I Run a custom TB spacer which is 60mm top side and tapers to 55mm bottom side. Helps keep air pressurized coming in which maintains torque through out the power band. Works for me.
that would be a nozzle which would increase velocity and decrease pressure according to Bernoulli equation (simplification)
take a look at the picture on right side of the wiki link.
http://en.wikipedia.org/wiki/Bernoulli%27s_principleyou have to have the same flow rate through the entire system so when you go to a smaller diameter the velocity has to go up to maintain the flow rate. Bernoulli equation has velocity, pressure, and gravitational potential terms, in this case (1 inch drop) gravitational potential change is quite small, so for the increase in velocity you lose pressure.