General Motors is taking an approach similar to the folks behind the 6.7L Cummins that will be found under Dodge hoods. The new LMM version of the Duramax 6.6L is much like the current LBZ, but with a diesel particulate filter (DPF) and a larger exhaust gas recirculation (EGR) cooling system along with some other improvements. Even with the added complexity and an exhaust scrubber before the tailpipe, the LMM is rated at 365 hp at 3,200 rpm and 660 lb-ft of torque at 1,600 rpm.

   Here is how the General has changed the Duramax to satisfy the Feds and also manage to create a clean engine with the most diesel power.

   New Boreless Turbo
   Unlike the previous Duramax turbos that have a compressor wheel with a hollow center and the shaft passing all the way through, the LMM uses a new design that is solid except where the shaft screws directly into the wheel. By eliminating the weak center of the wheel, the variable vane turbo should be able to better handle the stress created by the ultra-high 120,000-plus rpm the turbo will experience every day and greatly improve the long-term durability of the compressor.

New Injectors
All-new, solenoid-controlled, seven-hole injectors will be used in the LMM Duramax to improve combustion. The ULSD-safe injectors were designed by the GM Powertrain team to improve the atomization of fuel and lower emissions and noise through the use of multiple injection events during a single combustion stroke. These customized injections will also be occasionally used to increase the exhaust temperature to clean out the diesel particulate filter.

Intake Throttle Added
Another emissions addition to the Duramax is an electronically controlled intake throttle upstream from the turbocharger. While the engine is idling or the truck is traveling at low speeds with no load, the engine computer will limit the amount of air allowed to enter the engine to intensify the load. This will increase the temperature of the exhaust, which helps maximize the performance of the passive catalytic converter and the diesel particulate filter.

Larger EGR Cooler
To help reduce nitrogen oxide emissions, the exhaust gas recirculation system will have a larger heat exchanger than the one found in the LBZ engines. This larger exhaust gas cooler will lower cylinder temperatures and allow more nitrogen oxide-fighting gasses to work in the combustion chambers.

Closed Crankcase Ventilation
To further reduce nitrogen oxide emissions, gasses created in the crankcase are not released into the atmosphere. The LMM employs a sealed chamber between the cylinder banks to ensure crankcase gasses are held within the engine.

Diesel Particulate Filter
In addition to the passive catalytic converter, a "regenerating" diesel particulate filter will scrub exhaust gasses and trap particles before they are allowed to exit the tailpipe. To do this, the exhaust gasses must pass through an oxidizing catalyst, then into a ceramic "brick" that traps the particles known as soot. Pressure sensors before and after the DPF tell the engine when there is too much backpressure and alter engine functions to raise the temperature up to 1,022 degrees Fahrenheit to burn off the trapped matter.

New Electrically Controlled Fan
Speaking of temperature, the new LMM will be cooled by a fan that is electrically controlled by the 32-bit E35 engine computer. Engineers say this feature allows more precise control of engine temperatures, which increases efficiency, reduces emissions, and should slightly improve fuel mileage.

Computerized Oil Life System
To meet emissions requirements and prevent the DPF from clogging, the LMM requires the use of CJ-4 compliant low-ash oil. The amount of time between oil changes is up to the engine computer and the computerized "Oil Life System."

Full Power Fleets
Some good news for the drivers of fleet vehicles is that there will no longer be a low-output version of the Duramax LMM for fleet trucks. Unfortunately, the DPF increases the length of the exhaust system and may create packaging problems for specialty vehicles such as street sweepers and airport tugs.

The Last Duramax?
The LMM may not be the last engine to wear the Duramax name, but it may be the last to displace 6.6L. GM calls 2007 an "interim model year," so it may be safe to assume this engine is a stopgap to meet the EPA requirements until a new engine that meets 2010 requirements is ready. If it's anything like the future light-duty Cummins and GM engines we've been hearing about, then may have overhead cams, make more power, and have emissions cleaner than those nerdy gas-electric hybrids that are all the rage-for now.

2007i Duramax Diesel 6.6L V-8 Turbo (LMM)
Displacement: 6.6L 403 ci
Bore and stroke: 4.06x3.9 in
Compression ratio: 16.8:1
Valve configuration: Overhead valves (four valves per cylinder)
Valve lifters: Mechanical roller
Assembly site: Moraine, Ohio
Horsepower: 365 @ 3,200 rpm
Torque: 660 lb-ft @ 1,600 rpm
Fuel shutoff: 3,450 rpm
Block: Cast iron
Cylinder heads: Cast aluminum
Exhaust manifolds: Cast nodular iron with steel pipe extension
Main bearing caps: Cast nodular iron
Crankshaft: Forged steel
Camshaft: Steel
Connecting rods: Stress-fractured forged steel
Emissions controls: Cooled EGR,catalytic converter, DPF, ashless CJ-4 oil, and intake throttle