Turbo lag. It’s that uncool and quite scary moment when you mash the throttle of your turbocharged car and nothing happens. And then all of a sudden, the turbo finally comes alive and you’re swiftly going on your merry way. It’s something that drivers had to skillfully deal with in most early turbocharged vehicles. Cars like the Porsche 930 Turbo had the tendency to swap ends mid-corner when the turbo decided to wake up at the wrong time.
Advancements in technology have enabled car manufacturers to reduce or mitigate the effects of turbo lag. Ball bearings in the turbine shaft enable the turbo to accelerate faster and reduce lag. Clever turbine blade design also helps spin the turbo quickly even when exhaust pressures are low. Variable geometry alters the internal physics of the turbo and lets it produce boost at low engine speeds. However, the Mazda 2.5-liter Skyactiv-G engine uses simple tricks of the engineering trade in ingenious ways to practically eliminate turbo lag.
One of these tricks is shortening the exhaust manifold. The shorter the manifold, the shorter the distance the exhaust gases have to travel as well. Not only does this enable the exhaust gases to get to the turbo faster, it also preserves the heat and energy delivered by these gases. The amount of energy present in these gases will determine how fast the turbo will spin, so ideally you’d want energy loss to be kept to a minimum. This is like you taking shorter trips versus longer ones to visit your family during the holidays. The shorter the trip, the more energy you have left when you get to your destination. Which means you’re more able to spend quality time with your loved ones.
The Mazda 2.5-liter Skyactiv-G engine uses simple tricks of the engineering trade in ingenious ways to practically eliminate turbo lag
Another is a trick valve built into the exhaust manifold. Below 1,620rpm—when the engine isn’t producing lots of exhaust gases—this valve closes and essentially reduces the size of the piping which the exhaust gases are flowing through. The slow exhaust gases are now accelerated through this narrower path, and this helps the turbo accelerate faster at low engine speeds. Think of it like pinching the end of a garden hose. The same amount of water flows through, but it does so at a faster rate. You can now either knock down stuff with it or water plants from a distance. Above 1,620rpm, when there are enough exhaust gases being produced by the engine, the valve opens and the turbo is kept happily spinning.
Last but not least is the 4-3-1 exhaust manifold. Internal-combustion engines use a process called scavenging to push out spent exhaust gases and draw in fresh air for the next cycle. For an engine to run smoothly, all exhaust gases have to be shot out of the cylinder before any fresh air is ingested. If scavenging is incomplete, the ensuing combustion cycle will begin with a mix of exhaust fumes and will lead to poor running conditions.
Mazda designed the 4-3-1 manifold to optimize scavenging by pairing cylinders that are next to each other in the firing order. When one cylinder is at its exhaust stroke, another cylinder is about to begin its intake stroke. The cylinder at the exhaust stroke expels its gases at high pressure, while the cylinder about to begin its intake stroke still has some exhaust gases left at a much lower pressure. The high-pressure exhaust gases of the first cylinder will suck the low-pressure exhaust gases out of the second cylinder. This eliminates as much exhaust gases as possible from the second cylinder before it begins its intake stroke, and ensures that only fresh air is used for the next cycle.
Combined with the high compression ratio of 10.5:1 (which is inherent in all Skyactiv engines), the 2.5-liter Skyactiv-G turbo engine produces 250hp at 5,000rpm and 420Nm from as little as 2,000rpm. This amount of torque from such a low engine speed is something that you normally see on similarly sized diesel engines used in pickup trucks and SUVs. It’s no surprise that Mazda deemed this powerplant fit to do duty under the hood of the CX-9 seven-seat crossover.