2007 Ford Chevy Dodge Diesel Comparison - Big Boost Battlefield

GM Duramax 6600

Once past 1,600, Duramax delivers 600 lb-ft or better and remains the power champ (at least for now). Note the steep curve where the turbo begins to come on boost.

Dubbed the LMM family, the new Duramax wins honors for most power right now. Since the "old" Duramax had an EGR cooler and size is unchanged, engine weight gain is negligible but figure on another 80-120 pounds for exhaust and cooling upgrades. Although the oversquare bore and stroke are the same, compression ratio has dropped to 16.8:1. That was the lowest achievable value with good cold-start characteristics; the LMM uses both intake air heaters and glow plugs.

The variable-geometry turbo is a new boreless unit from Honeywell, oil pressure-powered with electronic control, and has a water-cooled center housing (in the cooling circuit, not a stand-alone, run-after-shutdown setup). Common-rail handles injection chores, at up to five events per cycle.

A GM engineer referred to the muffler as a "tuner," but it's more like a resonator than a true muffler. The DPF is shielded and the exhaust uses a patented "venturi" outlet and air gap "cones" at the ends. There is a warning in the upfitter's manual that there are some constraints with the "active exhaust system."

GM noted that DPF regeneration is based on a host of inputs, including exhaust temperature and backpressure ahead of and behind it, hours, miles, duty cycles, all calculated by an ECU (hence the big software improvements) to determine when regeneration occurs. On average it will happen once per tank of fuel consumed.

Cooling systems have all been upgraded, though typical operating temperatures are the same as current models so you need not rethink your warning zones. Integration with the automatic transmission allows the VGT to close vanes and the intake throttle to close, generating some engine braking, especially in tow/haul mode.

GM has not announced pricing for the pickup version of the Duramax LMM, but Top Kick/Kodiak C4500 models in '06 had LRX/LPD Duramax option prices of $6,946/$7,127 and the '07 LRX and LYE "new" engines are listed at $9,259 and $9,639, respectively. We're guesstimating $6,000-$6,500 on Silverado and Sierra HDs.

Apart from lower compression ratio, Duramax long-block has changed least of the Big 3. Both glow plugs and intake heaters are employed for cold-start performance. Common rail systems work much like gasoline systems by metering fuel right at the injector rather than at a pump some distance from the injector. Typical injection pressures would erode most materials not designed for 25,000+ psi.

The New Look in Ashtrays

A DPF essentially traps leftover particulate matter in the exhaust system and automatically regenerates itself by burning off those particulates, all controlled by the air and fuel management in the engine and the catalyst immediately upstream of the DPF. The DPF does not require routine service though after time it may need to be cleaned of built-up ash, a process we expect won't be too pricey, nor inconvenient. Regeneration is transparent to the driver and requires no action on anyone's part, but when it happens, temperatures in the DPF may well exceed 600 degrees C, so before you go crawling around under there looking to see what you're stuck on, or decide to route the pipes through the bed for the "stack" look, be aware of what you're getting into.

1/2-Ton GM Diesel Coming

Last fall GM announced they will have a diesel V-8 engine for use in vehicles under 8,600 GVW "after 2009." The four-cam, four-valve engine will use aluminum heads with integrated manifolds, an iron block, and the latest common-rail injection system and will take up no more space than a small-block gas engine. Said to be best-in-class against benchmarks that were the best of "everything GM could buy," expect this new diesel to be offered first in high-end pickups and fullsize SUVs.

Specifications