Cobb Downpipe Construction:
I just received my Cobb downpipe and after looking it over I was kind of questioning some of the design and how it pertains to exhaust flow. I attached an exaggerated illustration.
The end that connects to the turbo is fitted and welded over top of the main pipe. So when looking down through that end you see the main pipe edge and even some small gaps between the outer wall of the main pipe and inner wall of the portion that connects to the turbo (portion what is welded to the main pipe). So will this not affect the efficiency of airflow? Why not have the portion coming off the turbo go inside of the pipe so that the air has no abrupt edge to flow over?
I noticed that too... I'm sure they thought of it, but either way you're restricting flow to make a smaller bell mouth that fits inside the pipe, or using a huge pipe to fit over the current bellmouth.
I did contemplate having a shop grind it down smooth though.... Just a thought.
What you're describing does sound like a non-optimal design. I suspect it was a cheaper way to make the part, but even with that design flaw it's better than stock and is still a good upgrade. Maybe cleaning up those ends would have added $50-$100 to the cost or something so they opted to ignore it.
Assuming I'm reading it right and the box around "Airflow" is the downpipe, that's not how air flows out of the turbo. Air is only coming out of the wastegate and the exhaust outlet (is there another term for this hole?), not the entire back side of the turbo.
I may be reading what you said wrong though as I'm a little confused reading it.
Basically the reason to build it that way would be because they probably didn't have the money to make the pipes from castings or formed metal. Instead they welded two pipes together. But, I agree that it should've been done a little bit better.
378awhp/373wtq Custom Dom 3 & CBRD tuned 2006 STI.
"When in doubt...flat out" - Colin McRae
I haven't contacted Cobb yet to ask them about this little flaw. Everyone else I have asked states that it is still better than the stock downpipe, but I would think Cobb would have made this a little better. I have thought about trying to grind some of it down, but it makes me nervous thinking I might screw something I spend alot of money on. If one were to grind that protrusion on the pipe the downpipe would have to be hanging from above so that the shavings wouldn't fall down and go into the cat.