learning: turbo exhaust sizing

Ok, so I'm very new to turbos, but now I have one as part of my cheap green saab project. EDIT: Link to Build

It needs basically a brand new exhaust, turbo back. If I'm going to fab an exhaust, I want to do it once. My main question is sizing this stuff.

Pertinent (or maybe not) info:

1. Stock exhaust piping looks to be 2 1/4 diameter.
2. Exhaust outlet of turbo (both stock t25 and the slightly bigger TD04 I want to run) measures 2 1/4 inch diameter opening - it could likely be ported a bit, since the flange seems to be a bit of a pinch point, but not much.
3. The only 'tuner' pre-made exhaust stuff I can find is 3 inch downpipe and 3 inch cat back.
4. One of my main goals is keeping daily driveability, so I will run a cat (IL, so I think I can get away with a 'race style' or 'high flow' cat), and some sort of resonator/silencer or muffler.
5. I will have the ability to re-tune the engine to maximize whatever I need to with the new exhaust.
6. Other goals are: to be able to run the turbo to its max output, low cost, low weight.

My Questions:

1. How do I know what sizes to use? downpipe and catback the same size? 3 inch? Seems silly to be THAT much bigger than the turbo opening, but maybe going that big helps the turbo?
2. Does length matter (total exhaust system length or space between the turbo and cat, or cat and muffler, etc)?

Go big. 3" or bigger. You want as little back pressure as possible because the turbo has a ton built into it already. It also helps with noise quite a bit.

For weight savings, you can shorten it and dump it out the side, in front of the rear wheel, also avoid as many bends as possible and go with mandrel bends.

Get a modern, performance cat that has very little restriction and stick to mufflers that are fairly wide open, so something like a Dynomax Race Bullet, not a Super Turbo or chambered muffler.

Here's some actual data (while a bit aged) to look at:

http://www.gusmahon.org/html/mufflers.htm

http://www.thedodgegarage.com/caravan_backpressure_testing.html

Not sure if it applies to your turbos/engine, but on old Turbo Dodge 2.5 cars, if you ran a big exhaust(like 3in) it would flow so much that the wastegate becomes too small to regulate the amount of exhaust gas flow, so would boost creep like hell.

Just FYI, no clue about Saabs. But the 2.5 and 2.2 T1 cars had a Mitsu TE04 turbo.

2.5" is what the standard aftermarket performance exhausts are, and are usually paired with a 3" down pipe. For higher than stock power I would shoot for 3" all the way back. I'll be watching what you come up with. My exhaust is in good shape, but is the next bottleneck.

I agree with other posters. A 3 inch down pipe but then a 2.5 inch exhaust for a stock or mostly stock engine. A full 3 inch for a modified one.

One thing I've read is large down pipes help break up and slow the spinning motion that the exhaust gases have when they exit the turbo.

I have a case study for you...

I had a 2001 Saab 9-3 2.0T, TD04 turbo. Car made 320whp.

The exhaust was a 3" SS Genuine Saab downpipe to 2.5" Saab sport exhaust. The exhaust had a nice burble under normal driving, but had no drone. From a performance/comfort balance standpoint it was ideal.

From a power standpoint the intake pipe is very valuable as well (J pipe I think it was called?).

I'm also curious as to when it would be best to go from a 2.5" to a 3" exhaust, and down pipe only Vs. full 3". I've got a full 2.5" system on my Galant VR4. The turbo is a TD06-20G at 20 psi, I'm sure a 3" would free up some power. But the car sounds nice as it is, doesn't drone, and doesn't hurt for power. I'm wondering if a down pipe only would be the best of both worlds.

That said, I've seen some cars make very good power on 2.5" exhausts. I think the overall quality and construction is as important as size. Minimize bends, and make them large radius and smooth. Based on the original post, I'd say 2.5" would be just fine- unless you plan on more serious upgrades and a larger turbo in the future.

I've also seen small turbo cars make more power with a 3" exhaust, but not directly due to the exhaust. The boost level went up because the wastegate could no longer keep up.

In reply to flatlander937:

Just port the wastegate.

Also the answer is to just run more boost. :)

NordicSaab wrote: I have a case study for you... I had a 2001 Saab 9-3 2.0T, TD04 turbo. Car made 320whp. The exhaust was a 3" SS Genuine Saab downpipe to 2.5" Saab sport exhaust. The exhaust had a nice burble under normal driving, but had no drone. From a performance/comfort balance standpoint it was ideal. From a power standpoint the intake pipe is very valuable as well (J pipe I think it was called?).

I love this case study. thanks for responding. May I ask what else you had in your setup? Stock intercooler?

In reply to Stefan (Not Bruce):

thanks, your info is helpful.

Thanks for the replies so far all - keep em coming.

I'm having trouble understanding why a 3 inch downpipe would be any benefit at all over a 2.5 inch if the cat back is 2.5 inches... can anyone help there? Seems like the 2.5 inch catback would be the bottleneck so the extra space in the downpipe would be next to useless.

maybe because the catalytic converter is the most throttling piece of the system so a 3 inch cat for an otherwise 2.5 inch system is better?

Exhaust gases slow down as they cool, stepping the diameter down should keep it warmer longer and help it move more quickly in the pipe.

The challenge is knowing how much to step and where to maximize exhaust velocity, not too mention how to construct the system :)

Hence the simple answer to run the same size pipe all the way out to make it simpler to build, etc.