Compact Sport Coupe Engine Comparison

Three approaches to 100 hp per liter
One of those benchmark numbers, 100 hp/L is a threshold beyond which only the most pedigreed of high-performance engines reside. Notably, our trio that power the Mini Cooper S, Pontiac Vibe GT, and Volkswagen New Beetle Turbo S make the cut and by three completely different means: supercharging, complex valve timing, and turbocharging, respectively. Notice anything? Yep, performance per liter boils down to moving maximum air through an engine, while burning as many molecules as you can. To visualize these engines' diverse means of hyperventilated power, we romped on the throttles in second gear and put their acceleration under a microscope as they pulled from 1000 rpm to redline.

In Cooper S guise, the Mini's 1598cc engine's Roots-type blower and hood-scoop-fed intercooler tweak its output from an unboosted 113 hp to a growling 163. And our second-gear acceleration graph portrays just what you'd expect from supercharged enhancement: a small engine magnified into feeling bigger, yet with only a hint of supercharger whine at low rpm. Here's an eminently useable powerband, no acceleration lumps or hollows, and with virtually instant-on power delivery when you pop open the throttle.

The Vibe GT's furious little 180-hp engine is a transplant from the Toyota Celica GT-S' engine bay and achieves its 100-hp/L status via the Japanese company's VVTL-i (Variable Valve Timing and Lift with intelligence) valvetrain, a DOHC setup featuring dual sets of cam lobes: one for ideal mid-rpm breathing, the second with lift and duration optimized for high-rev punch. The point here is to achieve, via the physics of air resonance within the induction tract, what the other two engines accomplish by resorting to compressor hardware; it's analogous to the amplifying power a trumpet has on the sound of the trumpeter's lips. An elegant concept, but one with a high-rev payoff (from 6000 to 8200 rpm) that isn't always useable.

The 180-hp New Beetle Turbo S inhales its extra air via an exhaust-driven centrifugal compressor. By comparison with the Cooper S' belt-driven, linear-output supercharger, the 1.8L Turbo S holds an efficiency advantage by extracting some extra energy from its otherwise wasted exhaust heat (think of it as having a two-stage expansion: first pistons, then turbo), but suffers from a typical non-linear wind-up of boost pressure (see graph). Note that this isn't the once-dreaded turbo-lag (that spongy response to quickly opening the throttle), only a reflection of the power curve's natural shape. Is there a winner here? No, there's no need to hold your breath in anticipation. The Beetle's an easy choice, but you have to admire the smaller-capacity Cooper S' masterful use of its lung power.--Kim Reynolds