Turbo Diesel V8 6.7L

What it is

Bore is 107 mm, stroke is 92 mm and the compression ratio is 17:1, which comes to 6,618 cc across eight cylinders in a 90 degree vee. No production diesel carries that bore with that stroke. The Ford 6.7 Power Stroke and the GM Duramax 6.6 share the architecture, and that is the whole of the claim. Torque on the dyno comes out of a two-part heat release, a sharp premixed spike near top dead centre carrying 60 percent of the heat and a slower diffusion tail carrying the rest, solved against crank-slider geometry with the plenum, the runners and the collector filling and emptying around it.

The roster carries a naturally aspirated version of this engine, and the two match part for part: the same bore and stroke, the same 17:1, the same 1.266 bar of friction MEP, injection starting 9 degrees before top dead centre on both, a breathing peak at 2,000 rpm on both. A compressor and a cooler are the only difference. Compression stays where it is once the turbo arrives, because a diesel lights on the heat of compression alone and no end gas sits ahead of a flame front waiting to go off. The petrol V8s here hand compression back the moment a blower or a turbo shows up. Run the two diesels one after the other and boost is the only variable you changed.

Everything on this turbo derives from displacement except the turbine housing. Throat area is set at 0.12 of the wheel disc against the 0.17 the sizing rule picks on its own at a 2.4 bar target, and tight is how a working diesel gets matched: more backpressure, more turbine power out of weak exhaust, an earlier peak, a worse top end. The compressor measures 97.6 mm and the turbine 85.9 mm, and the intercooler takes 72 percent of the compressor's temperature rise back out.

Things to try

• Open Housing A/R from 0.12 toward the 0.25 the 7.3 Power Stroke on this roster runs. The turbo goes lazy off idle, breathes better near the limiter, and peak torque walks past 3,000 rpm.
• Take Boost target from 2.4 bar absolute down to the 1.0 bar floor. The curve walks back toward the naturally aspirated V8 on the same shelf, which is this engine with nothing bolted to the exhaust.
• Pull Intercooler from 72 percent toward zero. Charge temperature climbs, density falls, and fuel falls with it, since diesel fuel mass here is air mass divided by one smoke limit. No KNOCK chip arrives, because compression ignition in this model cannot knock.

Seen in

Ford Super Duty (F-450)

Same layout

2011 onward. Ford's own 6.7 Power Stroke replaced the bought-in Navistar diesels here, and it has been the only diesel in the Super Duty range since.

Photo: Matt Weissinger · Pexels

Chevrolet Silverado HD

Same layout

2001 onward. The Duramax 6.6 has been the diesel in the 2500HD and the 3500HD, and in the GMC Sierra HD beside them, for its whole life.

Photo: Zak Mogel · Pexels