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Discussion Starter · #1 ·
I have ordered the Hirsch cat-back system for my 9-5 (the single pipe one, but still I can't wait till it arrives). Hirsch tells me that the diameter of the pipe connecting to the standard cat is 7 cm, or 2.756".

My question would be: How do I connect the cat-back to the 3" downpipe that's also on my wishlist? Any suggestions?

It may have been more practical to get a full 3" system (front to back), but for whatever reason, the authorities here in Singapore seem very particular when it comes to the silencers, but I'm told don't bother with downpipes and the likes. That's why I went with the "approved" cat-back.
 

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The likes of the JT downpipe will come with an adaptor to fit to a standard system. Thus if your Hirsch one mates to the standard system, the two should be compatible
 

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Speaking from ignorance I can understand having a 2.5" pipe running into a 3" but the other way 'round wouldn't it restrict the gas flow, causing a back pressure? Surely 3" thru makes sense or 2.5" thru, or 3" into 2.5". As I have a hole in my downpipe do I replace it with 3" @ ?cost or go for standard. Keep in mind that I intend upgrading on a need basis due to cashflow prob's, ie; no work at the mo'

Keith.
 

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Discussion Starter · #4 ·
The likes of the JT downpipe will come with an adaptor to fit to a standard system. Thus if your Hirsch one mates to the standard system, the two should be compatible[/b]
Mark, thanks for this. Probably could have thought of it myself eventually, but posting to this forum is just more convenient than figuring it out on my own Anyway, I'm going for a Maptun downpipe and that comes with an adapter for a standard exhaust, too.
 

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Speaking from ignorance I can understand having a 2.5" pipe running into a 3" but the other way 'round wouldn't it restrict the gas flow, causing a back pressure? Surely 3" thru makes sense or 2.5" thru, or 3" into 2.5". As I have a hole in my downpipe do I replace it with 3" @ ?cost or go for standard. Keep in mind that I intend upgrading on a need basis due to cashflow prob's, ie; no work at the mo'  [/b]
Your right in saying having a 3" system all the way back would be best but on turbo cars having a 3" front pipe helps get the exhaust gasses away from the turbo/head more quickly. This helps spool up time and possible damage to head/valves espacially where ECU's have also been upgraded for more power. I run a 3" front pipe into standard exhaust back boxes but with no CAT - not the best but needs must at present.

Mike
 

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Originally posted by RedSaab formerly Carly Keith:
[qb]Speaking from ignorance I can understand having a 2.5" pipe running into a 3" but the other way 'round wouldn't it restrict the gas flow, causing a back pressure? Surely 3" thru makes sense or 2.5" thru, or 3" into 2.5". As I have a hole in  my downpipe do I replace it with 3" @ ?cost or go for standard. Keep in mind that I intend upgrading on a need basis due to cashflow prob's, ie; no work at the mo'       [/qb][/b]
The entire exhaust system generates back pressure on the turbo. Generally speaking, any reduction in the backpressure by lowering flow resistance will give a performance benefit, so any larger section anywhere in the system will help. However, the resistance is proportional to the flow rate and nearer the turbo output the gasses are at their hottest and moving quickest, so the downpipe gives most benefit. Further along the system the gasses have cooled somewhat and are moving slower, and thus the benefit of a larger dia is less.
 

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Hmmmm ... I think there might be some de-bunking that needs to be done. I may start a new thread for that.

Suffice to say, the exhaust system need only be slightly larger at it's minumum diameter, than your turbine discharge. 2.5" should be more than enough. According to Corky Bell, 2.5" is good enough for upwards of 400 bhp.

The reason a 3" is beneficial straight out of the turbine discharge is because it breaks up the spiral flow of gasses out of the turbine.

This quote is from "Turbochargers" by Hugh MacInnes:

"Exhaust gasses exiting the turbocharger will be spinning like a helix. The direction of the spiraling can be either the same, or the opposite of the turbine-wheel rotation. Which direction depends on exhaust gas speed.

The turbocharger is used over a broad range of engine speed and power. Consequently, the exhaust gasses will sometimes be going faster than the turbine-wheel exducer, and sometimes slower.

When the exhaust gasses are going faster than the turbine wheel, they will rotate in opposite direction from the turbine rotation. When they are going slower than the turbine-wheel, they will rotate in the same direction.

In either case the path for the exhaust gasses is considerably longer than if they were coming out axially. For this reason it is desirable to break up the swirl and turn the gasses into turbulent flow. This should be done as soon as possible after leaving the turbine housing.

One way to do this is to have a sharp diffuser angle on the turbine housing. This requires a large-diameter exhaust pipe from the turbine housing. Once turbulent flow is established, usually after 18-24 inches, exhaust pipe diameter can be reduced.

A smooth reduction in exhaust-pipe diameter will not create much additional backpressure and will help quiet exhaust noise. In some cases it will do such a good job that a muffler will not be required."

Dubbya~
 

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Discussion Starter · #10 ·
Gee, this forum is great! Not only do get practical advice and moral support, I actually learn something that perhaps someday I can use to impress some unsuspecting soul.

So, thanks a lot on all counts, Dubbya.
 

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As an aside and not involving theory - I lost ALL boost on my Speedparts Stage 1 GM900T recently and having been through ALL the usual checks and even into Saab dealer who could find nothing wrong replaced both back boxes yeterday and full boost now restored. Can only imaging that whatever was causing blockage the exhaust gasses could not escape fast enough thus preventing the turbo from spooling up at all.

Mike
 

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Originally posted by RedSaab formerly Carly Keith:
Speaking from ignorance I can understand having a 2.5" pipe running into a 3" but the other way 'round wouldn't it restrict the gas flow, causing a back pressure? Surely 3" thru makes sense or 2.5" thru, or 3" into 2.5". As I have a hole in my downpipe do I replace it with 3" @ ?cost or go for standard. Keep in mind that I intend upgrading on a need basis due to cashflow prob's, ie; no work at the mo'    [/b]
Am I glad I wrote the above. To all who have read these reports any doubt anybody had about which way 'round and what regarding exhaust should be cured. Said B4 and I'll say again, " The depth in knowledge within Saabscene is
" and thanks to everyone with a special to Adrian W.
 
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