From the August 2016 issue
The Dynamic Pressure Turbo system fitted to the CX-9?s 2.5-liter engine combines elements of twin-scroll and variable-geometry turbos in one small package. Like a variable-geometry turbo, the CX-9 constrains exhaust flow at low rpm to accelerate the gases, which helps spool up the turbo. It produces the same result as putting your thumb over the end of a hose, but instead of doing so inside the turbine housing, as in a variable-geometry turbo, Mazda puts its ?thumb over the hose? upstream. The valve body fits between the cylinder head and the turbocharger. Below 1620 rpm, the three valves are closed, forcing exhaust gas into the smaller passages above the three butterflies. The constricted path accelerates the gas into the turbine and improves the turbo?s low-rpm responsiveness. At 1620 rpm, the exhaust volume is great enough that the valves open and the turbo operates normally. To manage the exhaust pulses as in a twin-scroll turbo, Mazda separates the engine?s waste gases into three distinct branches with a 4-3-1 exhaust manifold built into the head. The two inner cylinders blow their exhaust into a common runner, while the outer cylinders push their exhaust through individual pipes. Separating the gases has two effects. First: Scavenging uses the fast-moving exhaust gases from a cylinder beginning its exhaust stroke to help draw out the remaining low-pressure waste gas from the cylinder just starting the intake stroke in the adjacent passage. ...