It's amazing how small engines these days can pack a whole lot of power. Back then, you had to tune beyond the limits to be able to extract as much power as possible, sometimes at the expense of reliability. These days, it's the manufacturers coming up with such engines.

The latest small capacity high output engine hails from Mercedes-AMG, and they've managed to extract over 400 horsepower from a 2.0-liter engine. Not only that, they fitted that engine to their smallest AMGs, the all-new A 45 S and CLA 45 S. That means it makes well over 200 horsepower per liter, all thanks to boost. It even comes with a warranty.

Exactly how much power does it make? Mercedes-AMG's new, boosted 2.0-liter puts out an impressive 421 PS, doling out 210.5 horsepower per liter. Even more impressive is the torque, which is rated at 500 Nm, substantial for a gas-powered engine. This helps the A 45 S rocket to 100 km/h from a standstill in 3.9 seconds. That puts it toe to toe with the V8-powered, 510 PS C 63 S. In the CLA 45 S, it can do the same task in exactly 4.0 seconds. Top speed for both is rated at 270 km/h, electronically limited.

Transferring all that power to the road is a new eight-speed dual-clutch transmission. It then sends it to all four wheels via Mercedes-Benz's 4Matic system. The 4Matic system has been re-tuned by AMG to send a little bit more power to the rear. Also, it comes with more performance oriented suspension with stiffer springs and dampers, along with a lower ride height. Brakes are bigger too and measure a whopping 360 mm in diameter in the S models.

Unlike the AMG A 35 and CLA 35, the 45 S siblings look far from subtle. Both get red brake calipers, large wheels and low profile tires, the brash 'Panamerica' slatted grill, and a pair of aerodynamic aids on the front bumpers called cannards. In the A45 S, it gets a massive tailgate wing to go along with the other aero aids.

But if 421 PS is a little too much, there's the standard A 45 and CLA 45. It makes less power than the S variants but power output is still worth a mention nonetheless. In standard trim, the A 45 can do 0 to 100 km/h in 4.0 seconds while the CLA 45 does it in 4.1 seconds. The speed limiter does come in a little earlier though, kicking in at 250 km/h.