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2010 2011 BMW 5 Series Forum F10 F10 Technical Topics Engine, Transmission, Exhaust, Drivetrain, ECU Software Modifications Downpipe Question
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      11-30-2014, 09:50 PM   #1
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Downpipe Question

Can someone explain how an aftermarket downpipe makes additional horsepower in comparison to a stock unit? I totally understand turbocharging, but don't really understand the role of the DP. Thanks.
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      12-01-2014, 01:41 AM   #2
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depending on whichever you go will give you different power gains.

Catless being the most power gain vs. catted being less of a gain

Downpipes give you more power because they let the exhaust from the motor flow more freely (catless flows more freely than catted hence the power gain).

picture a hose with a kink in it being your stock down pipe…. then remove the kink from the hose thats your aftermarket down pipe…… flows better without the kink right? Thats the dummy way of explaining it lol
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      12-01-2014, 06:45 AM   #3
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Quote:
Originally Posted by phil7027
depending on whichever you go will give you different power gains.

Catless being the most power gain vs. catted being less of a gain

Downpipes give you more power because they let the exhaust from the motor flow more freely (catless flows more freely than catted hence the power gain).

picture a hose with a kink in it being your stock down pipe. then remove the kink from the hose thats your aftermarket down pipe flows better without the kink right? Thats the dummy way of explaining it lol
I understand that a catless DP makes more power because you are removing the restrictive cat, but if we are talking a catted DP, is the increase in HP that significant? Just seems like the pipe is so short, I don't see how it makes huge gains.
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      12-01-2014, 06:56 AM   #4
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I've been just reading about the same, so please have a look at this very simple and clear article: http://forums.nasioc.com/forums/showthread.php?t=445812
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      12-01-2014, 09:32 AM   #5
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Quote:
Originally Posted by Valery.BMW View Post
I've been just reading about the same, so please have a look at this very simple and clear article: http://forums.nasioc.com/forums/showthread.php?t=445812
Good article !
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      12-01-2014, 10:32 AM   #6
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Originally Posted by badblack550xi View Post
Good article !
Thanks, bud! Good to see you like it, you owner of the brutal black bullet with catless DP, methanol injection and race exhaust!
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      12-02-2014, 01:29 AM   #7
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Yes you will still see gains even with an aftermarket catted DP because it still flows better than the stock cat. My hose reference explains it the best. Any free up in the restriction of an exhaust system (especially a turbo motor) will see an increase in horsepower.

Before I got my 550 2 weeks ago I had a 2012 mustang GT. Back when I deleted my cats before adding headers, i gained 20rwhp just from deleting them.
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      12-02-2014, 07:54 PM   #8
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Yep I get. So gain is mainly realized with the removal of the cat, or replacement of the OEM cat with a less restrictive one. Basically what I was getting at is there is more to it that just replacing a very short length of exhaust piping. My next question is, are our cars able to pass emissions testing with the catted DP options? I already know they won't pass without cats.
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      12-03-2014, 02:03 AM   #9
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Quote:
Originally Posted by bake View Post
Yep I get. So gain is mainly realized with the removal of the cat, or replacement of the OEM cat with a less restrictive one. Basically what I was getting at is there is more to it that just replacing a very short length of exhaust piping. My next question is, are our cars able to pass emissions testing with the catted DP options? I already know they won't pass without cats.
I see your in NY so your in the same boat as me. In NY as long as the 02 sensors are all turned on and there is no CEL, you will pass.

You can pass with the catted down pipe, Catless will be more of a hassle, but it can be done. I am going carless on mine
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