what's the best RPM to cruise

SD diesels were widely available in the US in the 1981-86 Datsun/Nissan 720 pickups, and in Canada through '87 in the D21 pickup.

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philip1
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what's the best RPM to cruise

#1

Post by philip1 » 11 years ago

I'm looking at this from a fuel milage standpoint so heres the question. Is it better to cruise at the torque peak or stay below it?
Phil
1981 HL510 (in WA)
I will be installing a diesel in this thing

Bob Drucker
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#2

Post by Bob Drucker » 11 years ago

I have been told that the lowest rpm that a internal combustion engine can pull and still keep the vehicle moving at the wanted speed with out surging or shuttering will give the best mpg. The less you use- the better the mileage. comments?
roverdiesel

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kassim503
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#3

Post by kassim503 » 11 years ago

i would say the best mileage achieved would in a diesel is whatever rpm results in the least amount of pedal down. Will say nearly the same for a gasser as well
'83 maxima sedan, l24e, a/t, black

227K SOLD 6/7/2012

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asavage
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#4

Post by asavage » 11 years ago

kassim503 wrote:i would say the best mileage achieved would in a diesel is whatever rpm results in the least amount of pedal down. Will say nearly the same for a gasser as well
A problem with that theory:
Assume a fixed accelerator position. Other factors being equal, an engine will use less fuel at 1x RPM than at 2X RPM. Even if the accelerator position is the same, the higher RPM engine will use more fuel.

IOW, throttle position alone is too limiting a factor in determining the most fuel-efficient RPM to run.

==================
Just something to toy with your head: in a throttled engine, the pumping losses go down as the intake throttle is opened: the engine becomes a better air pump, and less of the engine's power is used to maintain a vacuum in the intake. One reason why gasser EGR systems don't kill off MPG as much as you might think: EGR opens, combustion efficiency drops, power drops, driver opens throttle to compensate loss of power, pumping losses reduced, overall efficiency is not as bad as you thought it would be.
Regards,
Al S.

1982 Maxima diesel wagon, 2nd & 4th owner, 165k miles, rusty & burgundy/grey. Purchased 1996, SOLD 16Feb10
1983 Maxima diesel wagon, 199k miles, rusty, light yellow/light brown. SOLD 14Jul07
1981 720 SD22 (scrapped 04Sep07)
1983 Sentra CD17, 255k, bought 06Jul08, gave it away 22Jun10.

ocd
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#5

Post by ocd » 11 years ago

do we have brake specific fuel consumption data on the sd22?

IIRC many diesels use the least amount of fuel when producing the most amount of torgue. so some would say that cruising speed should be just above peak torgue so when a climb begins rpm's drop and torque increases therefore ascentablility is maintained. on a 6.2/6.5l gm/detroit diesel brake specific fuel consumption is least at peak torque at 1850rpms
-Noah

i deliver blends of biodiesel -no more!.

82 datsun 720 KC w/sd22
85 volvo 760gle sedan turbo diesel
85 peugeot 505s wagon turbo diesel & parts car
83 chevy k20 suburban silverado 6.2 n/a diesel

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kassim503
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#6

Post by kassim503 » 11 years ago

Thanks for clarifying that idea Al, I wasnt really thinking of rpm's which is a major factor in fuel consumption
'83 maxima sedan, l24e, a/t, black

227K SOLD 6/7/2012

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