Saabscene Saab Forum - Saab Technical Information Resource banner
1 - 20 of 44 Posts

·
Registered
Joined
·
597 Posts
Discussion Starter · #1 ·
My 1999 9-3 has a custom 2.5 inch exhaust (inc downpipe) with a high-flow cat followed by a vortex with a single silencer at the back. The builder said it would be good for well over 300 bhp.

Could I risk a BSR stage 3 ECU (I have stg 1) or would I need a 3 inch downpipe? If I do need the pipe, what distance from the turbo would need to be 3 inch?
 

·
Registered
Joined
·
2,041 Posts
I would go for a 3" downpipe into your exhaust - ideally you should go for 3" all the way but if you have spent money on the rest then this would be best alternative. I run a 3" downpipe into a standard exhaust set up except that I have no CAT.
The front pipe will help quicker spooling up and will help get the hotter gasses casued by the ECU upgarde away from the head more quickly.

Mike
 

·
Registered
Joined
·
1,219 Posts
The MapTun dealer in Australia here, has a Stage 4 Aero 9000 with the Abbott 2.5" Stainless turbo back exhaust.. he only wants the 3" if he goes over 350 bhp.
 

·
Registered
Joined
·
11,343 Posts
Originally posted by Grentarc:
[qb]he only wants the 3" if he goes over 350 bhp.    :thumbsup:   [/qb][/b]
Really That seems pretty short-sighted.

The downpipe is, perhaps, one of the most significant non-software upgrades that you can do to a turbo engine. Personally, I would not want an engine tuned to this degree that was not able to expel the exhaust-gasses.
 

·
Registered
Joined
·
1,448 Posts
Originally posted by /john:
[qb]Really        That seems pretty short-sighted.

The downpipe is, perhaps, one of the most significant non-software upgrades that you can do to a turbo engine.  Personally, I would not want an engine tuned to this degree that was not able to expel the exhaust-gasses. [/qb][/b]
According to Corky Bell a 2.5" exhaust pipe should be sufficient in most cases for around 400 bhp.
 

·
Registered
Joined
·
11,343 Posts
Originally posted by Adrian W:
[qb]According to Corky Bell a 2.5" exhaust pipe should be sufficient in most cases for around 400 bhp. [/qb][/b]
That's as maybe. A larger downpipe allows hot exhaust gasses to escape quicker and there is a significant increase in power, so it's a no-brainer.
 

·
Registered
Joined
·
348 Posts
I think Hirsch uses the stock downpipe on all their Troll models, even for over 300 hp applications.

But I agree that in my case, the 3" DP made my turbo spool up much quicker.

Yours,

Philip
 

·
Registered
Joined
·
597 Posts
Discussion Starter · #9 ·
The main reason for my question is that boost control is still not great and a new control solenoid only lasted 6 months before sticking. Turbo and actuator have been inspected and are fine, but every now and then boost drops to about 0.6 bar for around five minutes of driving, and hard acceleration uphill in 4th or 5th makes the max boost oscillate between 1 and 1.2 bar, while under other conditions it holds at 1.0 bar (bb = 0.35). This behaviour preceeded the last solenoid failure.

So, having read other threads, I suspect that the ECU may in some way be at fault. Rather than send the ECU back to BSR to check, I may as well move up to stage 3 (£100 or so). Cost-wise, I'd prefer the 3 inch downpipe to come a bit later, though I don't want to risk problems.
 

·
Registered
Joined
·
132 Posts
Originally posted by philip hs:
[qb]I think Hirsch uses the stock downpipe on all their Troll models, even for over 300 hp applications.

But I agree that in my case, the 3" DP made my turbo spool up much quicker.  

Yours,

Philip [/qb][/b]
Hi Philip

Hirsch uses for the 305Ps version a sport cat as you can see here but i don't know the dimension of the downpipe (3", 2.5", 2.75")?

http://www.hirsch-performance.ch/pdf/9-5%20Aero%20305PS.pdf

9-5 aero
 

·
Registered
Joined
·
348 Posts
Hi 9-5 Aero,

yes, I am aware of the Sport cat. It is also used in the Hirsch stage III for Viggen.

But I think they use it on the stock downpipe, this is no problem, the cat on both 9-3 and 9-5 is not welded to the downpipe but only connected using clamps.

Yours,

Philip
 

·
Registered
Joined
·
1,448 Posts
Originally posted by philip hs:
[qb]But I think they use it on the stock downpipe, this is no problem, the cat on both 9-3 and 9-5 is not welded to the downpipe but only connected using clamps.[/qb][/b]
Seems to be welded on my car. Remember, Philip, your 99 Viggen has a different downpipe to all the other MY Viggens, 9-5's, and 9-3's. 2000+ got dual catalysts.

In regards to the "no brainer" about exhaust size ...

There's little advantage to letting the exhaust slow down too quickly. Remember that a faster moving fluid has a lower internal static pressure. I'm NOT trying to say that a smaller pipe will usually flow better. But if you bolted a 6" diameter pipe to your turbo you would likely lose power. There's a happy medium here, and I wouldn't jump to the conclusion that 3" is it.

Adrian W~
 

·
Registered
Joined
·
4,895 Posts
There's a happy medium here, and I wouldn't jump to the conclusion that 3" is it.  [/b]
Nor would I jump to the conclusion that all people who have upgraded from a standard (or perhaps even 2.5") to a 3" downpipe are deceiving themselves when they notice a performance increase.

I would agree that over-size exhausts are of little benefit and are probably detrimental to the cars performance and fuel economy. If you use the 1" per litre + 0.5" for a turbo* thumb rule, you can say that the optimum size for a 2 litre car is 2.5". But for those of us with 2.3 litre engines, we would find it tricky to obtain a 2.8" inch system.

I personally feel that it's always safer to go over-spec rather than under-spec. Since there is little difference in the cost of 2.5" and 3.0" downpipes sure it makes economic sense to go for a 3.0" downpipe and save yourself the cost and trouble of possibly upgrading it later on?

Or am I over-simplifying things.
 

·
Registered
Joined
·
11,343 Posts
Originally posted by Adrian W:
[qb]But if you bolted a 6" diameter pipe to your turbo you would likely lose power.  There's a happy medium here, and I wouldn't jump to the conclusion that 3" is it.          
[/qb][/b]
The fact is that nobody here has bolted a 6" diameter piper to the turbo (AFAIK). So that part of your post is a red herring.

The fact is that there is a significant increase in power as the result of using a 3" downpipe. This fact was verified by rolling road sessions before and after the 3" downpipe.
 

·
Registered
Joined
·
1,448 Posts
Originally posted by /john:
[qb]The fact is that nobody here has bolted a 6" diameter piper to the turbo (AFAIK).  So that part of your post is a red herring.

The fact is that there is a significant increase in power as the result of using a 3" downpipe.  This fact was verified by rolling road sessions before and after the 3" downpipe. [/qb][/b]
AFAIK no-one has made a back-to-back comparison of a performance 2.5" and 3.0" on the same car. Sure a 3" downpipe will flow more than a stock, dual-catted (or even just stock single catalyst), 2.25" pipe ... but the original question was whether a performance 2.5" pipe would suffice in place of a 3" pipe. For under 400 hp I believe it would do just fine. At the very least, whether it would suffice is more likely to come down to the radii of the bends, and flow through the replacement catalyst.

Also, the 6" pipe analogy was not a red-herring. It was an attempt to illustrate the concept of overkill on exhaust downpipes.

Adrian W~
 

·
Registered
Joined
·
1,448 Posts
Originally posted by Jason (Mr Torque Steer):
[qb]Nor would I jump to the conclusion that all people who have upgraded from a standard (or perhaps even 2.5") to a 3" downpipe are deceiving themselves when they notice a performance increase.        [/qb][/b]
I don't know why you guys are jumping on me over this. The original question was whether a 2.5" performance exhaust would suffice in place of a 3.0" performance exhaust on a car that is going to push between 250-300 bhp. This is NOT an argument over whether a 3" flows better than the stock exhaust.

Here's the page from Corky Bell's book on exhaust diameter. It's based on Exhaust gas velocity, which is probably a better method of estimating proper size than engine displacement. (I'm sure the 750 hp 2.0L Viggen hillclimb car needs a larger exhaust than my mostly stock 2.3L Viggen does.)



Adrian W~
 

·
Registered
Joined
·
14,305 Posts
The original question was whether it was necessary to have a 3" downpipe for a stage 3 ECU.

The simple answer is that if the tuners normally supply a 3" downpipe, then yes, you should fit it.

It then moved on to a discussion on theory vs reality.

Whilst a 3" DP may not be absolutely necessary for power below 350hp, those that have fitted one have noticed both seat of the pants and dynoed improvements in performance.

To say that it is not worth it is therefore incorrect.

In the particular case quoted by Grentarc, there is already a performance exhaust system fitted with a radiused exit bend from the turbo, removing most of the restriction. It's probably true to say in this case that a further reduction in downpipe resistance increase is not necessary.

Having said that, I had the very same system and changed just the downpipe for a 3" one and noticed and improvement in spool up.

So, there's theory and the real world. Whilst no-one is wishing to argue with the complex theory, practical experience has shown that a 3" DP is of benefit at levels below 350hp. I don't have to have 4 pot caliper brakes, but they are better than standard...
 

·
Registered
Joined
·
288 Posts
One other thing to keep in mind is what SW was used for the comparison. If it is a T5 car the results will vary according to back pressure due to reaction it has on the speed density system...

In a real "comparison" one should optimize the SW for both the 2.5" and 3" and then do the comparison. Just by looking at some dyno values wont get you far when speaking of "generialization"...
 

·
Registered
Joined
·
11,343 Posts
Originally posted by Adrian W:
[qb]AFAIK no-one has made a back-to-back comparison of a performance 2.5" and 3.0" on the same car.  [/qb][/b]
I have.

Originally posted by Adrian W:
[qb]Also, the 6" pipe analogy was not a red-herring.  It was an attempt to illustrate the concept of overkill on exhaust downpipes.      
[/qb][/b]
It is not an illustration, it is hyperbole.
 

·
Registered
Joined
·
1,448 Posts
Originally posted by /john:
[qb]I have. [/qb][/b]
And what was the difference in horsepower? 5? 6? 10? I saw 7 bhp difference in 3 consecutive runs on a dyno with the SAME car and no physical alterations.
Had I made some physical alteration it sure would have looked like it made some serious horsepower ... and that is exactly the problem I have with all these claims of "dyno proof".

I don't recal anyone putting pressure probes on their exhausts to compare the sizes either; lower backpressure can cause just as many problems as higher backpressure due to the effects of backpressure on wastegate control.

Adrian W~

p.s. ANY illustration of severe overkill will be, by definition, hyperbole. That does not make it a red-herring argument. It does, however, make the statement that it is hyperbole moot.
 
1 - 20 of 44 Posts
Top