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Fuel efficiency related to engine size #4847999 08/22/18 06:41 PM
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5AcresAndAFool Offline OP
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So there was a discussion at work and it was said that the 5.4L F150s get better fuel economy than the 4.6L in a truck with identical equipment aside from engines. Similar remarks have been noted on the silverado line.

So I looked on the EPA fuel economy website and the 2008 f150 with the 4.2l V-6 and the 4.6L V-8 indeed get the same fuel mileage in testing, the 5.4 either gets slightly below or the same as the other two depending on trim package, which maybe has something to do with rear axle ratio or something.

I know there are many other examples of this across the automotive landscape.

I have always understood that engines are most efficient at low rpm and wide throttle openings. Now the econominder gauges you would sometimes see in the 1970s or 1980s, which was basically a glorified vacuum gauge would tell you otherwise.

Anyway it seems that its very simple, a gallon of gas has a certain amount of energy in it and it takes a certain amount of energy to move a vehicle that weighs xxxx.xx lbs down the road. This is assuming that the different engines have a similar volumetric efficiency.

Now take certain vehicles such as a 2008 V-6 Camry and it definitely swills more fuel than the I-4, these seem to be the anomalies though.


So why would a smaller engine not save fuel. That is my question.

Re: Fuel efficiency related to engine size [Re: 5AcresAndAFool] #4848021 08/22/18 07:07 PM
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I dont know the math behind it, but I think its mostly to do with torque curves. The faster you spin the engine the more fuel it drinks. I hope one of our resident gear heads can chime in because Id love to understand this.


2010 Ford Fusion SE 3.0L V6, 181 miles M1 HM 10w-30
Re: Fuel efficiency related to engine size [Re: 5AcresAndAFool] #4848022 08/22/18 07:07 PM
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d00df00d Offline
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Any engine will get its best fuel economy within a certain range of load and RPM. Sometimes, a larger engine has a range that lines up nicely with a certain usage pattern, whereas a smaller engine would end up out of that range with the same usage (e.g. it has to rev too high). In that case, the larger engine could well be more efficient.

Some small engines are just really high-strung, which makes them inherently less efficient than they could be. An engine like that often doesn't get any better fuel economy than a bigger one that didn't have every last hp/L squeezed out of it. I think Honda S2000 vs. Corvette is an extreme example of that.

Many engines are optimized too much for the EPA test cycle (or the equivalent in another market) and not enough for real-world conditions. AFAIK that was the problem with the Smart Fortwo and the Ecoboost F150. If you drove them exactly within the parameters of the EPA test cycle, you got the EPA-rated fuel economy. But if you strayed just a little bit outside those parameters, mpg dropped like a rock.

Lastly, as you suggested, final drive could explain everything. If the bigger engine gets a taller final drive, that might boost its mpg accordingly.

Tangential: weight means a lot for mpg at low speeds, but aero drag counts for a lot otherwise.


2008 BMW M3 Sedan 6MT
Re: Fuel efficiency related to engine size [Re: d00df00d] #4848036 08/22/18 07:31 PM
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CR94 Offline
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Originally Posted By: d00df00d
... Tangential: weight means a lot for mpg at low speeds, but aero drag counts for a lot otherwise.
"Weight means a lot for mpg" at both low and high speeds whenever braking, consequent acceleration, and hill climbing are involved. Not as much in steady level travel, when aero drag normally dominates.


2011 Toyota Prius now at 107K
1981 Mazda GLC (323) retired at 606K
1972 Subaru DL retired at 190K
1954 Chevrolet retired at 121K
Re: Fuel efficiency related to engine size [Re: 5AcresAndAFool] #4848046 08/22/18 07:47 PM
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IndyIan Offline
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Originally Posted By: 5AcresAndAFool
So why would a smaller engine not save fuel. That is my question.

If the gearing was the same on both engines and you drove them at the same throttle openings and shift points, the smaller engine would be always more efficient, but a bit slower.
Typically the smaller engine gets run at higher rpms and if it also has a shorter top gear it could get worse mileage just going 60 down the highway. Watching the CVT work with instantaneous mileage displayed on the Outback really opened my eyes on how important keeping the rpms low is for good mileage. Basically you get your best mileage running the lowest rpm possible for the load. The Outback will run with the TC locked at 1100 rpm in town if you just need 5-8% throttle opening to maintain your speed or even accelerate a bit.
I suspect I could get the Outback up to 33-35mpg on my commute equal to my Focus, just because it can be driven at so much lower rpms all the time.


07 Focus ZXW, 5spd manual, 245km M1 5W30
18 Outback 2.5 CVT 35km 0W20
Re: Fuel efficiency related to engine size [Re: 5AcresAndAFool] #4848065 08/22/18 08:07 PM
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My 3.5 Litre V6 in the Highlander gets better fuel economy than my Dodge Journey with the 2.4 Litre 4 cylinder did. Because it keeps the RPM much lower most of the time thanks to having 4 more speeds in the transmission.


'18 Caravan - 45k KM - AMSOIL SS 0w20, Fram Ultra, TC-W3 500:1
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Re: Fuel efficiency related to engine size [Re: d00df00d] #4848075 08/22/18 08:15 PM
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CR94 Offline
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Originally Posted By: d00df00d
Any engine will get its best fuel economy within a certain range of load and RPM. Sometimes, a larger engine has a range that lines up nicely with a certain usage pattern, whereas a smaller engine would end up out of that range with the same usage (e.g. it has to rev too high). In that case, the larger engine could well be more efficient. ...
Yes, that right. The usage pattern is critical to whether a larger engine will be more efficient or not. In general, engines are most efficient at moderately high load and moderate speed.

Also in general and other things being equal, a larger engine is theoretically more efficient than a smaller one if both are working equally hard in proportion to size. That's because the larger one's combustion chamber has less surface area in proportion to displacement. Therefore, it has proportionately lower friction and lower heat losses to the metal surfaces.

Last edited by CR94; 08/22/18 08:17 PM.

2011 Toyota Prius now at 107K
1981 Mazda GLC (323) retired at 606K
1972 Subaru DL retired at 190K
1954 Chevrolet retired at 121K
Re: Fuel efficiency related to engine size [Re: CR94] #4848078 08/22/18 08:24 PM
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eljefino Offline
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Originally Posted By: CR94
[quote=d00df00d]
Also in general and other things being equal, a larger engine is theoretically more efficient than a smaller one if both are working equally hard in proportion to size. That's because the larger one's combustion chamber has less surface area in proportion to displacement. Therefore, it has proportionately lower friction and lower heat losses to the metal surfaces.


True, and this is why utility vehicles like potato chip trucks have ~3-liter, 4 cyl turbo diesels. The Jag V12 was a disappointment due to so much internal friction.

Re: Fuel efficiency related to engine size [Re: 5AcresAndAFool] #4848082 08/22/18 08:26 PM
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clinebarger Offline
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Trucks are heavy & have the aerodynamics of a barn, You can't compare them to lighter, low slung sedans with better aerodynamics.


2001 Chevy Camaro L92/4L80E
2006 Chevy 2500HD LBZ/Allison 1000
2010 Toyota Corolla 2ZR-FE/U341E
2000 Toyota Avalon 1MZ-FE/A541E
Re: Fuel efficiency related to engine size [Re: 5AcresAndAFool] #4848131 08/22/18 09:10 PM
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It's all about weight they are having to move and at rpm

Last edited by FermeLaPorte; 08/22/18 09:11 PM.
Re: Fuel efficiency related to engine size [Re: 5AcresAndAFool] #4848213 08/22/18 10:37 PM
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I remember watching a Top Gear episode where they had a really cheap, small economy car go "fast" around a race track, and it was followed by a super car. I think the eco was 3 cylinders. Afterwards, he said the eco ate more gas than the super car, so you're better off with a bigger engine.

Re: Fuel efficiency related to engine size [Re: 5AcresAndAFool] #4848250 08/23/18 12:39 AM
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I think it's worth mentioning thermal efficiency here. A cylinder with twice the volume won't have twice the surface area. The less surface area in the cylinder, the less of both frictional losses you have (ring area vs. piston surface area) and thermal losses (hot gas, distance from cylinder wall to center, dissipation of heat into the surrounding metal vs. doing work)


2005 Tahoe - 140k
2003 Forester(the swagger wagon) - Rehomed
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Re: Fuel efficiency related to engine size [Re: IndyIan] #4848342 08/23/18 05:24 AM
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BMWTurboDzl Offline
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Originally Posted By: IndyIan
Originally Posted By: 5AcresAndAFool
So why would a smaller engine not save fuel. That is my question.

If the gearing was the same on both engines and you drove them at the same throttle openings and shift points, the smaller engine would be always more efficient, but a bit slower.
Typically the smaller engine gets run at higher rpms and if it also has a shorter top gear it could get worse mileage just going 60 down the highway. Watching the CVT work with instantaneous mileage displayed on the Outback really opened my eyes on how important keeping the rpms low is for good mileage. Basically you get your best mileage running the lowest rpm possible for the load. The Outback will run with the TC locked at 1100 rpm in town if you just need 5-8% throttle opening to maintain your speed or even accelerate a bit.
I suspect I could get the Outback up to 33-35mpg on my commute equal to my Focus, just because it can be driven at so much lower rpms all the time.


Agree, My current ride is programmed with a driving mode called "eco pro" and the rpm/gear is exactly as you describe. My only quip is that I loose responsiveness because I don't have any excess torque (probably why I should have a diesel, but the goshdarn emissions systems are weak and the I4 variant is currently prone to catching fire).


“It took untold generations to get you where you are. A little gratitude might be in order. If you’re going to insist on bending the world to your way, you better have your reasons.”

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Re: Fuel efficiency related to engine size [Re: KnicksGiants] #4848392 08/23/18 06:17 AM
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Originally Posted By: KnicksGiants
I remember watching a Top Gear episode where they had a really cheap, small economy car go "fast" around a race track, and it was followed by a super car. I think the eco was 3 cylinders. Afterwards, he said the eco ate more gas than the super car, so you're better off with a bigger engine.

It was a Prius driving as fast as it could around the test track, and an E90 M3 following it. At those speeds, the Prius was obviously way outside its comfort zone and likely exhausted its battery pack, but the M3 was barely awake.

Not exactly a realistic scenario, but it does illustrate the point.


2008 BMW M3 Sedan 6MT
Re: Fuel efficiency related to engine size [Re: 5AcresAndAFool] #4848529 08/23/18 08:48 AM
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meep Offline
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Lots of good, and very partial answers above. I looked into this years ago, and found that the dynamics of all of the above answers and more all work together. Look up the bore and stroke of those two engines. If the gearing ratio and transmission ratios are the same, pay attention to the stroke.

Yes, smaller displacement engines are generally more efficient. BUT, longer stroke engines, in my armchair research, yield better MPG in comparison of several different vehicles with engine options. This of course, is also by itself a partial answer, but it was quite consistent what I found when I looked into it before.


2018 F150
2015 crv (wifey!)
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