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 rpm, then floored the throttle and measured the time it takes the vehicles to reach a longitudinal acceleration of 0.2 g. To prevent either car from having an advantage with a lower gear ratio, we used fourth gear, where the Mustang’s overall ratio is just 0.3 percent shorter than the Camaro’s.
Starting from 1500 rpm, the Camaro walks away from the Mustang, reaching the 0.2-g mark 0.8 second before the Ford. The Camaro holds on to its advantage when starting from 2500 rpm, where the Mustang makes its peak torque of 320 pound-feet, but there the gap narrows to just 0.2 second.
By 3500 rpm, however, the Ford’s lag is minimal enough to capitalize on its flat torque curve, matching the Camaro in the race to the acceleration benchmark. The higher initial rpm increases the exhaust-flow rate and requires a larger throttle opening to maintain a constant speed, so at 3500 rpm, the Mustang’s turbocharger is nearly up to boost speed even before the throttle swings completely open.
The lesson? Turbocharged engines may claim fat torque curves starting from low rpm, but they’re significantly more responsive if you drive them as you would a naturally aspirated engine: Downshift, then accelerate.
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