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tbirddillon 04-06-2009 09:30 AM

Fuel economy And Rpm
Does A 200-300 Rpm Difference Have A Signifigant Affect On Fe. When I Am Going 55mph It's About 1500 Rpm. When I Hit 60 It's Then Around 1700-1800 Rpm. I Know It Will Make Some Difference I Am Just Curious As To How Big Of An Impact It Has.

MetroMPG 04-06-2009 10:35 AM

The difference in fuel consumption in your example comes from both increased aerodynamic drag from increased speed, AND higher RPM. You can't easily separate the two to figure out the RPM hit alone.

However, for a given constant speed, lower RPM is better.


Showdown: testing RPM vs. MPG at a fixed speed -

Speed kills: testing MPH vs. MPG in top gear -

theunchosen 04-06-2009 12:17 PM

You could separate them, but you'd have to run a dyno test while measuring your FE and then subtract the power needed to overcome wind resistance.

Not worth it. Dyno time is 80$+ an hour.

wagonman76 04-06-2009 12:39 PM

As a general rule, lower RPM is better for FE. But there comes a point where you start to use more fuel to maintain forward motion than if you were to use the next lower gear. It is different for each vehicle, you would have to experiment and see. With a mpguino you would be able to tell instantly.

Superturnier 07-31-2009 06:15 AM

Finally got my instrumentation installed. It's a Zemco fuel computer (I have carb engine)
I'm slowly getting the calibration quite close to reality, and after a few days of driving I'm surprised what this instrument is showing.

The faster I drive the better the mpg!?

At 40mph (65km/h) its appr. 24mpg (1800rpm, highest gear)
At 70mph (115km/h) its +30mpg (3100rpm , highest gear)
Coasting neutral did not improve (instant mpg)readings. With gear engaged its even worse.

I really was convinced that slow speed always gives you better mpg, but maybe this does not apply with older cars.

I have already read topics here about the BSFC charts and about the optimal piston speed and so on, but I will not give up yet.

There are few things that could be the reason:

-The fuel flow sensor is maybe not accurate at low load. Mechanical fuel pump will not give even flow. It pulsates back and forth. I need to make a slow test drive with a separate fuel canister to measure used fuel accurate.

-A/F ratio tends to be rich at idle and small throttle openings. I already have the smallest idle jet that can be found. If I bleed more air to the intake the idle rpm goes higher.

-Ignition advance could be higher at low load. I think I'm far away from the knocking treshold. I cannot add static advance any more, since that causes hot start problems. Maybe an adjustable vacuum advance canister would help.

It would be SOOOOOO easy with programmable fuel & ignition system like Megasquirt. Just put new values to the map and go for a test drive.
But for some reason I like to keep this car old and mechanical.

Any opinions?

robchalmers 07-31-2009 08:32 AM

I think the RPM difference will make less difference than the load induced by Drag increase at speed. also depends how your fueling - as above is you're running a distributor/carb set up it'll be a world away from a lambda aligned ECU controlled fuelling and ignition system. Mega jolt and Mega squirt are very cool yet accessible systems with amazing 3D mapping potential!! kinda surprised I haven't heard much about folkes on here adapting it for eco-ing rather than POWER!!!!!!!!!!!!!

Superturnier 07-31-2009 08:58 AM

I have to admit that I have dreamed about a hidden Megasquirt system converted for 6Volts (yes I have 6 volt system) and injector(s) hidden inside carburetor.
Lambda sensor I already have and monitoring it with multimeter. The ECU is my head and the feedback is my hands and screwdriver... a bit slow system...

I know Megajolt also.

And the same thing I've been wondering - why everybody wants Megasquirt only for power purposes?

robchalmers 07-31-2009 09:59 AM

I guess because no-one really promotes it (here in the UK at least) as a eco oportunity but more as a cheap way to control Throttle bodies.

MetroMPG 07-31-2009 02:00 PM


Originally Posted by Superturnier (Post 118765)
Coasting neutral did not improve (instant mpg)readings. With gear engaged its even worse.

You're right to suspect the gauge isn't working properly. Your instant fuel economy in neutral (at speed) will be vastly higher than steady state speed MPG.

Superturnier 09-21-2009 05:38 AM

I think I finally solved the mystery, why my fuel computer showed better values when driving faster.

Leaving home on a cold morning, during the first 5 minutes, the mpg readings were lower than I was used to at 40mph.

When the engine temperature got hotter the readings were back to "normal"
That's when I realized that my mechanical fuel pump heats the fuel, because there is no adequate insulation between the pump and the engine.
And as the fuel boils very easily, it starts creating bubbles in the fuel line.

And the fuel sensor has an optical eye that counts the revolutions of the circulating ball.
And this eye counts also the bubbles coming from the pump.
So when driving faster there is more fuel flow and the heat has less time to boil the fuel and mpg readings look better.

So, I placed the sensor before the pump in the fuel line and quess what was the result;)
Now my fuel computer encourages me to drive slower as the readings go very much with the load.
Even driving behind a truck makes a difference!

Now I don't have to prefer the highways any more. I can choose the easy country roads and relax and enjoy driving:)

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