UNYSOC

  6. #5
    Admiral Ackbar the 1st mycologist's Avatar
    Join Date
    Oct 2005
    Location
    Late Devonian
    Posts
    13,314
    I Support ClubWRX
    Great FAQ Mathew. I'm sure Ron, Rick, or Scott will stumble upon it eventually.
    "From a little spark may burst a mighty flame." - Dante
    "The stitch is lost unless the thread is knotted." - Italian proverb

  7. #6
    Registered User
    Join Date
    Apr 2010
    Posts
    31
    quick question about the bosal shorty dp, i see it has a secondary pipe coming off of it then adjoining on each end, but this hks dp doesnt, whats the difference, what should i get? both same price
    Last edited by coolstorybro; 04-28-2010 at 09:49 AM.

  8. #7
    Master Baiter EJ257's Avatar
    Join Date
    Aug 2007
    Location
    On a stream bank somewhere
    Posts
    19,852
    I Support ClubWRX
    Quote Originally Posted by coolstorybro
    quick question about the bosal shorty dp, i see it has a secondary pipe coming off of it then adjoining on each end, but this hks dp doesnt, whats the difference, what should i get? both same price
    The Bosal is what's referred to as a "shorty" downpipe, which retains the midpipe cat. You can buy a secondary pipe which has a HFC or not.

    The difference in design is that the HKS has a plate which blocks wastegate gases, where the Bosal design separates the wastegate gases and dumps them into the exhaust stream further down the exhaust.
    2005 WRX STi (Mods | Virtual Dyno)

    Resident Tuner @ WTF Tuning

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

    UNYSOC

  9. #8
    Moderator Donkey's Avatar
    Join Date
    Apr 2005
    Location
    Da boonies
    Posts
    6,295
    I Support ClubWRX
    How did I miss this? Good job Matthew!
    Resident Adviser @ WTF Tuning, LLC
    2002 FORESTER S
    1998 FORESTER S




  10. #9
    Registered User SwampAssassin's Avatar
    Join Date
    May 2010
    Location
    Lakewood, CO
    Posts
    29
    i just installed a downpipe on my 06 sti and for some reason the inside of my car has some weird smoky film on the windows everytime i get back in it. does this have anything to do with no heat shield installed yet or perhaps just burning off the spray i put on the bolts to loosen them?

  11. #10
    Registered User devik's Avatar
    Join Date
    Jun 2006
    Location
    PHX Metro
    Posts
    276
    Quote Originally Posted by SwampAssassin View Post
    i just installed a downpipe on my 06 sti and for some reason the inside of my car has some weird smoky film on the windows everytime i get back in it. does this have anything to do with no heat shield installed yet or perhaps just burning off the spray i put on the bolts to loosen them?
    I've had the smoky film develop before, from what I can tell it's burnoff of various chemicals that you may have used under the hood. It seems to make its way through the defroster vents. Clean it off good, because it could be harmful to you or passengers, and make sure it doesn't come back.

  12. #11
    Registered User
    Join Date
    Apr 2010
    Posts
    31
    Hey,

    I have a great opportunity to buy a cobb catted downpipe, is it worth the price or are there other downpipes that are cheaper and get the same or even better gains over stock
    Last edited by coolstorybro; 06-16-2010 at 01:00 PM.

  13. #12
    Master Baiter EJ257's Avatar
    Join Date
    Aug 2007
    Location
    On a stream bank somewhere
    Posts
    19,852
    I Support ClubWRX
    Quote Originally Posted by coolstorybro
    I have a great opportunity to buy a cobb catted downpipe, is it worth the price or are there other downpipes that are cheaper and get the same or even better gains over stock
    Quote Originally Posted by EJ257
    Pretty much any brand will perform the same (any difference you see by switching downpipes is minimal enough to be equated to "dyno error").
    2005 WRX STi (Mods | Virtual Dyno)

    Resident Tuner @ WTF Tuning

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

    UNYSOC

  14. #13
    Registered User lytlec's Avatar
    Join Date
    Jan 2009
    Location
    Central PA
    Posts
    4,310
    Now that I have my EGT guage installed how do I use the thing?! I see the temp moving up and down depending on how much throttle I give it...but where should it be?!
    <-------- Taylor Swift.
    2006 WRX 330whp. 300wtq. Slow.
    Hawk-Eye Alliance #137
    First Mods...what to do.
    Subaru Guide

  15. #14
    Master Baiter EJ257's Avatar
    Join Date
    Aug 2007
    Location
    On a stream bank somewhere
    Posts
    19,852
    I Support ClubWRX
    Quote Originally Posted by lytlec
    Now that I have my EGT guage installed how do I use the thing?! I see the temp moving up and down depending on how much throttle I give it...but where should it be?!
    How does this have anything to do with exhaust? You should know better j/k

    My EGTs are in the 1300-1500° range, depending on my driving. I've got my warning set to 1600°
    2005 WRX STi (Mods | Virtual Dyno)

    Resident Tuner @ WTF Tuning

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

    UNYSOC

  16. #15
    Registered User lytlec's Avatar
    Join Date
    Jan 2009
    Location
    Central PA
    Posts
    4,310
    Quote Originally Posted by EJ257 View Post
    How does this have anything to do with exhaust? You should know better j/k

    My EGTs are in the 1300-1500° range, depending on my driving. I've got my warning set to 1600°

    its.....Exhaust Gas Temperature......thats why I posted it here......
    <-------- Taylor Swift.
    2006 WRX 330whp. 300wtq. Slow.
    Hawk-Eye Alliance #137
    First Mods...what to do.
    Subaru Guide

+ Reply to Thread
Page 1 of 9 123456789 LastLast

Quick Reply Quick Reply

Register Now

Please enter the name by which you would like to log-in and be known on this site.
If you do not want to register, fill this field only and the name will be used as user name for your post.

Please enter a password for your user account. Note that passwords are case-sensitive.

Please enter a valid email address for yourself. We strongly suggest that you stay away from using aol, yahoo, msn, and hotmail accounts. Sometimes the mail server blocks the emails from our server. As a result you will not receive any notifications including the confirmation email.

Log-in

Human Verification

In order to verify that you are a human and not a spam bot, please enter the answer into the following box below based on the instructions contained in the graphic.


Posting Permissions

  • You may post new threads
  • You may post replies
  • You may not post attachments
  • You may not edit your posts
  •