Now that the plastic side of the diffuser is in, time to remove the two mufflers, build a cat back exhaust, then install, then finish the driver side of the diffuser.
I was able to reuse some of the stock bends. The stock pipe was 2" od and stainless and cleaned up nice. I bought a 2" 90 degree bend, and 2 - 18" straight pipe. Welded, then painted with high heat paint.
Then install with some clamps. I do have a slight rub problem. More on that later.
Then finished the driver side diffuser in sheet metal and installed. Then the two bumper undertray pieces and then spray bomb in green.
I left the tailpipe exit protrude long, since the diffuser angle will probably change and the end of the diffuser will be lowered. I wanted to have the exhaust exit near flush to the diffuser surface, as this should be the lowest pressure area. I am trying to get a few thermal efficiency bumps on the motor. The TDI motor is not any louder at all with both mufflers removed and I probably shed 40 to 50 lbs. I have read that reducing back pressure is bad for mpg in a TDI and I have heard that it is good for mpg in a TDI. I am going to find out.
I will do a future write up on how each of these mods effected my mpg and drivability. I record my every day commute each way trip mpg. I drive it the same every day at the same mph, so I can see what the mods I do, do to mph.
So far, I found that with the catback, my driving in cruise mode boost pressure is higher than before, like 16 - 17 psi (absolute) versus the 15.5 to 16 psi before. That is the opposite of what I wanted to accomplish.
But, the mpg going up hill went up 10-15%, so the lack of back pressure helped the turbo to spool more efficiently, it appears. So long hills that I used to get 30 mpg going up at 55 mph, I am not getting 35 mpg or so. My level cruising mpg seems the same. But my one-way trip mpg went up consistently 2 mpg each way. Some of this could be because the diffuser and bumper undertray is now finished.
Of course I left the catalytic converter on. Removing it is illegal. I do have a bad tail pipe rattle, where it is hitting the subframe brace. Sounds like somebody is hammer drilling concrete inside the car, at idle. Goes away when in gear on the motor. I need to fix that, it sucks.