Im trying to fabricate my own downpipe for my turbo b18c. For reference, stock block looking to make 350whp no more than 400 in the future. Also I have absolutley no welding experience, but do have a friend with welding experience guiding me. I've already bought all the equipment, just need supplies and practice now. Im running a STC hunter bb turbo w/ a 2.5" outlet. Today I went to measure my existing exhaust piping where the downpipe is going to connect and found out its 2.5", seems to be there's a test pipe where the downpipe will connect. It looks like the rest of my exhaust is 3". So my dilemma is do I just do a full 2.5" down pipe or ditch the test pipe and make the downpipe another 1-1.5' longer?

 

Ditch the test pipe, do a 2.5" downpipe with a flared section to 3", then include a 3" flex/mesh section somewhere in the DP (possibly the end).

 

Agreed. Ditch that downpipe and extend 1-1.5" and be 3" all the way.

 

Thanks for the response guys. When looking for flex piping I stumbled across flex bellows, anybody running one of these or have input on them?
https://www.aceraceparts.com/collect...nt=11002653316

I'd like to run a 2.5" elbow first and then flare it to 3" on the way down. Is that going to be restrictive in any way?

 

I just put 4-1 tri racing header on my 92 cb7 with a 2.5 downpipe. with a flowmaster.. sounds nasty. I love it!

 

It won't be restrictive, if anything it should keep the exit velocity up while still allowing room for expansion immediately afterwards. Should be great.

Can't say anything about the bellows - I've actually been curious about these before as well. I've always just used the woven mesh sections.

 

^edited...we do similar setups in that manner every other day

 

I think I'll give it a shot. According to the description they are designed for turbo applications and greatly reduce vibration, (hopefully) saving the manifold. Worst scenario I have to cut it out and replace with a mesh section. Definitely will place it down stream behind the motor.

 

Pulled the test tube off today, found a number of interesting things.



Where my old down pipe connected, the beginning of the test tube, a whopping 2" ID. I can't imagine how restrictive my exhaust was after going through all of this.


After the test pipe, on the section of the exhaust with the resonator, they welded a reducer to match the test tube.


I also found out I have 2 o2 sensors? The closest one pictured runs into the engine harness, assuming into the ecu and the farthest one is for my afr gauge. Do I need to weld 2 bungs on my downpipe for both of these then or can I get away with just 1 somehow?

 

One is your wide band the other is probably the factory O2. You need both depending on how its tuned.

 

I ordered 2 bungs just in case. Does placement matter much? Planning on putting the flex bellow at or behind the motor a bit, then an 02 sensor, then maybe another foot or so and the second 02 sensor?

 

O2 sensor placement should be pretty much inconsequential for you. I'd just make sure you have easy access to both and that they're angled in a way that provides good clearance around them.