Curious on restrictive exhaust flows

clairvoyant · 10-29-2011, 11:04 AM

Which one is the choke point (restrictive flows on the exhausts EXCLUDING the OEM downpipes)?

-The secondary cats (on the midpipes)
or
-The twists and the bends of the entire exhaust system

Thank You

thebigragu99 · 10-29-2011, 11:10 AM

Quote:

Originally Posted by clairvoyant

Which one is the choke point (restrictive flows on the exhausts EXCLUDING the OEM downpipes)?

-The secondary cats (on the midpipes)
or
-The twists and the bends of the entire exhaust system

Thank You

The rear cats!!

secretsquirrel · 10-29-2011, 11:41 AM

I'd be willing to bet that the oem dp's are the most restrictive considering that aftermarket ones net you 25hp or so.

cn555ic · 10-29-2011, 11:42 AM

IMO any cats is more restrictive than a couple of bends. Have you ever looked into the oem cats, they are restrictive

Tiago@VRSF · 10-29-2011, 01:31 PM

The cats are the most restrictive part of any turbo car.

turb0mik3 · 10-29-2011, 01:38 PM

Agree'd, cats are no good. I have an exhaust question I will be posting up soon as well.

secretsquirrel · 10-29-2011, 01:38 PM

I agree that the cats are the most restrictive, but I think the first cats (which are replaced by aftermarket dps) are more restrictive than the rear cats.

Edit: I just read the question over. I previously skimmed the question on my phone. I fail.

clairvoyant · 10-29-2011, 01:50 PM

Assuming I keep the OEM downpipes and remove the secondary cats, how much hp gain (realistically)?
Thank You

turb0mik3 · 10-29-2011, 01:52 PM

Quote:

Originally Posted by clairvoyant

Assuming I keep the OEM downpipes and remove the secondary cats, how much hp gain (realistically)?
Thank You

I would assume 3whp or so. Not a big gain.

nitehawk · 10-29-2011, 01:58 PM

Quote:

Originally Posted by clairvoyant

Which one is the choke point (restrictive flows on the exhausts EXCLUDING the OEM downpipes)?

-The secondary cats (on the midpipes)
or
-The twists and the bends of the entire exhaust system

Thank You

Have you seen the cross section of a catalytic converter?

It is by far the most restrictive...

tibra1 · 10-29-2011, 02:01 PM

The restriction nearest to the engine/turbos will always be the worst..hence swapping the OEM DPs gives the best gain..then removing the secondary cats..then going to free flow exhaust setup

clairvoyant · 10-30-2011, 12:56 PM

Since someone mentioned the catalytic converter cross section, the next question will be:
Do the downpipes' catalytic converters have the same cel densities as those of the secondary catalytic converters on the midpipes?
Thank You

Quote:

Originally Posted by nitehawk

Have you seen the cross section of a catalytic converter?

It is by far the most restrictive...

10-29-2011, 11:04 AM	#1
clairvoyant Major 79 Rep 1,032 Posts Drives: Bavarian Manure Wagon (BMW) Join Date: Oct 2007 Location: 45°19′20″N, 75°50′0″W iTrader: (5) Garage List 2007 335i [5.24]	Curious on restrictive exhaust flows Which one is the choke point (restrictive flows on the exhausts EXCLUDING the OEM downpipes)? -The secondary cats (on the midpipes) or -The twists and the bends of the entire exhaust system Thank You
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10-29-2011, 11:41 AM	#3
secretsquirrel Colonel 130 Rep 2,291 Posts Drives: f80 Join Date: Dec 2010 Location: Far North Dallas iTrader: (2)	I'd be willing to bet that the oem dp's are the most restrictive considering that aftermarket ones net you 25hp or so.
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10-29-2011, 11:42 AM	#4
cn555ic 441 Rep 18,331 Posts Drives: 335i Join Date: Jun 2007 Location: US iTrader: (6)	IMO any cats is more restrictive than a couple of bends. Have you ever looked into the oem cats, they are restrictive
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10-29-2011, 01:31 PM	#5
Tiago@VRSF 2095 Rep 43,350 Posts Drives: F90 M5 Join Date: Dec 2007 Location: Doral, FL iTrader: (99) Garage List 2014 F30 328xi [0.00] 2015 F82 BMW M4 [0.00] 2013 Ducati 1199 Pa ... [0.00] 2008 BMW 335i [0.00]	The cats are the most restrictive part of any turbo car.
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10-29-2011, 01:38 PM	#6
turb0mik3 242 Rep 2,781 Posts Drives: E90 335i Racecar Join Date: Jan 2010 Location: San Diego, CA iTrader: (1) Garage List 2010 BMW 335i [8.90]	Agree'd, cats are no good. I have an exhaust question I will be posting up soon as well. __________________ HG Motorsports \| Motiv Motorsport \| Sparta Evolution \| BBS Motorsports \| Sabelt \| JRZ \| NASA ST1 SoCal --- 554whp/487wtrq --- 3200lbs <<YouTube Channel w/ Race Videos>>
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10-29-2011, 01:50 PM	#8
clairvoyant Major 79 Rep 1,032 Posts Drives: Bavarian Manure Wagon (BMW) Join Date: Oct 2007 Location: 45°19′20″N, 75°50′0″W iTrader: (5) Garage List 2007 335i [5.24]	Assuming I keep the OEM downpipes and remove the secondary cats, how much hp gain (realistically)? Thank You
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10-29-2011, 02:01 PM	#11
tibra1 Banned 127 Rep 6,773 Posts Drives: 2011 ZCP M3 - 2007 335i crashd Join Date: Apr 2009 Location: NYC iTrader: (4)	The restriction nearest to the engine/turbos will always be the worst..hence swapping the OEM DPs gives the best gain..then removing the secondary cats..then going to free flow exhaust setup
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