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Old 10-02-2017, 12:33 AM   #1
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Maximum torque vs maximum hp

I have been reading older threads and wanted to bring up what seems a contradiction. The thread claimed that max mpg happened at 200-300 rpm after maximum torque. Another thread stated that running a diesel at/above maximum torque shortens the life of the diesel. Can anyone comment on this? Also, does adding a second turbo to a diesel push the rpm at maximum torque down - lower rpms - or raise it?

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Old 10-02-2017, 06:53 AM   #2
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Originally Posted by pengyou View Post
I have been reading older threads and wanted to bring up what seems a contradiction. The thread claimed that max mpg happened at 200-300 rpm after maximum torque. Another thread stated that running a diesel at/above maximum torque shortens the life of the diesel. Can anyone comment on this? Also, does adding a second turbo to a diesel push the rpm at maximum torque down - lower rpms - or raise it?
I can answer the bit about the turbo ...

Adding a second would have very little effect on either the power or the torque. It might affect how quickly the engine responds, a little.

Turbos add pressure. The engine doesn't care whether that pressure comes from one turbo, or six, all it sees is a pressure increase on the intake. However, smaller turbos react more quickly and can be cheaper, so it might respond more quickly (at a price).

You can alter the boost pressure from a turbo, which adds horsepower at the expense of reliability and longevity.

I'm sure there are other, minor effects, but that's the basics.

A bigger intercooler could possibly have a greater effect than more pressure.
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Old 10-02-2017, 09:55 AM   #3
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The "sweet spot" for nearly all diesel engines is (as you indicate) just above the max torque rpm point (two-strokes maybe a little higher). The sweet spot being defined as the rpm level that will provide optimal energy to move the load while yielding maximum mpg's and putting the least wear and strain on the engine.

And like just about any type of engine, running it consistently above that point will certainly have a negative impact on longevity as well as mpg's.

Most of the builders who add turbos (I have seen little 4BT Cummins with four turbos) are not building for longevity or reliability. While that engine is capable of producing well mover 800 HP and massive amounts of torque...it was purpose built for short bursts of extreme HP & torque with the full understanding that it will either be totally rebuilt every few runs...or blow completely apart during a pull or race.

The best way to add a few ponies and up the torque for most over the road engines is to finesse the fuel screw and timing. My 130 horse motor can easily (and cheaply) be pumped up to 200-250 HP this way with barely any impact on reliability or mpg. In fact, a good diesel tuner can usually squeeze out a few more mpg's while increasing hp & torque.

Best bet is to find a tuner very familiar with your engine and develop a build up plan that is balanced to your needs and expectations.
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