At 178 horsepower per liter, the CLA45 just knocked off the Porsche 911 Turbo in claiming the title of most power-dense production engine in the world. AMG’s secret sauce is a BorgWarner twin-scroll turbocharger custom-made for this application and capable of producing up to 26.1 psi of boost. Thus strapped, the CLA45’s four-cylinder makes 355 horsepower at 6000 rpm and 332 lb-ft of torque at 2250 rpm. Although dimensionally identical to the CLA250’s turbo four, AMG’s M133 engine uses a stronger sand-cast—rather than die-cast—aluminum block. There’s also a low-temperature cooling circuit that feeds a water-to-air intercooler.

Unfortunately, those upshifts aren’t as quick or direct as we expect them to be. Mercedes’ new seven-speed dual-clutch automatic was the source of our largest gripe with the CLA250, and it remains so for the CLA45. The hardware is strengthened, but the engine interface still relies on a pair of dry-clutch packs. The Volkswagen Group’s wet dual-clutch transmissions shift faster and more crisply.

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Car and Driver: Mercedes-Benz CLA45 AMG Is Sterile and Lacks the Excitement Of Other AMG Models

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