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Originally Posted by bryson
My main question is whether load or RPM has a greater effect.
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RPM is the killer.
While it requires more fuel to fill the cilinders more often, higher rpm also lowers the relative engine load, meaning that you won't get your HP as efficiently.
This works as long as you don't get near the max. engine loading at that rpm.
Then it'll just pour on more and more diesel to try to maintain (engine) speed.
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I notice that towing my boat doesn't seem to affect the mileage nearly as much as I expected.
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That's because once at constant speed, your engine isn't being used efficiently.
It may be getting (relatively) good FE, but gets bad BSFC-numbers at the same time.
The good FE comes from the basic fact that the engine isn't producing many HP, and as a result, doesn't need much gas.
Hooking up the trailer loads up the engine a bit more, improving the BSFC a bit. You will need a few more horses, but you're getting each of them more efficiently at the same speed, due to the higher loading. Net result : you see little difference.