Doesn heat create the velocity in the exhaust gasses to spool the turbo?
No, heat doesn’t create velocity. Heat creates volume. If you look at any of the physics laws for gasses, you will find that pressure and volume and heat are related. PV=NRT is a popular one, The V isn’t for velocity, it is for Volume. The turbine housing is what creates the velocity. The scrolling design that reduces the volume of the exhaust chamber as it scrolls around causes the gasses to have to increase in velocity and pressure to maintain the same flow rate. Hotter gasses have more volume, thus requiring a higher A/R which in effect means that it starts at say 3″ and scrolls down to approximately 1″. Lower temperature gasses are denser and have less volume, so they require a lower A/R housing which would start at the same 3″ volume, as the turbine housings use standard flanges, and scroll down to say 3/4″. Now if you were to reverse the housings in application, the conventional turbo would spool up extremely quick, at say around 1500 rpm but would cause too much backpressure at
