Exhaust sizing question

At my last Rallycross, I managed to drop the exhaust off the Neon and get it completely bent around the rear control arms, so it's pretty much completely destroyed at this point. Good news is that I managed to get a couple seasons out of that particular set-up, so I feel like I got my money's worth!

I'm thinking that I'll just go from the turbo to a resonator and then out the rear. The turbo is the stock SRT-4, I'm pushing about 8 pounds of boost on the 2.0 DOHC and probably only pushing it to about 160-180 hp.

I don't know the ID of the O2 housing, but it has a flange on it that has a 2.25" ID stub attached (which is the stock SRT-4 and Neon exhaust size). Would I get any benefit from flaring back out after that stub to a 3" or 2.5", or is one of those things where I need to either go all the way and replace the O2 housing or just leave it at 2.25"? Obviously if cost was no option, I'd just do it, but I'd guess that I'm probably looking at an extra $200-$300 to replace the O2 housing and get an appropriately sized downpipe.

Making the exhaust pipe after the turbo bigger will always help. I have read that the exhaust gases are spinning as they leave the turbine and by quickly increasing the size of the pipe it breaks up this rotation and makes the gases flow straight down the pipe. This usually results in faster turbo spool up.

The sub-350HP Saab 2.3 turbo guys are all about running 3" downpipes with 2.5" cat-back systems, so it can't be that bad of a thing to do.

Yup, 2.5 or better 3" out won't hurt and might help. Only drawback is weight.

Good chart here

going from stock 2 inch after my downpipe to matching 2 1/2 caused a 2 psi increase in boost on my car without any other changes. so just keep an eye out for that if you go larger

4" stacks is the way to go.

I definitely want to go bigger on the exhaust (although I'm pretty sure that 4" might be a tiny bit of overkill and create ground clearance problems).

It sounds to me, then, that even if it is necked down to 2.25" by the existing O2 housing, if I go to something bigger directly after that I will still see some benefit. Not as much as if I was 2.5 or 3 all the way from the turbo, but enough that it would be worth the effort.

Now I just have to decide if I'm going to trim back the rocker panel to go out the side behind the door, or get things bent to go around the gas tank and over the control arms.

Thanks for the info! One more question: what do we think of these: http://www.drgas.com/Boom-Tube-Muffler_p_32.html ? 3" inlet to a 11.5x1.875 opening with a sorta muffler crammed in there. That would give me the ground clearance I need, but would it choke things back down too much?

wae wrote: I definitely want to go bigger on the exhaust (although I'm pretty sure that 4" might be a tiny bit of overkill and create ground clearance problems). It sounds to me, then, that even if it is necked down to 2.25" by the existing O2 housing, if I go to something bigger directly after that I will still see some benefit. Not as much as if I was 2.5 or 3 all the way from the turbo, but enough that it would be worth the effort. Now I just have to decide if I'm going to trim back the rocker panel to go out the side behind the door, or get things bent to go around the gas tank and over the control arms. Thanks for the info! One more question: what do we think of these: http://www.drgas.com/Boom-Tube-Muffler_p_32.html ? 3" inlet to a 11.5x1.875 opening with a sorta muffler crammed in there. That would give me the ground clearance I need, but would it choke things back down too much?

boom tubes are always the proper answer, even if they cost a little power compared to something else.

By way of an update -- and primarily for the benefit of future search queries... -- I just finished up. I have the turbo into the stock (for an SRT-4) O2 housing which is about 2.25", then into a 2.25" downpipe stub, into a 3" reducer, through a flex section, into a Magnaflow glasspack resonator, then through a couple bends and out the rear bumper. I decided against a side exhaust because I was concerned with ground clearance getting the car on and off the trailer.

Apparently, I was having serious restrictions in my old exhaust and didn't realize it. The turbo spools way faster, and there is a ton of additional power on tap. I wish I had easy and free access to a dyno so I could see exactly how much better it is at this point, but the butt dyno says that I was leaving a ton of power on the table previously.

I think my next purchase is going to have to be the 3" O2 housing and a flange to build a straight 3" downpipe. So, to answer my own question, yes, you will get significant benefit going to a 3" exhaust, even if it is necked down to 2.25" at the downpipe.