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Once the turbo is building up pressure, as a rule of thumb, a smaller engine that would otherwise be deemed underpowered and need to rev up higher to provide power and torque figures similar to a higher-displacement one can become more efficient. Now due to the widespread of direct injection, leaner burn can be implemented to go even further on fuel savings even though it might lead to an increase in nitrogen oxides (NOx) formation, which had been one of the Achilles' heels of Diesel engines. Cooling the air intake charge through a so-called intercooler or some water and alcohol injection is an effective way to deal with the NOx issue, and also to prevent knocks that would happen in a port-injected or a carburettor-fed turbocharged engine operating with a lean mixture at a high compression.
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