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Old 08-17-2014, 05:28 AM   #1 (permalink)
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Display shows increased mpg with higher rpm

I thought the general consensus is that higher rpm=higher fuel consumption, but if it is so, then why does my onboard computer display show my fuel consumption decreasing with higher rpm when accelerating in same gear?

I can only think and explain that in terms of fuel delivery rate, lets say 2 ml of fuel/second is delivered when the pedal is pressed at 50% at 2000 rpm in second gear at 30 km/h , and rising to 4000 rpm=60 km/h but with the same amount of 2 ml of fuel being burned.

So why should I change at lets say 2700 rpm if the instant consumption decreases as it accelerates to higher rpm ?

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Old 08-17-2014, 09:15 AM   #2 (permalink)
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Are you sure the display is accurate? Depending on the engine design and what kind of camshaft the engine has, it is possible to have higher efficiency at higher engine speeds, just not likely. Over 3-4 thousand rpm, friction increases and efficiency is lost as pressure in the exhaust builds up.
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Old 08-17-2014, 09:42 AM   #3 (permalink)
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Well I don't really know if this is true but my grandfather has always said that all cars have a "sweet spot" where you get the best fuel economy so perhaps that's the case.

But what kind of car do you have?
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Old 08-17-2014, 09:57 AM   #4 (permalink)
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MPG is a measurement of fuel usage compared to distance. When you begin accelerating you are covering less distance over a given amount of time. As the speed of the car increase it covers more distance on a given amount of fuel up to a point.
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Old 08-17-2014, 09:58 AM   #5 (permalink)
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1.5 japanese engine , suzuki sx4 2009 (torque power graph in attachement)

I dont have the gear ratios, all that I can say is that 1st gear max 50 km/h, 2nd 100 km/h, 3rd 140 km/h, 4th 180km/h, and 5th .
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Old 08-17-2014, 10:07 AM   #6 (permalink)
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Quote:
Originally Posted by ksa8907 View Post
Are you sure the display is accurate?

Over 3-4 thousand rpm
Yes.

I dont really remember instant fuel consumption above 4000 rpm, but below 4000 the instant fuel consumption definitely decreases as rpm increases to 4000. Yeah in this topic I'm reffering to instant mpg or instant fuel consumption not averages.
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Old 08-17-2014, 10:12 AM   #7 (permalink)
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Quote:
Originally Posted by nick666 View Post
Yes.

I dont really remember instant fuel consumption above 4000 rpm, but below 4000 the instant fuel consumption definitely decreases as rpm increases to 4000.
But how does the consumption compare at say 4000 in first to second at the same road speed?
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Old 08-17-2014, 10:36 AM   #8 (permalink)
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Quote:
Originally Posted by nemo View Post
But how does the consumption compare at say 4000 in first to second at the same road speed?
Its higher in 1st at 4000 than in 2nd at same speed.
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Old 08-17-2014, 10:52 AM   #9 (permalink)
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Since at 4k it uses more fuel than in 2nd you would want to be in 2nd. Now make those comparisons at 2K, 2.5K, 3K, 3.5k and in each gear. This will give you an indication of you best shift point vs FE. One other consideration not to accelerate too slowly.
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Old 08-17-2014, 11:38 AM   #10 (permalink)
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It looks like you get peak torque at 4100 rpm, and from the jumpiness of the torque below 4k im going to assume the engine has variable intake runners. From what I have seen, most engines achieve their best bsfc (brake specific fuel consumption), while operating below peak torque and not within 200rpm of peak torque. For your engine that would be 3100 - 3900rpm.

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