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Old 08-28-2015, 06:21 AM   #1
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Best RPM

I have a 350 Cat Diesel pusher running my 40" coach. I would like to know what RPM will give my best milage?:

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Old 08-28-2015, 06:31 AM   #2
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This might help you, from Caterpillar:


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Old 08-28-2015, 07:17 AM   #3
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Originally Posted by Dr. Q View Post
This might help you, from Caterpillar:

Attachment 105010
Dr Q,

U DA MAN!!!!! What a great document. I am sure that nearly all of the info contained relates to Cummins and other diesels, as well. Very informative and proves (and disproves) alot of info that is seen on IRV2 and other forums.

After reading the whole document, I was really, really excited to discover, on page 13, that it provided confirmation from what I had thought/"kinda calculated" over the last 5 years. That is my Cummins 6.7L 350hp, 750lb diesel had a "sweet spot" at about 1750-1800 RPM (where 1750 is my usual target RPM).

Great reference material, thank you for sharing!
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Old 08-28-2015, 07:29 AM   #4
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Originally Posted by wingman139 View Post
I have a 350 Cat Diesel pusher running my 40" coach. I would like to know what RPM will give my best milage?:


I do have some experience in this field from spending 20+ years working on and driving our diesels, new and older. From experience a lot can be said for the age and technology used in any motor so let's assume that you travel a set distance at the same speed, first in 3rd gear, then in 4th.
You can see that the engine will turn more times in the lower gear. Each revolution of the engine will "consume" the same amount of air/fuel mixture. Therefore conventional wisdom says that you must consume more air/fuel mixture at the lower gear.
Now this being said, a lot of modern engines are getting smarter about feeding fuel so the assumption of a constant a/f mixture may not be valid. With intelligent fuel metering the mileage difference may be small. The biggest difference to fuel consumption is your rate of acceleration. If you like to feel some acceleration and take pride in your ability to get to 60mph you will see improvement by playing "old lady" for a while. However, your motor does do more work when doing 1200-1500rpms to 60mph vs 2500rpms to 60mph. This increased work MUST be reflected in fuel consumption.

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Old 08-28-2015, 08:51 PM   #5
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Typically it will be whatever RPM barely gets you into the top gear (overdrive). Maybe 1-2 mph over that. If you aren't at a steady speed in top gear, mpg is probably out the window anyway.
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