So what's the consensus on wide open throttle for gas mileage?

fuzzybabybunny

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I read here once that gently accelerating to cruising speed actually isn't the best for gas. For example, to get to 60mh you'd shift to 1, 2, 3, 4, and then cruise in 5th. With WOT you'd gun it throughout 1st, gun it in 2nd to 60, and then shift to 5th to cruise at 60. So which is best for better gas mileage?
 
Feb 24, 2001
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In the same vein, my car makes the most power between 2800-3800 RPM. Does that mean efficiency wise it doesn't help much to go over 3800 RPM? Given your above example, wouldn't I be better getting to cruising speed while keeping it under 3800 RPM but above 2800?
 

f1r3s1d3

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Feb 18, 2006
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Who the hell told you that? WOT sucks up gas like no other. Shifting gears doesn't consume more fuel. Try it yourself. Start with a full tank, clear your trip and WOT yourself up to 60mph. On the third day, head to the gas station and fill up + calc. your MPG. Then drive 2 more days conservatively/normally. On the 5th day, head back to the station and do the same thing.
 

AnnonUSA

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Nov 18, 2007
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Short shifting, or getting out of the low gears quicker will save fuel on stop and go travel. This is easy to do with an automatic and your right foot, easier with a stick. If your car has a Manumatic, you can use manual mode to select and lock a higher gear while cruising to improve mileage.
 

Zenmervolt

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Oct 22, 2000
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WOT is the most efficient way for an engine to produce power. That's simply not disputable.

However, that doesn't mean that it's most efficient for mileage to run WOT to redline in every gear. RPM is the "800 pound gorilla" in fuel usage. RPM plays far more of a role than throttle position and it is less efficient to have high-RPM with a small throttle opening than it is to have low RPM with WOT.

Theoretically the best efficiency is from using WOT and short-shifting. I.e. use WOT but upshift as soon as is possible.

Now, modern EFI cars disrupt this somewhat since they will enrich the fuel mixture at WOT, so in practical application the real-world best mileage is obtained by using 3/4 to 7/8 throttle and upshifting as early as possible.

ZV
 

Zenmervolt

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Oct 22, 2000
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Originally posted by: BrunoPuntzJones
In the same vein, my car makes the most power between 2800-3800 RPM. Does that mean efficiency wise it doesn't help much to go over 3800 RPM? Given your above example, wouldn't I be better getting to cruising speed while keeping it under 3800 RPM but above 2800?

Not necessarily. It depends on gearing and how much your car weighs.

Remember that power is not really related directly to miles traveled, so it's a different sort of efficiency that's being measured. Remember too that an engine only makes rated power at WOT. So when you're cruising down the freeway at 65 mph at 2,500 RPM with only a light throttle, your engine is really only making between 25 and 35 hp.

The engine alone is measured for efficiency in terms of power produced per unit of fuel burned, and this peak will always occur at WOT.

The car as a whole is measured for efficiency in terms of miles traveled per unit of fuel burned, and this peak will not normally occur at WOT.

The measures are very different and even though both are "efficiency", they cannot be used interchangeably. It's kind of like the heating element in my oven. It is very inefficient as a light source, but it is significantly more efficient as a heat source. Both are measures of "efficiency", but they are not useful to compare against one another.

ZV
 

Dman877

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Jan 15, 2004
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Why is rpm the 800 pound gorilla? Isn't load on the engine what determines fuel usage? If you gun the engine in neutral, you aren't using as much gas as you are when you're WOT in a gear because there is no load on the engine. This is in fuel injected cars of course...
 

Howard

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Oct 14, 1999
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I might be wrong but fuel consumption is mainly a function of throttle position and engine speed - i.e. mass flow.
 

PandaBear

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Aug 23, 2000
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Engine's efficiency band is usually at 40-50% maximum rpm (redline) and that means usually WOT is not as efficient than having your pedal 40-60% down. Your engine has to retard timing and run richer when you WOT, and the engine is usually tuned to the most commonly used band, around 1500rpm to 3500rpm.
 

Dman877

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Jan 15, 2004
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Originally posted by: Howard
I might be wrong but fuel consumption is mainly a function of throttle position and engine speed - i.e. mass flow.

And load on the engine. If your car is parked and in neutral, you can rev the engine to redline and it won't use as much fuel as it would maintaining speed going up a hill. Your car's ecu will run the engine lean until it needs power (it's under load), and then it will max out fuel in the mixture.
 

Zenmervolt

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Oct 22, 2000
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Originally posted by: PandaBear
Engine's efficiency band is usually at 40-50% maximum rpm (redline) and that means usually WOT is not as efficient than having your pedal 40-60% down. Your engine has to retard timing and run richer when you WOT, and the engine is usually tuned to the most commonly used band, around 1500rpm to 3500rpm.

Once again, peak power efficiency is NOT synonymous with peak mileage efficiency.

Further, throttle position is not linear with RPM. If you are in a high gear, it's possible to hold WOT at low RPM with no acceleration (i.e. 100% throttle with only 30% of maximum RPM).

WOT allows an engine to operate most efficiently through the reduction of pumping losses. An engine with a restricted throttle opening is literally throwing efficiency away. Diesel engines lack a throttle butterfly, instead of controlling a butterfly valve, the accelerator of a Diesel engine modulates the injector pulse duration. The big efficiency gain of a Diesel engine is largely due to the fact that it has vastly lower pumping loss when compared to a gasoline engine. Yes, Diesel fuel has a higher energy content, but not enough higher to account for the majority of the efficiency advantage that a diesel enjoys.

Again, the most efficient real-world scenario is to use 75% to 90% throttle and upshift as soon as is possible without lugging the engine.

Additionally, engines are tuned for WOT operation in most cases. As I said before, when you're cruising down the highway on level ground, your car is only making 25-35 hp. That's all. No matter where in the RPM range you are at and no matter what your engine's actual rating is at that RPM. HP and torque ratings are always taken at WOT, at cruise, you close the throttle to intentionally choke the engine and reduce power efficiency in order to prevent constant acceleration. The reason that mileage efficiency stays acceptable is because mileage efficiency is, as previously stated, unrelated to power efficiency.

ZV
 

Zenmervolt

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Oct 22, 2000
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Originally posted by: Dman877
Why is rpm the 800 pound gorilla? Isn't load on the engine what determines fuel usage? If you gun the engine in neutral, you aren't using as much gas as you are when you're WOT in a gear because there is no load on the engine. This is in fuel injected cars of course...

I should have clarified.

RPM is the 800 pound gorilla for a given amount of work being done.

Revving the engine in neutral means there is essentially no work being done (aside from overcoming internal friction of the engine).

Unless you plan to be jettisoning ballast while driving though, there's no effective way to reduce the load on your engine. In essence, it's not a variable in the real world.

Whether the car is in 2nd gear at redline or 5th gear at 2,000 rpm, if the speed is the same in both cases, the amount of power required is the same for a given weight and even though both have reduced pumping losses (assuming both are WOT), 5th gear at 2,000 rpm will yield better mileage because of reduced friction in the engine (as engine speed increase, internal friction increases).

So yes, load does determine fuel use to a large extent. However, it's not something that is practical to consider to be a variable in the real world. The ways to reduce load are things like not towing a trailer everywhere or not keeping 500 pounds of concrete in your trunk. If you are towing the trailer, it's because you need to. If you have 500 pounds of concrete in your trunk, it's because you need to (in most cases).

Assuming that one is already taking practical measures to prevent hauling excessive amounts of junk in the car, engine load really cannot be varied easily "on the fly", but using the WOT + short-shifting technique can be, and it yields noticeable benefits.

Note that I am also assuming largely level roads. If there are hills, engine load can be varied to some degree by allowing the car to lose speed during uphill sections and only accelerating on downhill sections. However it should be noted that such practices may, depending on the road, number of lanes, and traffic density, severely disrupt the overall traffic flow, so I recommend using them intelligently and doing one's best not to cause problems for others.

ZV
 

PlasmaBomb

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Nov 19, 2004
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Some of the more modern EFI cars are beginning to run direct injection (DI) too, although some use DI in combination with a conventional throttle/injector (twinport and such like).

At very low engine speeds you also loose some thermodynamic efficiency as the expanding gas has relatively longer to lose heat to the engine block.
 

yelo333

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Dec 13, 2003
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Originally posted by: Zenmervolt
Originally posted by: PandaBear
Engine's efficiency band is usually at 40-50% maximum rpm (redline) and that means usually WOT is not as efficient than having your pedal 40-60% down. Your engine has to retard timing and run richer when you WOT, and the engine is usually tuned to the most commonly used band, around 1500rpm to 3500rpm.

Once again, peak power efficiency is NOT synonymous with peak mileage efficiency.


WOT allows an engine to operate most efficiently through the reduction of pumping losses. An engine with a restricted throttle opening is literally throwing efficiency away. Diesel engines lack a throttle butterfly, instead of controlling a butterfly valve, the accelerator of a Diesel engine modulates the injector pulse duration. The big efficiency gain of a Diesel engine is largely due to the fact that it has vastly lower pumping loss when compared to a gasoline engine. Yes, Diesel fuel has a higher energy content, but not enough higher to account for the majority of the efficiency advantage that a diesel enjoys.

Again, the most efficient real-world scenario is to use 75% to 90% throttle and upshift as soon as is possible without lugging the engine.

ZV

Perhaps my car (MT 07 Civic LX) is an exception to your general rule of 75% to 90% throttle w/out lugging?

1) It has this i-VTEC thing, which allows the throttle butterfly to remain almost all the way open during low load by delaying the closure of the intake valves until the piston is a good way into the upstroke phase. (did I say this right?)

2) It has a DBW throttle, which maps throttle in a nonlinear way, to make the engine feel more responsive. In the real world, it means I get minimal additional throttle response by going WOT compared to when the throttle is halfway down. If I were following your suggestion, I would be shooting for 75-90% throttle, but not necessarily 3/4 of the way down, right?

I'm thinking that these two things would mean the following if I'm trying to achieve maximum gas mileage:

Get the economy mode to kick in ASAP, and give it only as much throttle as will keep it there. I can feel it change over, since there is a (very) slight jerk and change in engine pitch. Still shift at the lowest speed that doesn't cause lugging.

In practice, there it takes such a small amount of throttle to get it back out of economy mode that it takes me about as long as it would for a fully loaded semi to get up to speed when I try this. It just isn't worth it. So your recommendation is probably still the best, even for my car.
 

Zenmervolt

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Oct 22, 2000
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Originally posted by: yelo333
Originally posted by: Zenmervolt
Originally posted by: PandaBear
Engine's efficiency band is usually at 40-50% maximum rpm (redline) and that means usually WOT is not as efficient than having your pedal 40-60% down. Your engine has to retard timing and run richer when you WOT, and the engine is usually tuned to the most commonly used band, around 1500rpm to 3500rpm.

Once again, peak power efficiency is NOT synonymous with peak mileage efficiency.


WOT allows an engine to operate most efficiently through the reduction of pumping losses. An engine with a restricted throttle opening is literally throwing efficiency away. Diesel engines lack a throttle butterfly, instead of controlling a butterfly valve, the accelerator of a Diesel engine modulates the injector pulse duration. The big efficiency gain of a Diesel engine is largely due to the fact that it has vastly lower pumping loss when compared to a gasoline engine. Yes, Diesel fuel has a higher energy content, but not enough higher to account for the majority of the efficiency advantage that a diesel enjoys.

Again, the most efficient real-world scenario is to use 75% to 90% throttle and upshift as soon as is possible without lugging the engine.

ZV

Perhaps my car (MT 07 Civic LX) is an exception to your general rule of 75% to 90% throttle w/out lugging?

1) It has this i-VTEC thing, which allows the throttle butterfly to remain almost all the way open during low load by delaying the closure of the intake valves until the piston is a good way into the upstroke phase. (did I say this right?)

2) It has a DBW throttle, which maps throttle in a nonlinear way, to make the engine feel more responsive. In the real world, it means I get minimal additional throttle response by going WOT compared to when the throttle is halfway down. If I were following your suggestion, I would be shooting for 75-90% throttle, but not necessarily 3/4 of the way down, right?

I'm thinking that these two things would mean the following if I'm trying to achieve maximum gas mileage:

Get the economy mode to kick in ASAP, and give it only as much throttle as will keep it there. I can feel it change over, since there is a (very) slight jerk and change in engine pitch. Still shift at the lowest speed that doesn't cause lugging.

In practice, there it takes such a small amount of throttle to get it back out of economy mode that it takes me about as long as it would for a fully loaded semi to get up to speed when I try this. It just isn't worth it. So your recommendation is probably still the best, even for my car.

Yes, if the throttle butterfly is non-linear with the accelerator pedal, then you want to aim for 75%-90% throttle opening, not necessarily 75%-90% accelerator pedal position. :)

ZV
 

JulesMaximus

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Jul 3, 2003
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Originally posted by: Dman877
Why is rpm the 800 pound gorilla? Isn't load on the engine what determines fuel usage? If you gun the engine in neutral, you aren't using as much gas as you are when you're WOT in a gear because there is no load on the engine. This is in fuel injected cars of course...

Simple, at 7000 rpms your engine is gulping in more fuel and air than at 2000 rpms.
 

Dman877

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Jan 15, 2004
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If you are traveling up a hill at 2000 rpm and accelerating, you will use more fuel than if you are coasting down a hill at 7000 rpm.
 

Gamingphreek

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Mar 31, 2003
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Originally posted by: Dman877
If you are traveling up a hill at 2000 rpm and accelerating, you will use more fuel than if you are coasting down a hill at 7000 rpm.

Are you sure about that? I honestly don't know for sure, but that doesn't seem right.
 

mwmorph

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Dec 27, 2004
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Originally posted by: Gamingphreek
Originally posted by: Dman877
If you are traveling up a hill at 2000 rpm and accelerating, you will use more fuel than if you are coasting down a hill at 7000 rpm.

Are you sure about that? I honestly don't know for sure, but that doesn't seem right.

Coasting down a hill at 7krpm means you're probably in 1st gear, engine braking.
 

alpineranger

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Feb 3, 2001
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The i-vtec thing on the civic's engine is even better. In low load operation it uses one of the vtec cam profiles to run super high overlap with the intake vales closing very late. In this mode it is an atkinson cycle engine, with correspondingly higher efficiency but lower power density. Since you aren't running at very high load to begin with the dbw throttle can open wide and give you the amount of power you were expecting. Not only are pumping losses reduced but the atkinson cycle is more thermodynamically efficient as well.

At WOT, it is safer and produces more power to run intentionally rich. But since most cars have narrowband lambda o2 sensors, they also have to drop out of closed loop operation (where the o2 sensor readings are considered to compensate for wear and other deviations from stock programming conditions).

http://www.tc.gc.ca/programs/e...ort/image/Final_28.gif
Study this graph, which is representative of a standard engine. The isocontours show BSFC (brake specific fuel consumption, how much fuel is consumed per hp generated). The plot points represent samples from a real world driving test. As you can see, you can generate power more efficiently by operating in the low-mid rpm range at very high load (represents almost WOT operation).
 

alkemyst

No Lifer
Feb 13, 2001
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Yeah these are some retarded car questions lately fuzzy.

Gas consumption at WOT is crazy...

In basics the least rpms you turn the engine the better mileage you are going to get though. If you shift as soon as you can without bogging the engine you'll get great mileage. The trade off is a slow start.

Personally I wind it out to 4K+ usually. My drive is short, I like it fun. My mileage is only about 17-19mpg when 'city' for my car is 22, highway 28.
 

Assimilator1

Elite Member
Nov 4, 1999
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Zenmervolt
The big efficiency gain of a Diesel engine is largely due to the fact that it has vastly lower pumping loss when compared to a gasoline engine.

Actually that's one of it's smaller gains ,the bigger gain is from their higher compression ratios.

Yes, Diesel fuel has a higher energy content, but not enough higher to account for the majority of the efficiency advantage that a diesel enjoys.

No it doesn't ,Petrol (gasoline:p) has a higher energy content hence part of the reason a petrol engine with the same aspiration setup & engine size etc will have a higher power output.
 

Zenmervolt

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Oct 22, 2000
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Originally posted by: Assimilator1
Zenmervolt
The big efficiency gain of a Diesel engine is largely due to the fact that it has vastly lower pumping loss when compared to a gasoline engine.

Actually that's one of it's smaller gains ,the bigger gain is from their higher compression ratios.

Yes, Diesel fuel has a higher energy content, but not enough higher to account for the majority of the efficiency advantage that a diesel enjoys.

No it doesn't ,Petrol (gasoline:p) has a higher energy content hence part of the reason a petrol engine with the same aspiration setup & engine size etc will have a higher power output.

Nope, sorry. Wrong on both counts.

An increase in compression ratio will also allow an increase in efficiency, but the larger gain is from the lack of a throttle butterfly in terms of mpg efficiencies. A Diesel's increased thermal efficiency is indeed due largely to its higher static compression ratio, but the player in mileage efficiency is the absence of a throttle butterfly and the corresponding absence of any pumping losses. It takes a great deal of energy for a gasoline engine to maintain manifold vacuum.

Diesel has more energy per unit volume than gasoline. A gallon of Diesel fuel contains, on average, 147,000 BTU, while a gallon of gasoline has, on average, 125,000 BTU. Diesels have lower HP ratings because the engines are tuned to produce high torque at low RPM, a design consideration that necessarily restricts total HP output. The nature of Diesel engines requires heavier components than gasoline engines, which also limits RPM.

Thanks for playing though.

ZV
 

Howard

Lifer
Oct 14, 1999
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Originally posted by: Dman877
If you are traveling up a hill at 2000 rpm and accelerating, you will use more fuel than if you are coasting down a hill at 7000 rpm.
Don't forget about throttle...
 

Dman877

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Jan 15, 2004
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Accelerating = more throttle than coasting. EFI supplies the engine with an amount of fuel based on throttle position. When you lift your foot off the gas, whether the engine is spinning at 2k or 7k, the amount of fuel going into the cylinder drops to almost nothing. This is why engine braking works, with so much air in the cylinder and so little fuel, the engine actually drags on the car because it is producing no power. This is the advantage of fuel injection. Your engine only produces max power when it's called on to do so.