Hi-Flow cats Vs. Straight pipes all the way through. - BMW M5 Forum and M6 Forums
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post #1 of 12 Old 3rd October 2008, 06:20 AM Thread Starter
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Hi-Flow cats Vs. Straight pipes all the way through.

I was wonder what were some of the power benefits of have Hi-flow cats (100-200 cell) compared to just having straight pipes all the way through the exhaust system. I am not intrested in emission as the state I live in there are no smog test or anything like that. Any info is really appreciated and thanks..
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post #2 of 12 Old 3rd October 2008, 10:50 AM
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Most street engines need backpressure to give a proper street torque curve, and each car/motorcycle's exhaust system is a series of checks and balances, designed with almost everything else first and power last.

Open pipes are loud, smell atrocicous, and often only make additional power in the upper revs. Try to go out for an evening on the town, and while the engine is warming fiddle with the trunk. Forget your Issey, you now smell like a mechanic

Apparently, the M5's primary "light off" forward cats are the primary source of restriction, and eliminating those seem to give the most power benefit. I'm sure a small amount of low end suffers here too, but as the revs build the exhaust system fills and proper backpressure is acheived.

ASR Miami has an exceptional, complete exhaust system, which is purported to achieve good results in power and sounds great in person.

Hi-flow cats are a solution to a problem for those who want extra power w/o running afoul of the law (I believe there are still laws being broken by messing with anything related to emissions, just less with them unless certifed, blah-blah-I'm not sure). On the M5, it might help you get some extra power w/o massive loss of low end. Higher flow cats and straight pipes usually shine in a road race, where heat buildup is a big issue-removal of those heat potatoes at the end of the M5 headers should really reduce heat buildup close to the engine.

I have heard of random smog checks points in other states, though I'm, not sure how realistic that is (if you take a cross country/state trip, etc.)

Sorry for the book-

Last edited by MiamiDave; 3rd October 2008 at 10:57 AM.
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post #3 of 12 Old 3rd October 2008, 11:41 AM
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in the link below, you can see my comments about the straight pipes. Yes they do smell like gasoline, but the power increase is great. The firts race link (with video) was before I installed the straight pipes. The race with the GTR and 997 TT was after. i think the straight pipes helped my car make th emost of the headers and intake mods! if you can get the same from racing cats, then do it cause they are more civilised, but they are also 4x the price!

raced 997 turbo (auto) and the mighty GTR!

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post #4 of 12 Old 3rd October 2008, 03:07 PM
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I tried straight pipes without cats and I had to put them back. They become ridiculously loud. It was no longer amusing. I got negative stares from people. I thought I was macho about it. Honestly, I don't think so because I was annoying people and setting off alarms. And without ECU mod, the car chokes a lot at low rpm.

I just changed a straight sec. 2 without resonators and TUBI exhaust. I think it still sounds louder than most cars already. But live-able.

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I am really annoyed every time I open my hood to worship my beast's manly organs.

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post #5 of 12 Old 3rd October 2008, 04:07 PM Thread Starter
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Thanks for the comments. I was wondering because I ran straight pipes on my E46 M3 and it was loud and smelled like gasoline, well all the regular stuff. Thanks again. I know with straight pipes you do lose some low end TQ.
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post #6 of 12 Old 3rd October 2008, 04:48 PM
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I haven't listed them yet, btu I am about to sell some RPI high flow cats, brand new in the box. PM me if youre interested...

2008 Space Gray/Black Full Merino Leather/Alum. trim
Loaded - every option (minus heated steering wheel)
Interior:Hardwired Valentine One with Concealed Display, Blinder Xtreme M25 (J16) Laser Jammer, Bimmian Interior Xenon Lighting Kit and ///M Racing Pedals
Exterior: Hartge Front Spoiler Lip, Hartge Rear Diffuser, Vorsteiner Vented and Domed Hood (Double-Sided CF), Hamann Roof Spoiler, H&R Springs and 15% Tint
"Performance": RPI GT Section 3 Muffler, RPI GT Section I, ASR CAI, Evosport Power Pulley and Hybrid Rear Badge

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post #7 of 12 Old 3rd October 2008, 05:12 PM
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Hey G///McGregor K,


Why did you decide to sell them? Just curious how much those things cost?

EMVY

2007 M5
Interlagos Blue
www.bmwforum.ph


I am really annoyed every time I open my hood to worship my beast's manly organs.

ACS style carbon chin with splitters / CF kidney grill / CF side vents / CF rear diffuser / Reiger sider skirt with CF / Reiger trunk spoiler / DPE S20 - 19" / H&R lowering springs / Evosport sec. 1 & sec. 2 / Tubistyle Rumore / Evosport Pulley / RED CF Inlay ENGINE COVER / GRUPPEM CF airbox
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post #8 of 12 Old 3rd October 2008, 06:21 PM
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I had straight pipes meaning no secondary cats and no resonators, sounded kind of raspy till it warmed up, i didint like it to much. I ended up throwing a Meisterschaft GT on it and it was wayy to loud, so I went to any race shop and got 2 2.5" Magnaflow High flow cats- 90 bucks each, and 2 Dynomax bullet resonators - about 50 each. Quited it down and I gained some torque down low because you loose back pressure unless you throw an X-pipe where the resonators should be.
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post #9 of 12 Old 3rd October 2008, 06:42 PM
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Quote:
Originally Posted by kaninja99 View Post
in the link below, you can see my comments about the straight pipes. Yes they do smell like gasoline, but the power increase is great. The firts race link (with video) was before I installed the straight pipes. The race with the GTR and 997 TT was after. i think the straight pipes helped my car make th emost of the headers and intake mods! if you can get the same from racing cats, then do it cause they are more civilised, but they are also 4x the price!

raced 997 turbo (auto) and the mighty GTR!

I forgot to mention that I put the stock resonators back in to quieten it down.

___________________________________________
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post #10 of 12 Old 3rd October 2008, 07:16 PM
G ///McGregor K
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Quote:
Originally Posted by EMVY View Post
Hey G///McGregor K,


Why did you decide to sell them? Just curious how much those things cost?
I'm going with ASR's Stage II setup and need straight pipes. With teh Stage II setup, I need straight pipes to cool down the temps of the exhaust coming out....The secondary cats go for $1020 retail, but I think I'm going to sell mine for less. PM me if your interested in them...

2008 Space Gray/Black Full Merino Leather/Alum. trim
Loaded - every option (minus heated steering wheel)
Interior:Hardwired Valentine One with Concealed Display, Blinder Xtreme M25 (J16) Laser Jammer, Bimmian Interior Xenon Lighting Kit and ///M Racing Pedals
Exterior: Hartge Front Spoiler Lip, Hartge Rear Diffuser, Vorsteiner Vented and Domed Hood (Double-Sided CF), Hamann Roof Spoiler, H&R Springs and 15% Tint
"Performance": RPI GT Section 3 Muffler, RPI GT Section I, ASR CAI, Evosport Power Pulley and Hybrid Rear Badge


Last edited by G ///McGregor K; 4th October 2008 at 06:57 AM.
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