When Mercedes-Benz sends a model to AMG for high-performance finishing school, it’s hardly news. But until the CLA, nearly every car to make the trip to Affalterbach was packing an engine with a “V” cylinder layout. (The C30 five-cylinder diesel was the lone exception, and it never made it stateside.) So when the power brokers at AMG began dropping not-so-subtle hints that they’d already had their way with the CLA’s inline four-cylinder powerplant, our curiosity was piqued. Would the result be a roguish, hair-triggered toy or a true CLS63 AMG mini-me, blending breathtaking power with Mercedes style and luxury? If you’re expecting the deep-throated burble of its bigger AMG brethren on startup, you’ll be disappointed. The CLA45’s exhaust note at idle is more akin to an air compressor kicking on in the neighbor’s garage than a menacing V-8. But it would be a mistake to write it off on tone alone. AMG has managed to coax 355 horsepower and 332 lb-ft of torque from the 2.0-liter engine, much of it on the shoulders of a twin-scroll turbocharger capable of force-feeding the engine with up to a whopping 26.1 psi of boost. To make sure the internals stay contained under all that duress, AMG uses a special crankshaft, pistons, and a sand-cast aluminum block, which is stronger than the die-cast unit found in the pedestrian CLA250. A dual-clutch seven-speed automatic transmission with three driving modes funnels the prodigious output through a standard all-wheel-drive system. Thanks to a butterfly valve plumbed into the exhaust, the CLA45’s song begins to sound sweet around 3000 rpm and gets better as the revs climb. The power comes on early, pulling with the steady determination you’d expect from an engine that produces all 332 lb-ft of peak torque from 2250 to 5000 rpm; redline is 6700 rpm. Stay into it until it’s time to shift—automatically or initiated by the steering-wheel paddles—and you’ll be rewarded with a downsized version of the familiar bruuuump that accompanies gear swaps in the bigger AMG models. Using launch control, which allows the engine to climb to 4000 rpm before storming off the line catapultlike, we posted 0-to-60 times of 4.2 seconds and knocked off the quarter-mile in 12.8 with a trap speed of 110 mph. Although the CLA has a front-drive bias, torque is variable up to a 50/50 split, the distribution based on a laundry list of factors: vehicle speed, steering angle, gear selection, accelerator position, lateral and longitudinal acceleration, and any differences in wheel speed. All that processing power translates to confidence-inspiring stability on the straights and mild understeer in the corners, traits that had us wondering if the CLA45 might be too sterile in our first drive. We found the speed-sensitive electric power-steering assist to be accurate if not chatty.