From the June 2016 issue
Turbocharged engines make big torque at low rpm, but those two numbers, peak torque and its corresponding rpm, don?t tell the full story, as boost pressure isn?t solely a function of engine speed. A turbocharger?s ability to make boost (and inflate torque) depends on the energy in the exhaust stream that spins its turbine.
The peak engine output that automakers advertise and owners brag about is measured in laboratories where engines are held at a constant rpm with the throttle wide open and the turbo can take its sweet time spooling. That time is a key characteristic, rarely reported but highly telling, of a boosted engine?s responsiveness. Porsche engineers acknowledge that the newly turbocharged 911 Carrera needs a full three seconds at wide-open throttle to achieve its maximum torque at 1800 rpm. On the road, that delay is actually shorter because the revs rise as soon as you mat the throttle. Still, the hesitation can be felt during the transition from steady-speed cruising with a small throttle opening to full-wood acceleration. It takes time, a.k.a. the infamous turbo lag, before the engine?s pistons pump enough hot exhaust to accelerate the turbine and produce boost from the compressor.
To quantify what a driver feels from behind the wheel, we instrument-tested the naturally aspirated V-6 Chevy Camaro and the turbocharged four-cylinder Ford Mustang and tracked how quickly their engines respond to sudden transitions. We cruised at constant r...