I know, I 'm was just playing off the joke.
I too thought of an oval shaped pipe but to create was was too much trouble. My best plan on that was two 2.5" pipes cut open lengthwise, then spread apart and tack welded together, finally flattened down a bit with a hammer to the right shape and then seam welded. Or buy something if they have it but I couldn't find anything convenient.
Internal wastegate makes it more difficult. You would need a 3" to make good use of it. You could transition to a 2.5" exit but it would suck for high flow applications I think.
3" downpipe huh? Not a chance that it will fit and still be able to make the turn down, the radius is too big. Unless you get a less than true 3" pipe and contort it to make the turn. With the full size turbo you have the fact that you would want to keep it level for long bearing life, then the intake is a full 3" that must be able to make that turn and hook up, then the up pipe must provide clearance for the transmission oil fill plug to be serviced....can be a pain if you forget that one! :idea:
Then the AC accumulator must be at least a 1/4" away and room for heatwrapping on the pipe so you don't rub through it or conduct heat right into it....so I just can't see it working without removing something from the car itself like the upper radiator support member and the AC accumulator.
Besides, the normal discharge diameter of the T3 flange is 2.5" + whatever size the wastegate is. I've seen people use the 5 bolt flange adapter assembly and plumb a 2.5" pipe along with a small 1.5" pipe or similar size for the outlet of the wastegate. Not worth the trouble really.
An external wastegate allows the port to be ducted as in the sketch, and then combined at any convenient point later, or atmosphere. Way more flexible for just a couple hundred more dollars.
Now if you actually had ROOM there it would be a different story and I'd use an internal gate.
As far as combining back into a 2.5" two feet from the turbine, I'm not sure how benefiical that would be.
Sure the fluid speed will change with the differen't sections, slowing down then speeding up. But I tend to think of it in terms of fully developed flow with regard to the throughput of the system as a whole is most important.
Maybe you've got a point but I know this much for certain, there is enough exhaust pressure build up on the pressure side of the turbine to push open the 4lb wastegate springs on their own without help from the boost line, at just 10-12psi pressure on the intake side. This tells me that with over 4psi exhaust pressure you need to have a bigger setup on the outlet of the turbo to really flow well. And we know that it is the pressure differential that spins the turbine anyway.
