Why two small turbos are better than a single large one – sometimes
The Ford 3.5L EcoBoost is a twin-turbo engine, with the pair of turbochargers helping the engine deliver its 365hp rating and 420 lb-ft of torque. That's more than 100 horses per liter or, more specifically 1.7 horses for every one of the engine's 214 cubic inches. In an age where one horsepower per cube is still noteworthy, the EcoBoost's pressurized performance is unquestionably admirable, but its torque is the truly impressive trait.
The EcoBoost's peak 420 lb-ft is achieved by only 2,500 rpm, whereas Ford's 5.0L and 6.2L naturally aspirated V-8s need to reach at least 4,000 rpm to reach their peak torque (4,250 rpm for the 5.0L). You have to look at the monster Power Stroke diesel for an engine in Ford's truck lineup that makes its peak torque at a lower engine speed: 800 lb-ft at 1,600 rpm. Even more important than the low engine speed at which the EcoBoost torque peak is achieved is the fact that it holds it over a broad rpm range. Ford says 90 percent is sustained from 1,700 rpm to 5,000 rpm. That's pretty consistent with the general performance of most turbocharged engines.
One of the biggest reasons the EcoBoost achieves its maximum torque so low in the rpm range is its use of a pair of comparatively small Garrett (Honeywell) GT15 water-cooled turbochargers rather than a single, larger turbo. Smaller turbos generally "spool"—get up to boost-producing speed—quicker than larger turbos, which helps the engine make more power faster. The Honeywells can spin up to 170,000 rpm.
It's generally true that a larger turbocharger will make more maximum boost and consequently more horsepower in the engine, but because the spool time may be longer, that extra power – and torque – will come higher in the rpm band. That's fine for, say, a lightweight drag racer that doesn't need maximum torque at the starting line, but for a heavy, production pickup truck that needs low-rpm grunt, a larger single turbo with a
longer spool time wouldn't achieve the down-low torque output like a pair of smaller turbos.
The EcoBoost makes boost almost immediately off idle, giving the F-150 a strong feeling of acceleration. It's a textbook example of bigger not always being better.