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Discussion Starter #1
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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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 :checkeredflag:

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-
 

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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!

http://www.m5board.com/vbulletin/e60-m5-e61-m5-touring-discussion/125904-raced-997-turbo-auto-mighty-gtr.html
 

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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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Discussion Starter #5
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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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...
 

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Hey G///McGregor K,


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

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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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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!

http://www.m5board.com/vbulletin/e60-m5-e61-m5-touring-discussion/125904-raced-997-turbo-auto-mighty-gtr.html

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

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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...
 

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I've got no problems with my setup. As long as you keep the stock resonators, sound is perfect. Car definately feels faster, and in fact there is a dyno done by RPI showing gains in the mid range by deleting secondary cats.
 

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I am running Supersprint headers with the ASR Gen. II full exhaust. This means 3" pipes straight through to section 3 which has the minimal cans at the end.
There are NO cats anywhere.

Now I must say, naturally aspirated engines DO NOT need back pressure! Do I need to say that again?

My exhaust system makes rwhp and torque. There is NO power loss in the low end.

My M5 sounds awesome and the power is unbelieveable. The ASR exhaust truly makes power.

Ranger
 
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