September 17, 1999

Duramax 6600

First Look

By Jim Bigley

By now, most people familiar with the Diesel Page know that GM will introduce a brand new diesel engine for its HD pickup truck line next year as a 2001 model. This engine, named the "Duramax 6600", should move the GM trucks to the head of the pack in power, performance, reliability, durability, fuel economy, and quiet operation.

This engine is the result of a joint venture (DMAX Ltd.) between General Motors and Isuzu Motors. GM now owns nearly 50% of DMAX, and will produce the engines in a new manufacturing facility in Moraine, Ohio. Isuzu's experience in building diesel engines is unmatched, with a total to-date production of more than 13 million diesel engines spanning six decades.

The 6600 is a totally new 90-degree V-8 diesel engine (I'll talk about how new in a minute), utilizing 4-valves per cylinder and Bosch common-rail direct injection. Also new is the use of aluminum cylinder heads, which incorporate hardened steel valve stem guides, valve seat inserts, and injector inserts. Six head bolts surround each cylinder, which provide more than enough clamping load.

Best-in-class power/performance:

For competitive reasons, GM will not publish the power specifications until shortly before introduction next fall. However, according to GM, the Duramax 6600 has the highest horsepower and torque rating currently used in any diesel pickup truck.

To see the Duramax advantage, you need to look closely at the horsepower and torque peak graphs for the Powerstroke and Cummins. Some manufacturers suppress power to achieve high torque figures. This produces a high torque peak in the lower rpm range that drops off dramatically as engine speed increases. The Duramax, on the other hand, maintains the same high torque level across the entire power band, which provides superior off-the-line performance, acceleration and heavy-duty trailering ability.

Best-in-class durability:

The Duramax, while it is a new engine, is not a 30 day wonder. I first heard of this engine in June of 1997. Who knows how long it existed before then. I also know that field trials using test "mules" (old body style trucks with the new engine) have been underway for a long time. I received reports this summer from people in the southwestern US who saw several examples of these trucks. It appears GM has taken seriously the effect of extreme summer heat and heavy loads. The grapevine also reports seeing Duramax powered trucks pulling over the rated GCVW under maximum power for extended periods of time (and doing very well, I might add).

The primary cylinder block is made from an induction hardened gray iron casting. The deep skirted block (sides of the block extend below the crank centerline) provides additional strength, and helps reduce noise. The deep skirt also allows the use of side-bolted main bearing caps for even greater load-bearing capability. The cast aluminum crankcase (oil pan) adds structural rigidity, additional oil cooling, and helps suppress diesel engine noise.

Photo by James Fleming

Following the lead of the medium-duty market, the 6600 incorporates an integral "stacked-plate" engine oil cooler. This cooler is in-line with the engine cooling system, where the incoming coolant from the radiator first flows across the engine oil cooler before entering the cylinder heads and block. As we're discovering, maintaining control of engine oil temperature contributes to durability more than we previously thought possible.

The water pump is a new gear-driven design, instead of using the accessory drive belt to turn an external water pump. The primary advantages of this design are longer life, and the fact that you can still drive the truck if the accessory drive belt fails.

Allison 1000 Series
Photo by James Fleming

1000 Series® Allison Automatic Transmission

The Allison 1000 Series 5-speed electronically controlled automatic overdrive transmission draws it's heritage from the military and commercial market. Aside from the Duramax powertrain, Freightliner is reportedly using the 1000 series in new truck production. With a low first gear, an overdrive 5th gear, lock-up torque converter and narrow gear splits, the new Allison was chosen to provide the Duramax the best in both light-duty and heavy-duty performance.

Not having access to a suitable automatic transmission has been a limiting factor in the competitor's diesel power ratings. The Duramax will not be "detuned" when mated with the new Allison transmission. Input shaft rated at 545 lb-ft and 375 gross hp, the optional Allison transmission will make GM trucks the leader in diesel/automatic performance. For those who like to shift gears, a ZF 6-speed manual transmission will be standard in the Duramax powertrain (with a thorough "going-over" to improve shift quality for the Chevrolet/GMC application).

The new electronic TCM (Transmission Control Module) provides for two modes of operation; normal mode and tow/haul mode. Normal mode is useful for typical light-duty driving, providing excellent shift quality and a lockup torque converter function in 4th and 5th. The Tow/Haul mode is activated by a pushbutton on the end of the shift lever, and is intended for more heavy-duty applications.

If it's possible to anticipate the market, GM has tested this engine and drivetrain in every conceivable way to ensure the new GM truck owners won't be disappointed. The more you dig into the details of this engine and transmission; it becomes obvious that power, durability and reliability were the driving forces behind the design. It appears the sleeping giant is awake!

Jim Bigley

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