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Old 03-15-2009, 08:55 AM   #1
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back-pressure

so i bought an 88 spg and it had what looked to be a home-made cat-back exhaust on it. My mechanic said that, the 3 inch pipe coming off the cat was actually reducing HP, because of a back pressure issue because the pipe after the cat was larger than the pipe before the cat. Im not really knowledgeable about these things so i was hoping someone could tell me if this sounds right? i took the exhaust off, and replaced with a new stock turbo exhaust (i still have the other exhaust). it seems more responsive to the pedal now. less delay it seems. with the other exhaust if i feathered the pedal it would almost stall out at idle, doesnt seem to do that now....any thoughts?
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Old 03-17-2009, 09:37 PM   #2
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Hey Gunfighter,

Well, it appear as though the proof is in the pudding, so to speak. I'm no expert at all regarding the exhaust on these cars but looks like you already have your answers. If you can feel the difference in performance, then there's not much else anyone can say...

I'm actually, now that I have the Saaby back on the road, getting ready to upgrade from 2.5" to full 3" exhaust. From what I've been told, these turbos breath better with larger exhaust. This includes having a downpipe included in the system. I Recall that when I went and had my 2.5" exhaust installed, my muffler guy said the same thing.

Congrats...

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Old 03-18-2009, 03:56 PM   #3
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Well I have 3" custom exhaust, though i had a few problems with it scraping. But im sure with some extra hangers it would be fine, has a Flowmaster muffler, and it does sound nice (probably better with a complete 3" unit). Ill take some pics so you can check it out. Like i said i dont think it works well with a stock downpipe but probably would be nice for a complete 3". ill take some pics.
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Old 03-18-2009, 07:40 PM   #4
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That I'd have to agree with. There would be no way that I would do a 3" without the complete DP. Doesn't make sense. But who am I...just another Saab junkie...

Have a good one...

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Old 03-19-2009, 06:59 PM   #5
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NAs like Equal length headers, turbos dont really care, just want the Exaust to go away as fast as possible.

to my knowledge every NA saab i have had has been faster with open manafolds. the reason you need some type of exaust coming off the head is if you dont the valves heat up and cool down constantly and eventually break. thats what i was always told atleast

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Old 03-19-2009, 08:53 PM   #6
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http://www.mx-3.com/phpBB3/viewtopic.php?f=23&t=41098

Read and make your own judgment.

turbo really doesn't matter, but n/a it can make a night and day difference.
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Old 03-20-2009, 08:20 AM   #7
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Quote:
Originally Posted by '96Saab9kAero View Post
http://www.mx-3.com/phpBB3/viewtopic.php?f=23&t=41098

Read and make your own judgment.

turbo really doesn't matter, but n/a it can make a night and day difference.
Thanks,

Definitely and interesting read. Very informative.

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Old 03-20-2009, 08:54 AM   #8
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On a turbo car the turbocharger itself creates all the backpressure the motor needs. From there dump it into some big tubing and be happy.

It's an overly simplistic explanation, but for that most part that is just how it is.
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Old 03-20-2009, 10:53 AM   #9
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Quote:
Originally Posted by JK View Post
On a turbo car the turbocharger itself creates all the backpressure the motor needs. From there dump it into some big tubing and be happy.

It's an overly simplistic explanation, but for that most part that is just how it is.
You are absolutely correct. I found out after having my exhaust replaced. The muffler tech told me that I shouldn't go larger due to the backpressure issue. I learned later, from one of the Porsche forums that the turbo itself provides the needed backpressure. I currently run a 4" exhaust on my 930 with no issues whatsoever...

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Old 03-27-2009, 03:50 PM   #10
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well maybe the culprit was the cobbed exhaust. good to know about the turbo creating the back pressure...i had no idea. learn something new everyday here, thanks again
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