Displacement helps, but a turbocharger helps, too. That's how Subaru's 2.5-liter flat four-cylinder engine produces as much power as the V-6's in the Camaro and the 2011 Mustang.
More importantly, the WRX STi, with its relatively little engine, produces 290 foot pounds of torque, which is more than any of the other cars here. For most of us who drive our cars on the street instead of roaring around race tracks at top speed, torque is what matters. Horsepower lets you go fast but torque, an engine's twisting force, produces acceleration. In other words, it's the power you feel when the light turns green.
With fuel economy at a premium thanks to new government regulations, if not gas prices, you'll be seeing a lot more turbocharging on new cars. (The WRX STi has lowest mileage here largely because it has all-wheel-drive.) Turbochargers use hot exhaust gases to spin a turbine that forces air into the cylinder. That means higher compression inside the cylinder and, therefor, more power with every combustion.
One other fun feature of the WRX STi has to do with the engine control computer chip, a bit of technology that all cars have today. It controls how the car's engine runs and how it responds to the gas pedal. Normally, it's programmed at the factory. You can buy software packages to alter these chips, but doing so will usually void your warranty.
In this car, Subaru lets you control the chip easily and safely with a simple knob. Depending on your preferences, you can make the engine respond nice and easy to the gas pedal or you can make it quick and punchy. You can take it further by going the "void your warranty" route. With the proper programming tweaks, the STi, it is said, can deliver more than 350 horsepower with minimal changes. Not that we would recommend that.
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