Bellmouth vs. Divorced Wastegate catless
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This is a discussion on Bellmouth vs. Divorced Wastegate catless within the Engine Modifications forums, part of the Tech & Modifying & General Repairs category; Hey guys, so I plan on going stage 2 soon with a catless downpipe. Everything on the car is stock. ...

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    Bellmouth vs. Divorced Wastegate catless

    Hey guys, so I plan on going stage 2 soon with a catless downpipe. Everything on the car is stock. I was wondering what are the advantages/disadvantages of either downpipe (bellmouth vs. divorced wastegate). Is one more prone to boost creep or over boost? And also does anyone know where I can find good bellmouth catless dp's. I drive a 2013. Thanks a lot guys!!

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    MAINEiac 11blackSTi's Avatar
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    Credit to EJ257 on this...Exhaust FAQ

    "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."

    Also --->unibomber's manifesto goes into pretty good detail on this --->Exhaust FAQ: Read if you are thinking of buying one! - NASIOC

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    Registered User nsibanez's Avatar
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    Exactly this
    Just to add from my research is that unless you're pushing some boost on a bigger turbo (don't know how much or what size since it's not in the works for me at the moment) you shouldn't need a bell mouth unless you're pushing a good amount of boost and a bigger turbo so the exhaust gasses can flow freely where on the same setup the divorced would restrict it. But even still like said before the difference would be minimal

    Sent from my SCH-I545 using Tapatalk

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    zax
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    In theory the divorced design should reduce boost creep since it allows for a more effective pressure differential across the wastegate. In reality, it likely will not make much difference.
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    MTM
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    I got a divorced one. Because its "supposed" to be the better one, and it really wasnt that much more. $298 shipped I believe.

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    zax
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    Quote Originally Posted by MTM View Post
    I got a divorced one. Because its "supposed" to be the better one, and it really wasnt that much more. $298 shipped I believe.
    It was $298 more?
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    MTM
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    Quote Originally Posted by zax View Post
    It was $298 more?
    haha no. Total. I think it was like $25 or $30 more than bellmouth. Catless invidia.

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    Its a divorce of course its going to cost you
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    Quote Originally Posted by MTM View Post
    haha no. Total. I think it was like $25 or $30 more than bellmouth. Catless invidia.
    The real difference in price for DP's comes in when you are talking catless vs. catted.

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    Yeah so I think I may just end up going with the Invidia divorced wastegate because I don't plan on doing any further mods performance wise. Thanks for all the help guys! I really appreciate it!

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    Registered User WRX-maniac's Avatar
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    OP, are you getting a pro-tune done once you get your downpipe? If not, you need a downpipe with a high flo cat. Cobb's OTS maps were not designed to work with catless downpipes. And power gains are very minimal with a catless DP over a catted. Just a suggestion.
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    Quote Originally Posted by WRX-maniac View Post
    OP, are you getting a pro-tune done once you get your downpipe? If not, you need a downpipe with a high flo cat. Cobb's OTS maps were not designed to work with catless downpipes. And power gains are very minimal with a catless DP over a catted. Just a suggestion.
    Yeah I'm going to get it tuned with torqued performance. Eric said that it doesn't matter whether it's catless or catted. I mean in Michigan they have no emissions regulation and also catted downpipes are much more expensive compared to catless. So im going to try catless for now. Thanks for the suggestion!

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    Quote Originally Posted by Wrx>> View Post
    Yeah so I think I may just end up going with the Invidia divorced wastegate because I don't plan on doing any further mods performance wise. Thanks for all the help guys! I really appreciate it!
    thats what I have. Fit and finish is beautiful. I got the one with the extra 02 bung as well for a future AFR gauge.

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    Quote Originally Posted by Wrx>> View Post
    Yeah I'm going to get it tuned with torqued performance. Eric said that it doesn't matter whether it's catless or catted. I mean in Michigan they have no emissions regulation and also catted downpipes are much more expensive compared to catless. So im going to try catless for now. Thanks for the suggestion!
    Ok, got ya. Eric is very good, so im sure you will be pleased with the tune. Im actually thinking about going with torqued performance stage 2 as well bc he supports my perrin intake. Im currently running the perrin stage 2 for the intake, but I feel there is still room for improvement. And from all the satisfied people on here running a map from Eric, I think Ill give it a shot. Let us know how it turns out!
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    Quote Originally Posted by WRX-maniac View Post
    Ok, got ya. Eric is very good, so im sure you will be pleased with the tune. Im actually thinking about going with torqued performance stage 2 as well bc he supports my perrin intake. Im currently running the perrin stage 2 for the intake, but I feel there is still room for improvement. And from all the satisfied people on here running a map from Eric, I think Ill give it a shot. Let us know how it turns out!
    Will do! Hopefully the parts come in within the next week or so. I currently run the torqued performance stage 1 on my car. It's great! Eric is great with service and his map is truly spectacular! I can't wait to see how is stage 2 map is!

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