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Old 07-23-2018, 08:32 PM   #1
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ScanGage question does this sound right?

I drive a Sprinter FourWinds. I set the gauge to show % of engine power. If I am climbing a long not so steep grade it will show say 98-100% power at 2300rpms. I usually drop the gear to 4th, rpms increase to 2800 power drops to 80's%. So I am forcing a downshift which increases the rpms but lowers the engine output. I might run 4th gear a very long distance given hills or a headwind and 3rd on a steeper grade again to power in the 80s or below. I always keep an eye on RPMs trying to keep those below 3200 but generally run between 2200 and 2800 when cruising. Am I doing this correctly?
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Old 07-23-2018, 09:01 PM   #2
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High rpm will not hurt the engine (diesel or gasoline) nor increase the engine wear. Most engine wear (up to 90%) comes on the first 15 seconds of a cold engine start. It is much worse to lug and engine (low rpm at full throttle), so let the little diesel sing - that what is made to do. Higher rpm on diesel engines do not decrease fuel mileage the way it does on gasoline engines as diesels have not a "throttling loss".
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Old 07-23-2018, 09:07 PM   #3
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True...also look into your designed torque curve of the engine and drive within it. This is the best performance. You can run over this curve and lose power or under this curve and lug along...neither which is good long-term.
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Old 07-24-2018, 07:13 PM   #4
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Wow.. thanks so much.

Both comments will head me a in the right direction.
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Old 07-24-2018, 09:41 PM   #5
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Thanks again... is this torque curve you refer to?
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Old 07-24-2018, 10:40 PM   #6
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Ed, you are doing everything correctly. you definitely need to manually downshift when going uphill or pushing into headwinds.
Try to keep the % engine Load @ < 85-90% as much as possible and when pulling hard, keep the RPMs in the 2600-3200 RPM range.
A manual downshift with your engine and transmission combination increases the engine RPM by very close to 500 RPM in the
fifth gear (Overdrive 0.83:1) to 4th gear (1:1 no overdrive) so you can manually downshift (5th to 4th here) safely at any RPM < 3400 RPM.
Best fuel mileage (when pulling hard) is again in the 2600-3200 RPM range.
Use the GPH parameter on a Scan Gauge II to see if a manual downshift is actually getting your better fuel mileage.
Normally the % engine load will drop 5-15% and the GPH will decrease from > 4.0 to ~3.0 or less.
If i am not on an interstate I always just leave it in 4th. (and we try to stay off the interstates as much as possible). In 27000 miles I have averaged 15.4 MPG.
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Old 07-24-2018, 10:40 PM   #7
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Exactly...which means just over 2500 RPM you will start to loose "pulling power". Stay there or just below, when possible, and you'll get best performance. Mountain hills will wreck that theory but use it as a guideline and you'll be better served by the engine. HP get you there fast...torque keeps you there.
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Old 07-25-2018, 12:16 AM   #8
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Originally Posted by Mred View Post
Thanks again... is this torque curve you refer to?
Data in that graph doesn’t look correct to me. I tried to find curves directly from Mercedes but OEMs are starting to keep this type of info close to their vest.

You shouldn't directly compare engine performance at different speeds between engines of different displacement per cylinder. Physics being what they are, larger cylinders “normally” perform similarly at slower speeds, while smaller engines typically spin faster. In engineering school we compared engines more directly by measuring engine speed in piston speed. A larger engine with a stroke twice as long usually spins about twice as slow. I’ve had large diesels that redline at 1800 RPM. Very large engines may only go to a few 100 RPMs.

Just saying that you should take advice in right context. Your 3.0L V6 can spin to 4,000 RPM, while others can’t even come close to that. It also pays to know the difference between torque and power. They are often used interchangeably but shouldn’t be. And this is key in your question because you’re talking about different transmission gears and engine speeds.
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