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Because three-quarters of all Super Duty trucks are diesel-powered, the new oil-burner is the more interesting of the engines. There is also considerable interest because of the durability problems with the outgoing 6.0-liter PowerStroke engine. Those problems were attributable to the fuel injection system’s need for calibration that was tolerant of various levels of fuel quality, according to chief engineer Pete Reyes. By the 2007 model year, reliability scores for the 6.0 PowerStroke topped those of the tried-and-true 7.3-liter, he said.

Ford says the 6.4-liter PowerStroke is all-new, and that is mostly true. The engine is based on the same basic architecture as the 6.0-liter engine, but the parts are upgraded. Only the crankshaft is carried over from the old engine, with everything else replaced.

The significant change is the replacement of the old hydraulic fuel injection system with a modern common rail system featuring piezoelectric injectors. This high-tech system is key to the new PowerStroke’s ability to meet the tough gas engine-equivalent emissions rules.

The other critical part of the new induction system is the sequential turbocharger assembly. The combination of a quick-reacting variable-geometry small turbo with a high-volume large unit gives the PowerStroke the quick reflexes and maximum power that customers demand. Ford says that tests indicate the new engine powers the Super Duty to 60 MPH a second faster than the old 6.0-liter did. The injection system also contributes to a promised cold start capability down to minus 20 degrees F.

The large catalyst and particulate trap permit the truck to earn the clean bill of health from the EPA, but they do cost a bit of power because of the increased back pressure. Further, the system for cleaning the particulate trap involves sending extra fuel through the engine to help burn off the gunk in the trap, and that periodic fuel use puts a small dent in the truck’s fuel efficiency.

The engineering team balanced emissions, power and efficiency, but because of the new government regulations for diesel emissions, that factor had to take priority. Ford insists that unlike the 6.0-liter’s trouble-plagued launch, the new 6.4-engine is thoroughly tested, with more than ten million equivalent miles of testing in both trucks and on engine dynos.

Drivers who have no truck with diesels can still choose the 6.8-liter Triton V10 gas engine. Transmission choices are a six-speed manual or a five-speed automatic. I drove only the automatic, which is well-calibrated, with smooth shifts in casual around-town driving, while also delivering smart gearchanges in harness. Climbing hills, pulling trailers and hauling a load, the transmission makes good decisions about downshifts, when to hold a gear for a while, and when to upshift.

While Ford doesn’t have a name-brand transmission like GM’s Allison gearbox, the company has experienced good reliability with its much less-expensive in-house transmission, Reyes said.

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