Is lower RPM always more efficient? - Fuelly Forums

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Old 06-23-2009, 01:30 AM   #1
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Is lower RPM always more efficient?

One benefit to having a larger displacement engine is more power, if you tune it right you can get more low end torque which can mean lower revs but this leads to more fuel usage. A low displacement engine is great for better fuel economy because it allows for less fuel to be burnt and has less mass to move around but that leads to less power. One way to compromise is to add a turbo, you can get some of the fuel economy of the low displacement engine while having the power of the higher displacement engine. One article I read off of hotrod.com said it simply that having a turbo basically increases the displacement of the engine (not literally, but effectively since you're going about atmospheric pressure). This made me realize that a turbo is bad for when you want the benefits of low displacement as you'll have more fuel coming than you want but it also creates parasitic drag on the exhaust.

Alright, so here is my question:
In this SPECIFIC situation and no other, is it better for a low displacement engine vehicle at a fixed speed when meeting a grade to have to downshift, raising the RPMS with no turbo at all (anywhere) OR is it better to have a low speed turbo that allows the low displacement engine to maintain its speed despite the grade? For example instead of having to downshift and run the engine at 2500-4000rpm while climbing the grade, you could instead maintain your RPM of 1000-1500? My thinking is that being able to maintain the RPM of 1000-1500 with the usage of a turbo would be better due to not needing to move the mass in the engine so quickly and would save more overall despite the now extra fuel entering into the combustion chamber.
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Old 06-23-2009, 05:45 AM   #2
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On another forum, a driver with a Saab turbo swears that his fuel economy is terrible if he stays in a high gear and opens the throttle more, which apparently puts him in boost, and that his fuel economy is much better if he downshifts to run at higher RPM and stay out of turbo.

I'd guess that a modern turbo car with the computer-controlled turbo is more efficient if you do it like him, downshift enough to stay out of boost.

In my slightly torquey NA car, I've only found ONE situation where downshifting is more economical: Climbing a hill at WOT at 1200RPM and not accelerating, if I downshift one gear and climb at 1450RPM with less throttle, I get a good 40% fuel economy increase. Apart from that I have not found a situation where my car gets better fuel economy in a lower gear than necessary to keep the engine from growling.
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Old 06-23-2009, 07:51 AM   #3
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There is another thing to take into consideration when you are talking about throttle position and economy on top of what you mentioned.

The ECU. Almost all engine computers go into an open-loop power enrichment mode. I believe the normal throttle position for that is 80% or more. You also lose active timing advance and that's why the majority of cars take off faster from a stop at part throttle. Economy will drop substantially in that enrichment mode.

Take the Durango for instance, it makes plenty of torque down low and has no problem with hills but I can see better mileage by climbing a hill in 3rd gear revved to 3200 as opposed to at the edge of an overdrive downshift at 1900 rpm(at ~70mph).
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Old 06-23-2009, 12:56 PM   #4
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Originally Posted by dkjones96 View Post
There is another thing to take into consideration when you are talking about throttle position and economy on top of what you mentioned.

The ECU. Almost all engine computers go into an open-loop power enrichment mode. I believe the normal throttle position for that is 80% or more. You also lose active timing advance and that's why the majority of cars take off faster from a stop at part throttle. Economy will drop substantially in that enrichment mode.

Take the Durango for instance, it makes plenty of torque down low and has no problem with hills but I can see better mileage by climbing a hill in 3rd gear revved to 3200 as opposed to at the edge of an overdrive downshift at 1900 rpm(at ~70mph).
I am going to try that and watch my scangauge more closely.

I usually just get my speed up so I don't need to downshift and stay at 70+ going up and just blow by the trucks in the truck lanes....haha ....Then on the way down I just DFCO in 3rd to keep from using the brakes or I would be off the road.
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Old 06-23-2009, 03:12 PM   #5
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I am going to try that and watch my scangauge more closely.

I usually just get my speed up so I don't need to downshift and stay at 70+ going up and just blow by the trucks in the truck lanes....haha ....Then on the way down I just DFCO in 3rd to keep from using the brakes or I would be off the road.
Yeah I've noticed an improvement in fuel economy from driving a little faster up a grade in order to maintain the tallest gear than maintaining that slower speed and having the car downshift. In my FJ cruiser, when doing 5-7% grades, at 55mph it will tend to be quick to downshift as soon as possible, but those same grades at even 60mph I can maintain my gear. Also I've found that even really steep like 15% grades, if I accelerate to 70-75mph, I can maintain my gear better than at 55mph and I believe I've seen better fuel economy as a result but I can't tell cause I don't have a computer to check.

In the Volvo S60 I drive, on the 23% grade I have to drive up in order to get home, while the peak torque is 1500rpm, according to the computer, the way to get the best mileage is to accelerate up to 40mph before getting up the hill and then flooring it all the way up, slowly losing speed and hoping that you don't lose too much speed before you get to the top (24mph is the point where it'll downshift out of 3rd gear w/torque converter lockup).
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Old 06-23-2009, 04:34 PM   #6
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I would think there's a point at which the velocity of the air charge in the runners and ports would get too turbulent and make a motor lower its fuel economy...cruising in my old Focus, it got its best economy right at about 1500rpm. At 1200 it was about 10mpg less...

My Dad had a '73 Mach 1 with a 351 Cobra Jet motor. The damn thing had ports so big you could stick your fist in 'em, and under about 2k the air flow was so slow through those ports that the engine just didn't run (might have with port FI - fuel wasn't staying suspended in the air). It's for this reason, I theorize, that the 5-speed he put in didn't bring his FE up at all, at 60mph he was running 1700rpm and got the same 13mpg he got before.
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Old 06-23-2009, 05:37 PM   #7
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Actually one factor I totally forgot to realize was BSFC (Brake Specific Fuel Consumption). If you have poor BSFC numbers at a specific RPM, then at high load and in need of a lot of power, in order to get X amount of power at 1500rpm you'll need a lot more fuel than at a higher RPM where you so happen to have a good/better BSFC.. Also like another poster mentioned, if you don't have to experience the extra fuel enrichment from full throttle which you may need when you have a poor BSFC, then you could get better numbers.

I guess the solution would be to adjust the cam timing and other related factors in order to make peak BSFC at the desired RPM and not at the higher end where you don't necessarily want it. It's really possible that while the turbo was active at that poster's RPM, being at that RPM had a poorer BSFC score than at the higher RPM.
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Old 06-23-2009, 06:58 PM   #8
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BSFC isn't a factor; it's a measurement of everything else this discussion is about. If you have a 3d BSFC map of your engine then you don't need anything else we've discussed.
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Old 06-23-2009, 09:00 PM   #9
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BSFC isn't a factor; it's a measurement of everything else this discussion is about. If you have a 3d BSFC map of your engine then you don't need anything else we've discussed.
Is there anything wrong with this map? http://ecomodder.com/imgs/geo-1L-bsfc-chart-orig.gif

Or is that a 3D map plotted onto a 2D plane vs. the line chart with X and Y axis..?
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Old 06-24-2009, 05:50 AM   #10
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That chart is hard to see. It has RPM and torque, but does it have fuel rate?

The more I think about it, the more work it becomes trying to use a BSFC map to figure it out...but the data would be there. I wonder if it would be possible to feed a known 3d BSFC map into a program that's hooked up to OBDII along with some environmental sensors for detecting grade....well, that's way too complicated.
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