[97R855]

I think this is the tube that connects the turbo to the intake. Is this correct? Whats wrong with the stock part and what will you gain by changing it?

Also, are the cats themselves restrictive? If you are going to install cat backs only seems like this with now be the bottleneck in the exhast and will net only a few horsepower. Is it worth it to change just the cat backs without changing the cats or downtubes?

 

[dave850r]

The downpipe is the part that connects the exhaust turbine side of the turbo to the catalytic converter. It is is piece the oxygen sensor is screwed into. The advantage a bigger downpipe gives is less restriction, leading to quicker turbo spool-up times. Usually the biggest gain comes from the catback exhaust, then smaller gains from the downpipe and catalytic converter. Some catalytic converters come very free flowing from the factory, I'm not sure how the 850's is though.