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Discussion Starter · #1 ·
I am not a noob to modifying turbo cars, but I am a noob to Saabs. I have noticed that these cars tend to be extremely sensitive when it comes to modding them. If any small thing isn't right, it seems like it won't hesitate to throw a code and go into "limp mode". So my knowledge of what has worked on past turbocharged cars may not fully apply on these since this car is just to damn smart (well in comparrison to my turbo Mitsubishi's that is).

Here is my question. I plan on constructing my own turbo back exhaust for this vehicle. I have noticed something that seems quite odd to me though as far as exhaust sizing goes. I have read of a lot of people running a larger diameter downpipe (3"), and a smaller cat back (2.5"). That to me sounds completely backwards. On any other platform I've worked on, the methodology was to start from the back and work your way forward. As exhaust gets further towards the rear of the car, the more time it's had to cool off and become denser and slower moving in comparison to the air directly after the turbo. With that in mind, everyone always starts from the back with a large cat back (3"). As for downpipes, if you were still in a low/moderate power level, going with a 2.5" downpipe is very common. The air coming right out of the turbo is extremely hot, and moves much faster, so the smaller diameter pipe works fine. But once it starts reaching the back of the car, it becomes cooler and slower moving, which is why you'd step up the diameter a hair to allow for that. Once you start making some better power, well then a 3" downpipe becomes necessary.

So why do I notice people doing it completely backwards on these vehicles? It goes completely against anything I've ever ran into before. Does it have something to do with how "smart" the cars are? For my exhaust setup I was planning on doing my typical 2.5" downpipe to a 3" catback, then going with a BSR stg3 ECU mod, since at that level I'd consider it to still be in that low power range in comparrison to other platforms I've worked with.

Please share anything you know with me. Any tips or tricks as far as buliding my own exhaust goes. Is there something I'm going to have to do to trick the rear O2 sensor due to the increased exhaust diameter? Anything you have to share will be greatly appreciated. Thanks!
 

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Halon said:
I am not a noob to modifying turbo cars, but I am a noob to Saabs. I have noticed that these cars tend to be extremely sensitive when it comes to modding them. If any small thing isn't right, it seems like it won't hesitate to throw a code and go into "limp mode". So my knowledge of what has worked on past turbocharged cars may not fully apply on these since this car is just to damn smart (well in comparrison to my turbo Mitsubishi's that is).

Here is my question. I plan on constructing my own turbo back exhaust for this vehicle. I have noticed something that seems quite odd to me though as far as exhaust sizing goes. I have read of a lot of people running a larger diameter downpipe (3"), and a smaller cat back (2.5"). That to me sounds completely backwards. On any other platform I've worked on, the methodology was to start from the back and work your way forward. As exhaust gets further towards the rear of the car, the more time it's had to cool off and become denser and slower moving in comparison to the air directly after the turbo. With that in mind, everyone always starts from the back with a large cat back (3"). As for downpipes, if you were still in a low/moderate power level, going with a 2.5" downpipe is very common. The air coming right out of the turbo is extremely hot, and moves much faster, so the smaller diameter pipe works fine. But once it starts reaching the back of the car, it becomes cooler and slower moving, which is why you'd step up the diameter a hair to allow for that. Once you start making some better power, well then a 3" downpipe becomes necessary.

So why do I notice people doing it completely backwards on these vehicles? It goes completely against anything I've ever ran into before. Does it have something to do with how "smart" the cars are? For my exhaust setup I was planning on doing my typical 2.5" downpipe to a 3" catback, then going with a BSR stg3 ECU mod, since at that level I'd consider it to still be in that low power range in comparrison to other platforms I've worked with.

Please share anything you know with me. Any tips or tricks as far as buliding my own exhaust goes. Is there something I'm going to have to do to trick the rear O2 sensor due to the increased exhaust diameter? Anything you have to share will be greatly appreciated. Thanks!
usually the exhaust size is ramped down to something close to the original diameter to prevent pressure issues (popping, cracking, etc)

You can run 3" all the way back if you like, but it'll pop alot.

The O2 sensor should not throw a code for exhaust flow, but rather catalytic converter inefficiany. That is what the rear o2 sensor is for, to make sure the cats are working.

Larger the downpipe the better. I know I will be building a 4"in DP for my Turbo-X and probably scaling it down to 3" and 2.5" mufflers.
TIG here in a few weeks! woo! Professional looking work here-I-come! haha.


Funny... it's an ESAB too... swedish tig to make parts for my swedish car! :lol:
 

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lmpreza said:
usually the exhaust size is ramped down to something close to the original diameter to prevent pressure issues (popping, cracking, etc)

You can run 3" all the way back if you like, but it'll pop alot.

The O2 sensor should not throw a code for exhaust flow, but rather catalytic converter inefficiany. That is what the rear o2 sensor is for, to make sure the cats are working.

Larger the downpipe the better. I know I will be building a 4"in DP for my Turbo-X and probably scaling it down to 3" and 2.5" mufflers.
TIG here in a few weeks! woo! Professional looking work here-I-come! haha.


Funny... it's an ESAB too... swedish tig to make parts for my swedish car! :lol:
4" DP, sounds like a bit of overkill no?
 

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johnnyrocket said:
4" DP, sounds like a bit of overkill no?
No.

One of the highest quality downpipes for Subaru's was the TurboXS 4", Bellmouth designed, and it tapered down to 3" at the end of it.

This was mated up to a TD04L-13T, which is even smaller than the turbos equipped on the Saabs.

With Turbo-back exhaust, you want as little restriction as possible, so the bigger the exhaust, the better.
 

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Discussion Starter · #6 · (Edited)
Buschur did a test on their 700awhp Evo. Did back to back testing with a 3" and a 3.5" exhaust system. The results at one time showed a loss of a couple hp, then one with a gain of just a couple. Basically showed that any gains he would have gotten from it wouldn't have been worth the added weight of the 3.5" pipe. So he's still running 9's in that Evo, with a 3" turbo back. I don't plan on using anything larger than 3" in my toy car. This car definately won't use anything larger than 3". My goals for this car are not extreme. Just basically a stage 3 BSR setup. My toy car get's the real toys ( http://www.saabcentral.com/forums/showthread.php?t=120143 )


My backround is mainly with Mitsubishi's, and a little bit with turbo Honda's. I have never heard of anyone necking down a system.

Anyhow, my main reason for asking this question is if there are any known things NOT to do with the exhaust system on THESE cars. Not what anyone did on their other cars. THESE cars seem extremely sensative to any changes made, and are quick to throw the CEL. I've done plenty to other cars, but none of them are as "smart" as these ones. I fully understand the purpose of the 2nd O2 sensor. I also believe these cars utilize the standard front and 2nd O2 sensor setup, but also throw an EGT sensor into the downpipe. I've never dealt with a car that, from the factory, places an EGT sensor into the downpipe. I have no clue if these 3 sensors are used in combination to check for who knows what, and throws codes if something isn't quite right. From what I gather, putting an intake on these cars could cause a CEL if the piping isn't sized perfectly. I just want to make sure that nothing like this would happen by putting a larger exhaust on.

Thanks for the replies thus far.
 

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Halon said:
Buschur did a test on their 700awhp Evo. Did back to back testing with a 3" and a 3.5" exhaust system. The results at one time showed a loss of a couple hp, then one with a gain of just a couple. Basically showed that any gains he would have gotten from it wouldn't have been worth the added weight of the 3.5" pipe. So he's still running 9's in that Evo, with a 3" turbo back. I don't plan on using anything larger than 3" in my toy car. This car definately won't use anything larger than 3". My goals for this car are not extreme. Just basically a stage 3 BSR setup. My toy car get's the real toys ( http://www.saabcentral.com/forums/showthread.php?t=120143 )


My backround is mainly with Mitsubishi's, and a little bit with turbo Honda's. I have never heard of anyone necking down a system.

Anyhow, my main reason for asking this question is if there are any known things NOT to do with the exhaust system on THESE cars. Not what anyone did on their other cars. THESE cars seem extremely sensative to any changes made, and are quick to throw the CEL. I've done plenty to other cars, but none of them are as "smart" as these ones. I fully understand the purpose of the 2nd O2 sensor. I also believe these cars utilize the standard front and 2nd O2 sensor setup, but also throw an EGT sensor into the downpipe. I've never dealt with a car that, from the factory, places an EGT sensor into the downpipe. I have no clue if these 3 sensors are used in combination to check for who knows what, and throws codes if something isn't quite right. From what I gather, putting an intake on these cars could cause a CEL if the piping isn't sized perfectly. I just want to make sure that nothing like this would happen by putting a larger exhaust on.

Thanks for the replies thus far.
I'd wager the intake CEL due to improper sized piping is from a lean condition noted by the engine with the same airflow... or something along those lines.

I would not worry about a CEL from an exhaust upgrade myself, as for the weight gain from 3.5" pipe? I don't think it would be a noticeable amount.

I am a bit skeptical of Buscher myself, mainly after seeing them let a car leave with a major upgrade package and NO tune. (local kid has one of those Buscher evos.)
Though they will probably be the ones tuning my Turbo X as there arent many four-wheel dyno's around here.

You can always TRICK the car...

Measure the output of the o2 sensor under a warm idle condition.
Replace with new downpipe
measure output of o2 sensor agaun under a warm idle condition.
if there is a differance, add a resistor to bring the voltage down to the same level as with the stock exhaust.

(this is what we used on our WRX's to remove the cat inefficiancy CEL)
 

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lmpreza said:
I'd wager the intake CEL due to improper sized piping is from a lean condition noted by the engine with the same airflow... or something along those lines.

I would not worry about a CEL from an exhaust upgrade myself, as for the weight gain from 3.5" pipe? I don't think it would be a noticeable amount.

I am a bit skeptical of Buscher myself, mainly after seeing them let a car leave with a major upgrade package and NO tune. (local kid has one of those Buscher evos.)
Though they will probably be the ones tuning my Turbo X as there arent many four-wheel dyno's around here.

You can always TRICK the car...

Measure the output of the o2 sensor under a warm idle condition.
Replace with new downpipe
measure output of o2 sensor agaun under a warm idle condition.
if there is a differance, add a resistor to bring the voltage down to the same level as with the stock exhaust.

(this is what we used on our WRX's to remove the cat inefficiancy CEL)
The CEL caused by intakes is because of the difference in airflow at the maf. As for tricking this car. Let me know of how that works out for you because I have been wanting to build a turbo back for some time now, but like the OP am worried about codes being thrown.
 

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Discussion Starter · #9 ·
I am also extremely sketchy about trusting what Buschur says, but he does occasionally throw out good info. That test I do believe as he doesn't appear to have any way to profit from that test, nor is he trying to bash any of his competition.

Anyhow, as of right now, this will be a project for the summer. I'm going to talk with someone from BSR to ask them their opinions on sizing as I will be going with their tune. Right now though, I'm leaning towards a 2.5" downpipe, high flow cat, mated to a 3" catback. My buddy and I will be building it out of his shop.

Again, appreciate the feedback thus far. If you have any other relavent info, please share. Take care.
 

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BSR employees sporadically contribute here, most often when someone suggests there is a problem with their products. Then they hit us with a barrage of useful information.

I'm not sure exactly why we don't hear from them more often. I know there are language problems. Perhaps they contribute more regularly to STCS.

Halon, can you add your car MY and specs to your profile? It is most helpful to easily identify your year and engine.
 

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Discussion Starter · #11 ·
Ya sorry about that. I just picked it up Tuesday and never added it. Consider it done!
 

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Discussion Starter · #12 ·
lmpreza said:
I would not worry about a CEL from an exhaust upgrade myself, as for the weight gain from 3.5" pipe? I don't think it would be a noticeable amount.
I would agree with that, but the point I was making was that the difference in power he made was not noticable neither, and one time even made less power on his dyno. So based on his numbers, what you get from a 3.5" system was pretty much identical power, a little bit of added weight, and even less money in your bank account as 3.5" piping is a bit more then 3".
 

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In addition to weight, bigger pipe means less room in whatever channels Saab has formed in the underbody. Heat closer to the floor panels. Possibility of exhaust bumping here and there, etc.

I don't know how much room is there, but most of us don't want exhaust bulging off the bottom of the car, particularly if there is no power advantage.
 

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Discussion Starter · #14 ·
So I've been digging around on here quite a bit, trying to see what facts others have found regarding exhaust sizing on these cars. All I seem to find is a lot of theories, a lot of "my buddy has this on his supra", and just plain hear say. I do see the 3"DP / 2.5" Cat back to be regarded as a fairly standard setup. But again in my experience with Mitsu's, I've never heard of anyone doing this. I really wish there were some hard, factual results from people on these cars. Whatever I do build, I fully plan on dynoing the car completely stock, then immediately installing the exhaust and the BSR stage 3 ECU upgrade, then redynoing right then and there on the same dyno, just to get some numbers out there. I urge others to do the same with their setups so instead of throwing around theories, we can throw around real world numbers.

As of now, I'm completely undecided as to what I will do for sizes after all this reading. I am also highly considering going cat-less to save on cost. To prevent throwing an after-cat O2 sensor CEL, I would try the trick that is used by Evo owners who run into the same issue. They screw anti-foulers into that 2nd O2 bung, and screw the O2 sensor into that to remove the sensor from direct contact with the exhaust stream, and only allows a small portion of the exhaust to make it to the sensor.

We also intend on building a jig for this exhaust so we can make a few more to sell here and there as a cheaper alternative to some of these other exhaust I see selling for crack money.
 

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Good luck with the dyno runs. We had some members here long ago who planned to dyno the car before and after certain modifications, but not much came of that.

You can buy an O2 sensor simulator on ebay. I think it just generates a voltage in the expected range to keep the ECM happy. May not work with T8. Sometimes T8 checks the sensors under different conditions to make sure they are behaving as expected. There is usually pretty good detail in the WIS explaining these sensor "evaluations." Your chances of replacing the primary O2 sensor with a dumb device are poor. But the secondary sensor may be easy to replace with a simulator. The WIS may even tell you the "in-range" voltage.
 

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Discussion Starter · #16 ·
I am actually pretty close with the guys at the shop I will dyno at. They're fellow Mitsubishi enthusiasts, and I won't have any issues doing these tests on their dyno. I don't typically dyno there because my Talon is AWD and they have a 2WD dynojet, but it'll work perfect for this car.

We'll see how the anti-fouler method works for tricking the 2nd o2 sensor. It's a $2 mod that's definately worth a shot. If it turns out to not work, then I'll go from there. It works in Evo's, 2G Eclipses, as well as quite a few other applications such as the S2000, TSX. All it does is put the O2 sensor back out of the exhaust stream a bit, and has a small hole in it so that only a small amout of exhaust actually makes it to the sensor, so the air looks a lot cleaner.

In case anyone is confused by my explanation, here is a good read that shows the basic jist of how they do it on Evo's.

http://forums.evolutionm.net/showthread.php?t=114931
 

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So this mod depends on a steady state of oxygen trapped in the tubes. As long as the fittings don't leak it should work.
 

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ctrlz said:
So this mod depends on a steady state of oxygen trapped in the tubes. As long as the fittings don't leak it should work.
Not really. All you're doing is limiting the amount of CO2 that it can measure effectively simulating stock values.
 

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According to the WIS the 2nd O2 sensor performs two functions:

1) It detects failures in the catalytic converter's ability to store oxygen. Normally there should be little variation in the oxygen content of the post-cat exhaust. If the O2 amount starts to oscillate, Trionic logs a DTC.

2) The post-cat reading is used to make minor adjustments to the reading from the pre-cat O2 sensor.
 

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Discussion Starter · #20 ·
Where do the other DIYers get ahold of a flange for where the Dowpipe bolts to the turbo?
 
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