This came up in conversation with a fellow turbo car guy at work, so I thought I'd pass along the conversation and get some of your input.

Basically, let's say you have a dead stock turbo 2.... Say the inlet to the turbo is 2 1/4 inches, so let's start there. Is there any reason to increase the size of that pipe from the air filter? I doubt there is a downside to having a 4 inch pipe, but fitment would be an issue. Are there gains to be had with a 2 1/2 or 3 inch pipe? Considering the actual inlet of the turbo compressor is under 2 inches.

From there, the discharge from the turbo is maybe 1 1/2 inches, maybe 1 3/4 inches? From there, the factory pipes enlarge and reduce until it gets to the intercooler inlet that's maybe 2 1/4 inches. Is there any reason to go larger than the intercooler inlet diameter?

Assuming the intercooler outlet is the same 2 1/4 inches, and the throttle body is the same size, is there a reason to have a pipe with larger diameter?

When things are modified, the question gets more complicated, My personal feeling is the factory has price restrictions to work within, but we don't, so is there gains to be had in pipe sizing?