Twin Turbo V6 Vs. V8by Richard Rowe
That old adage about there being no replacement for displacement is beginning to look more time-worn than time-tested. Twin turbo V6's are making serious inroads into the performance community, competing with and even surpassing many competitive V8 designs in many ways. However, in 2010, the differences between the two aren't as great as you might think, at least in stock form.
The Nissan GT-R's VQ-based 3.8L V6 puts out between 480 and 500 horsepower (each engine is hand-built, so there's always some variation), making it a good match to the 7.2L Corvette LS7's 505 horsepower. Ford's new 365 horsepower, 3.5L Ecoboost V6 (as used in the Ford Taurus SHO) would similarly give the new Camaro's 400 horsepower, 6.2L L99 a run for it's money.
V8's like those mentioned above rely on the vacuum created by falling pistons and atmospheric pressure to draw air into their cylinders. This approach has worked well for many years, but such "natural aspiration" doesn't work efficiently all the way across the engine's RPM band; power is generally biased to either low-end torque or high-end horsepower. Turbocharged engines (which use a compressor to shove air into the cylinders) can get around most of the RPM biasing with careful tuning and good turbo selection.
Naturally aspirated V8s tend to have a fairly bell-shaped torque curve, which tends to start out fairly low and creep upward to a peak; after that peak, torque tends to taper off toward the engine's redline. Because they use two small compressors (turbos), twin-turbocharged engines tend to make huge torque at low RPM and maintain it well throughout the RPM band.
An engine's "powerband" is loosely defined as the area between the peak torque and peak horsepower. The wider the gap between these two peaks, the more useable and flexible the engine will be while driving. Twin-turbo engines (like the VQ and Ecoboost) triumph in this area, with a 3,000 and 2,000 RPM spread respectively. The LS7 and L99 only manage a 1,300 and 1,800 RPM powerband.
There's more to engine potential than just peak numbers; average power (power produced all through the RPM band) has a huge impact on performance. Although the LS7 and VQ may produce almost identical horsepower and torque figures, the turbocharged engine maintains over 400 ft-pounds of torque for a whopping 3,000 RPM (compared to the 1,100 RPM of the LS7). Although the LS7 and L99 aren't quite as "peaky" as older and less sophisticated designs, they still make comparatively little power outside their peak zones. The turbo engines ability to maintain more power across the RPM band makes for a quicker car on the racetrack and the street.
- corvettes image by michael langley from Fotolia.com