Ford and the Case of the Case of the Missing Compressor

We’re still wrapping our minds around Ford’s all-new 6.7-liter Power Stroke V-8 turbodiesel engine that debuts in the 2011 F-Series Super Duty trucks and learning about the differences of the two slightly different versions that Ford has planned.

The 6.7-liter PSD’s unconventional design features a reverse flow architecture that flips the intake and exhaust manifolds around so that the intakes are on the outside of the engine and the exhaust exits into the valley between the two cylinder banks - directly into the engine’s turbo.

This unique setup dramatically cuts the distance that exhaust gases travel to reach the turbo, improving turbo response while also protecting nearby powertrain components, such as the fuel pump and alternator, from excessive heat.

The 6.7-liter PSD’s turbo was designed with help from Honeywell’s Garrett division. Instead of using the current 6.4-liter Power Stroke V-8 diesel’s dual sequential turbo setup, the 6.7-liter PSD features what Ford calls a single-sequential turbo. The SST turbo places two compressors back-to-back inside the same housing. Why? As modern diesel mills have become more powerful, emissions limits have gotten tighter, and it’s become harder to match the turbo’s dual jobs of exhaust gas recirculation with acceptable responsiveness.

Ford F-250, F-350 and F-450 pickups will receive a high output version of the 6.7-liter PSD that meets very tough emissions standards for these lighter-duty HD work trucks – tested using a full vehicle dyno. F-550 and bigger chassis cab models will receive a lower output engine that meets less rigorous medium duty clean air standards – tested using an engine dyno only.

Here’s where it gets interesting. We already knew that the F-250, F-350 and F-450 pickups will use an aluminum “hot-side valve” that controls the volume of air into the engine’s EGR system, which helps control emissions. The F-550 and up chassis cab models will use an iron hot-side valve.

Now, thanks to the sharp eyes of our friend David Kennedy, editor of Diesel Power Magazine, we also know that the F-250, F-350 and F-450 pickups will be the only Super Duty models using the SST turbo. The chassis cab F-550 and bigger trucks will use a conventional turbo setup with a single compressor instead of the SST’s Siamese compressors.

Look at the pictures for a comparison. The first picture (top) is a full cutaway of the SST turbo. The second picture (bottom) is a close-up of the single compressor turbo. Where it’s smooth on the left-hand side, that’s where the other blades would be in the chassis dyno version.

We asked Ford about it.

“We have a single compressor wheel in the dyno certified version of the F-550,” said Adam Gryglak, Ford’s chief engineer for the 6.7-liter PSD engine program. “I can’t get into specific performance objectives, but the conventional single wheel met our objectives while delivering the EGR-driving capability for the dyno certification. The dyno cert engine will be rated lower than the chassis cert product.”

There you have it. The case of Ford's missing compressor, solved.

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