2019 Mazda CX-5 Turbo Winter Test Drive and Tech Dive

The other part of the equation to making big torque at low engine speed is their 4-3-1 pulse converter manifold to improve exhaust scavenging. Dave used the example of an airbrush used for painting. My example is when I was a kid and my parents made me vacuum the swimming pool. We had this vacuum where you hooked up a water hose and turned it on which created the suction force to suck all the leaves at the bottom of the pool into a net. The more proper principal term is fluid entrainment.

A typical 4-cylinder engine has the firing order of: 1-3-4-2. On the exhaust stroke, there’s a big exhaust energy pulse when the exhaust valve first opens and then, not so much as the cylinder pressure has been released and only the force of the piston moving up pushes out the remaining exhaust gas. There is overlap in the exhaust strokes from one firing cylinder to the next. So, Madza has paired the timing so that the big exhaust pulse from one cylinder helps suck/scavenge out residue exhaust gas from the previous firing cylinder. The more residue exhaust gas removed from the cylinder frees up space for fresh oxygen and fuel to burn for more bang. So basically, the way Mazda paired the cylinder exhaust flow paths in the head always has one cylinder scavenging from an adjacent cylinder improving the volumetric efficiency. How is this different from twin-scroll? Twin-scroll separates the flow paths from adjacent firing cylinders which helps prevents the pulses from interfering with each other preventing exhaust reversion, but it doesn’t do much in the way of scavenging as the flow paths are separated all the way to the turbine wheel.

Here are the parts that make the scavenging magic happen. Between the cylinder head and the turbo turbine housing is the valve assembly which closes off the big flow port of the turbine housing. The third little port in the corner of the turbine housing and head is for EGR flow.

The outer ports are for cylinders 1 and 4. Cylinders 2 and 3 share the middle ports. Those valves are made from inconel, the same stuff as turbine wheels, in order to handle the heat. If someone wanted to eek out some more power, they could probably do some port matching and making sharper edges on the flapper valves. The reason the head ports are smaller than the intermediate valve ports is due to casting tolerances. You never want a step smaller in the flow, so they have to oversize the ports in the valve assembly to make sure that never happens. On the valves, you can’t cast sharp edges as the metal won’t flow like that, so there’s a minimum cast radius.