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Optimal RPM...
Topic Started: May 2 2013, 10:38 AM (1,549 Views)
dayle1960
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Fastest Hampster EVER

dayle1960
May 3 2013, 07:34 PM
I'm now wondering if the graph test was run on a dyno or was it real life testing on roads. I'm doing a quick test going 3000 rpm's to see if the mpg's bump up a bit. Will be to see the results.
Well, I ran my mini-test for a half tank of gas. I was going 3000 rpms and attained a sustained speed of 65 mph. So for about 200 miles I was doing 65 mph.

The results----I got .8mpg less driving 3000 rmp at 65mph than when I ran a full tank of gas at 60 mph and around 2500rpm.

Moral of the story.......I think the 3000rpm theory is good if you run the metro on a static dyno with no aero drag, but once the drag comes into play then the mpg's automatically go down.

I'll stick to staying in the slow lane for a while longer.

Thanks for the idea, Coche. Sometimes it is good to live outside of the box to see what might happen. :cheers
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clarkdw


I would assume and we all know what that means, that the chart for BSFC shown in the first post is taken at WOT since that is the way most BSFC figures are arrived at. That means it is as good as useless for the purposes of determining what is the "sweet spot" for cruising down the road at best fuel economy.

As the engine % load is decreased the BSFC figures change dramatically. I have seen an instance where the BSFC for a specific engine increased by 117% from full load (WOT) to 25% load without changing rpm in the equation. That means that you will be using more than double the amount of fuel that would be calculated from BSFC tables to produce each kwh or hp (whichever suits your fancy)when you are running at 25% load as opposed to running at WOT. That is a huge loss in efficiency. How can that loss be corrected? That is where the gains are to be made

There are a lot of variable that would need to be considered here. Pumping losses (not frictional loses, don't confuse the two) are decreased dramatically as throttle openings become greater. In some cases to produce a given quantity of power as required to maintain speed the BSFC would be kept lowest by going up gears and using wider throttle openings but that can also be affected by cylinder filling efficiencies at a given rpm, cam timing, duration, etc, etc. The list of factors is very, very long.

I think this is a great thread but in order to get any useful information it must go way beyond just being a discussion of what rpm is the "best" for mileage.

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Turbo Dan-O
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Obsessive Car Detailer

clarkdw
May 5 2013, 07:37 AM
The list of factors is very, very long.

I think this is a great thread but in order to get any useful information it must go way beyond just being a discussion of what rpm is the "best" for mileage.

:gp Lots to think about... :hmm
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mwebb
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FOG

kiss version

you can use a vacuum gauge on any car , higher vacuum = better FE all else the same
while the above is not really true, in the real world it is true enough to use -

with an OBD2 car 1996 or later look at the calculated load PID on the scan tool
NOT absolute load
a lower value under the some conditions means better FE

(absolute Load should never be used for any calculation because it is always wrong and that will matter when you try to test for VE or flow at WOT )

or
on an OBD1 car 1995 and earlier use Intake vacuum OR the MAP PID on a
scan tool

higher vacuum (from zero) or lower value MAP = better FE

since MAP = Manifold Absolute Pressure
and Intake Vacuum + MAP = Ambient Baro pressure = 30"HG = 14.7Psi = 1 bar
at about sea level
Edited by mwebb, May 5 2013, 09:16 PM.
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texan821
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I am no longer a member

As a layman with anecdotal experimentation, I've gotta say that I'm terribly impressed with my results with higher RPMs, but only on the highway...going through a school zone in 2nd gear isn't FE at all.

My perspective is that the point is gears. Holding steady at 3300 RPMs in 3rd is far less FE than going the same speed in 5th. However, once you climb to highway speeds and run out of gears, the most FE spot to cruise is in the range illustrated in Coche's original post.

That's my experience after a couple tanks...this thread has really changed the way I feel about highway driving altogether and wanted to chime in with my experiences and reasoning.
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