All people are afraid of having too small tube ... its too rare the person who measures the pressure drop!
Bigger is better... no, the right size is compromise of restriction flow, because flow in a pipe make always a rectriction, too big is loss of space, loss of money for ridiculous flow gain.
If turbo output are 1.75", intake are 2"... why put 2.5" pipe between!
A bad quality intercooler scrap easely your bigger pipe gain.
A simple calculation gives you an idea of the right size, a simple test with gauge give you a confirmation of calculation.
http://www.pipeflowcalculations.com/airflow/index.htm
At 5000rpm (Ve:85%) 30psi intake=377ft3/min of air (very big TDI setup end of range!)
2" pipe 20ft long = 1.029psi drop
2.25" pipe 20ft long = 0.553psi drop
2.5" pipe 20ft long = 0.321psi drop
2.75" pipe 20ft long = 0.197psi drop
3" pipe 20ft long = 0.127psi drop
At 2500rpm (Ve:85%) 30psi intake=188ft3/min of air (very big TDI setup at spool up!)
2" pipe 20ft long = 0.261psi drop
2.25" pipe 20ft long = 0.144psi drop
2.5" pipe 20ft long = 0.085psi drop
At 2000rpm (Ve:85%) 15psi intake=106ft3/min of air (stock daily use)
2" pipe 20ft long = 0.088psi drop
2.25 pipe 20ft long = 0.049psi drop
2.5 pipe 20ft long = 0.029psi drop
Air temp 25c, intercooler eff:70% and 0.5psi drop (not2fast turbo calculator)
A take 20ft straight tube because the calculator dont take elbow!
Good reference to see how elbow and reducer make restriction.
http://www.wpb-radon.com/Piping_pressure_drop.html
Dieseleux
If you ever make it up this way, I'm sure we could figure something out, lol!
