Does downpipe and exhaust diameter influences the spool time and torque of the engine? Some people use wery short exhaust on turbo engines.

 

yes.. that depends about what setup ure runing.

 

straight 5" downpipe, anything smaller with a muffler wont fit the turbo outputs and give you a 5900º egt

 

You are talking about two different things. The diameter influences spool time, as does the length of the exhaust. Open downpipe is pretty much the most unrestrictive, and also the shortest lengthwise. Depending on your setup, a 2.5" or 3" is probably a safe bet. 3" becomes a bit of a restriction up above 450whp or so... So plan accordingly.