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Originally Posted by GlennWest
According to everyone on the Durmax forums, including dealers, Duramax's make peak torque at 2100 rpm. Many on the forum have complained about poor mileage pulling in 6th gear and were advised to use 5th and mileage increased, temps lowered.
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You did not provide direct references but my guess is "everyone" on the forums knows better than the people who make and test the powertrains. Think about it -- who in their right mind would want a diesel making peak torque at 2100 RPM? A few years ago peak torque was around 1800 RPM and that was found to be too high. Ford and Dodge led the way to reduce that to 1600 RPM and Duramax followed suit with their most recent versions.
The Duramax is considered to have a flat torque curve (see chart below). At 2100 RPM there is still significant torque (over 600 ft-lb) but when encountering an uphill grade there is some reserve as the engine slows down the torque will build to its peak at 1600. As RPMs fall below the peak level the transmission will shift to a lower gear to increase the RPM and get back into the optimum torque band. Running in the higher portion of the
power band (which is not flat) by running in a lower gear will produce more HP but will burn more fuel - not less. You will get the sense of better performance because the transmission will not shift as often on grades. There are many factors affecting fuel mileage including the objective of the driver. The most signifcant of these is speed if the load is constant.
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On Edit moved following comment outside limited space in uploaded file.
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General Motors horsepower and torque numbers have been certified by the Society of Automotive Engineers (i.e. SAE certified). A new voluntary power and torque certification procedure developed by the SAE Engine Test Code committee was approved March 31, 2005. This procedure (J2723) ensures fair, accurate ratings for horsepower and torque by allowing manufacturers to certify their engines through third-party witness testing.
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