Exhaust Back Pressure

[BEEB]

I'm looking to replace my resonator with a aluminized steel X pipe, stock exhaust pipe is 2-3/8" OD, with 1/8" steel wall, I can get 2-1/2" X pipe (with 2-3/8" ID) outlets. Now, with the pipes sitting inside the x pipe (see pic, similar theory), would it create lots of back pressure or turbulence? X pipe sits 6 ft away from the engine. Should it be connected with "step up" pipes to create a smooth transition ( stock pipe goes into a bigger pipe)?

My dilemma:

OEM X pipe $800
Aftermarket X pipe with welding job under $300

[320icar]

What kind of vehicle is this? Unless you have a completely built fully ported and polished, race prepped car I wouldn't worry about how the exhaust piping overlaps. Just my .02
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[BEEB]

I'm doing this on a twin turbo 335i. Stock pipe OD is 2-3/8"

I'd many experience with my previous cars, and the most problem I had noticed was on my B16 engine where a 2.5" pipe meets a 2.25" hole into the muffler. Power was dramatically lower in the higher rpm range.

[SpuGen]

Originally Posted by BEEB

I'm looking to replace my resonator with a aluminized steel X pipe, stock exhaust pipe is 2-3/8" OD, with 1/8" steel wall, I can get 2-1/2" X pipe (with 2-3/8" ID) outlets. Now, with the pipes sitting inside the x pipe (see pic, similar theory), would it create lots of back pressure or turbulence? X pipe sits 6 ft away from the engine. Should it be connected with "step up" pipes to create a smooth transition ( stock pipe goes into a bigger pipe)?

So you're going from a 2 1/4" to a 2 3/8"?

1/8th inch step shouldn't make that big of a difference. In your previous car, you had a 1/4 Inch step. Huge difference, even if it's just 1/8th more.

Assuming that pic you posted is the step in question, and you really wanted to create a smooth transition, you could just get somebody to run a weld bead along the step. Then bevel down the flange to match the 1/8" step for the larger pipe's flange.

[BEEB]

that's not my picture, but similar theory.
If anyone had the same experience, I would like to hear if there's any power loss especially on a higher power turbo car at higher rpm range

This is the similar thing I'm looking to do, but shops around town aren't quite friendly for normal work force. Needa take a day off to do so.

[sdubfid]

I am not quite sure what you are trying to say, but your stock exhaust is not 1/8" wall thickness unless bmw is preparing for the apocalypse.

FWIW- My old civic made around 215whp through a stock oem midpipe (no muffler) which necks down to 2". It made 160whp with stock oem muffler in place at the same boost. 55whp gain just by removing stock muffler.

Your stock exhaust is 2 3/8" od as you said which leaves the ID at 2 1/4" if it is not a double walled tube. So in theory your stock piping should support at least 430whp. Which makes sense as turbo s2000s with stock exhaust have made 400-420whp. Efficient-->No
Possible-->Yes

are there any other mods to it?

[SpuGen]

Oh.....

Get the exhaust guy to flare-end the existing exhaust pipes. Then get them to Butt-Weld it, instead of just slip fit + weld.

You only need to do the rear section. So:
- Existing pipe
- Slip fit into X pipe
- Flare Rear section of existing pipe.
- Butt-Weld Xpipe to Rear section.

It really shouldn't matter, but if you want the cheapest smooth transition, with the least amount of work, this would be the way.

Did you check the X-Pipe?
It should've been designed as a slip fit, so you're getting worried over nothing.
Check the inside to see if it flares down in size.

[dared3vil0]

Originally Posted by sdubfid

I am not quite sure what you are trying to say, but your stock exhaust is not 1/8" wall thickness unless bmw is preparing for the apocalypse.

FWIW- My old civic made around 215whp through a stock oem midpipe (no muffler) which necks down to 2". It made 160whp with stock oem muffler in place at the same boost. 55whp gain just by removing stock muffler.

Your stock exhaust is 2 3/8" od as you said which leaves the ID at 2 1/4" if it is not a double walled tube. So in theory your stock piping should support at least 430whp. Which makes sense as turbo s2000s with stock exhaust have made 400-420whp. Efficient-->No
Possible-->Yes

are there any other mods to it?

Do you have dyno charts of the 55 whp? Would like to see them... (I'm NOT doubting you, i just wanna see it lmao)

[BEEB]

It's only JB'd. Not looking for crazy mods as its a xDrive. I am happy if it can reach 400hp.

I guess spugen gave me the answer, slip on the front end and butt weld the rear end. Now I gotta find a shop that would take my car !

[Timpo]

I thought only 2 stroke engines need exhaust back pressure.

[BEEB]

Everyone thinks that their cars don't need back pressure. But I don't know how these people come up with that general idea...