Here is the test we did on 08+ intake system. all four runs performed within 20 mins, under stock tune without
maf scaling. red line is AEM cold air intake; green line is with stock air box and filter; dark blue is with no air box no filter nut with stock piping; light blue is with no air box but with stock filter.
http://www.turbobygarrett.com/turbob...pressure_ratio
more vacuum(intake depression) in the inlet, turbo shaft speed will be higher; at high air flow rate, it would further decrease efficiency. On my car, stock air box had 0.05 bar depression on stock boost, while AEM had 0.02bar.