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Old 10-27-2012, 04:20 PM   #1
IveBeenRued
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Default Does anyone know the fuel consumption at idle?

Out of curiosity, does anyone know the fuel consumption(gallons per hour) at idle for the 2012+ Imprezas 2.0i?
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Old 10-27-2012, 10:32 PM   #2
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On a warmed up engine with the A/C off, about 0.18 GPH.
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Old 10-27-2012, 11:48 PM   #3
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Quote:
Originally Posted by Commander Keen View Post
On a warmed up engine with the A/C off, about 0.18 GPH.

where did you get that number?
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Old 10-28-2012, 12:48 AM   #4
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Quote:
Originally Posted by Commander Keen
On a warmed up engine with the A/C off, about 0.18 GPH.
Kind of interested to find how you knew that number too.
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Old 10-28-2012, 10:34 AM   #5
Commander Keen
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At 10 MPH in neutral, the instantaneous MPG is 58.

The car will take 58 miles to consume 1 gallon.

The car will take 5.8 hours to cover 58 miles.

Therefore, the car will take 5.8 hours to consume 1 gallon.

1/5.8=0.1724 or 0.18, rounded up.

This number depends on the accuracy of the IMPG readout.
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Old 10-28-2012, 11:53 AM   #6
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lol that doesn't sound accurate at all.

i was trying to calculate this yesterday by taking the air to fuel ratio at idle which in most cars is 13 to 13.6 opposed to 14.2-14.6 cruising. this is why people say our cars runs rich at idle. assuming this, i took the size of the engine and divided by 13 because only 1/13 of the air filled in the car will be fuel. but it will only burn this fuel when all the cylinders have fired. how many rotations is that, 2. so it burns x amount of fuel for every 2 rotations. then you have "y" gas/rotation. then take this number times the idle rpm which would give you "z" gas/min. then times by 60 min/hour. then convert to gallons. this will give you gallons/hour. but the math just doesn't add up. logically makes since to me but i am missing something.
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Old 10-28-2012, 12:15 PM   #7
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You are missing something - volumetric efficiency. The amount of air in the cylinders varies constantly with RPM and throttle angle, so the amount of fuel will as well.

Our cars idle in closed loop mode, so a hot idle is probably around 14.7:1.
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Old 10-28-2012, 12:16 PM   #8
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You can just plug a laptop into the OBD2 port and look at how many cc the injectors are spraying at idle...

Also, if you have an automatic, idling in D will use a lot more fuel than idling in N due to the added load from the torque converter.

.2 is probably pretty close to what you use at warm idle with no load on a 2L, about .25ish for a 2.5L

-- Dave

Last edited by AcquaCow; 10-28-2012 at 12:22 PM.
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Old 10-29-2012, 08:53 AM   #9
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Thanks guys! It looks like my impreza will likely be powering my sump-pump via a power inverter if the power goes out for an extended period of time.
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Old 10-29-2012, 08:59 AM   #10
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Quote:
Originally Posted by AcquaCow View Post
You can just plug a laptop into the OBD2 port and look at how many cc the injectors are spraying at idle...

Also, if you have an automatic, idling in D will use a lot more fuel than idling in N due to the added load from the torque converter.

.2 is probably pretty close to what you use at warm idle with no load on a 2L, about .25ish for a 2.5L

-- Dave
I thought CVTs don't have a torque converter...?
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Old 10-29-2012, 09:38 AM   #11
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Originally Posted by goregasm View Post
I thought CVTs don't have a torque converter...?
How did you think the engine kept running in gear?
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Old 10-29-2012, 09:40 AM   #12
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Quote:
Thanks guys! It looks like my impreza will likely be powering my sump-pump via a power inverter if the power goes out for an extended period of time.
Unless you have a powerful inverter and a tiny sump pump, I have my doubts.
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Old 10-29-2012, 09:59 AM   #13
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Originally Posted by IveBeenRued View Post
Thanks guys! It looks like my impreza will likely be powering my sump-pump via a power inverter if the power goes out for an extended period of time.
If you don't have a well, you should get one of those water-powered sump pumps...
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Old 10-29-2012, 03:51 PM   #14
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Quote:
Originally Posted by auskip07 View Post
How did you think the engine kept running in gear?
I'm not a car expert so I'm not gonna pretend to know anything, but google "cvt torque converter".
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Old 10-30-2012, 06:07 PM   #15
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Good news guys. My power is back up, my basement is dry, and I have some relatively accurate numbers for you. My 2012 Impreza Sport Limited idled for 22.5 hours and consumed 3.544 gallons of fuel (including the 5.7 miles that it took me to go back and forth to the gas station). So including a few miles, my consumption rate at warm idle was .1575 GPH.

I had the car idling in my garage(with proper ventilation and a CO monitor running) with a 750/1500 watt RMS/Peak inverter hooked up and it kept my sump-pump running flawlessly. The pump would cycle on for 35 seconds every 5-6 minutes and would peak at 646 peak watts at startup then drop down to 344-362 watts for the remainder of the cycle.

My neighbor to my right has a flooded basement because his brand new generator failed and he had no backup system. Even though I have no backup system my basement is bone-dry thanks to my Subaru.

On a side note my Impreza also saved our Beta fish from freezing by running his 15 watt heater continuously via the same power inverter.
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Old 10-30-2012, 06:23 PM   #16
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Quote:
Originally Posted by IveBeenRued
Good news guys. My power is back up, my basement is dry, and I have some relatively accurate numbers for you. My 2012 Impreza Sport Limited idled for 22.5 hours and consumed 3.544 gallons of fuel (including the 5.7 miles that it took me to go back and forth to the gas station). So including a few miles, my consumption rate at warm idle was .1575 GPH.

I had the car idling in my garage(with proper ventilation and a CO monitor running) with a 750/1500 watt RMS/Peak inverter hooked up and it kept my sump-pump running flawlessly. The pump would cycle on for 35 seconds every 5-6 minutes and would peak at 646 peak watts at startup then drop down to 344-362 watts for the remainder of the cycle.

My neighbor to my right has a flooded basement because his brand new generator failed and he had no backup system. Even though I have no backup system my basement is bone-dry thanks to my Subaru.

On a side note my Impreza also saved our Beta fish from freezing by running his 15 watt heater continuously via the same power inverter.
Time for an oil change
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Old 10-30-2012, 07:00 PM   #17
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Originally Posted by flyboy1100 View Post
Time for an oil change
not likely. I ran our subie in march for about 20 hours of that we idled (caught in a snowstorm) from 10 pm to 6 am keeping warm and then drove from 6am to 12pm then waited 4000 more miles for an oil change and the oil sample came out fine. we averaged 29mpg also on that trip with the idling included.

as a side note, you just about don't have to worry about CO poisoning anymore because car exhaust is so clean now.
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Old 10-30-2012, 09:58 PM   #18
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It looks like your pump averaged 34W throughout the night. It's funny to think that small amount of power made the difference between a dry basement and a flooded one.

Low idle fuel consumption is another benefit of the boxer. I haven't seen an inline four that could idle as smoothly and slowly as our engines do.

Being able to idle three days straight on a full tank has its survival value.

Glad you pulled it off, and thanks for the numbers.

Last edited by Commander Keen; 10-30-2012 at 10:09 PM.
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Old 10-31-2012, 07:58 PM   #19
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Good news guys. .
This is awesome...
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Old 11-01-2012, 12:29 PM   #20
myrt1987
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Quote:
Originally Posted by IveBeenRued View Post
Good news guys. My power is back up, my basement is dry, and I have some relatively accurate numbers for you. My 2012 Impreza Sport Limited idled for 22.5 hours and consumed 3.544 gallons of fuel (including the 5.7 miles that it took me to go back and forth to the gas station). So including a few miles, my consumption rate at warm idle was .1575 GPH.
Idling in neutral or park?

That seems pretty good for an overestimation. Nice.

Quote:
Originally Posted by Commander Keen View Post
At 10 MPH in neutral, the instantaneous MPG is 58.

The car will take 58 miles to consume 1 gallon.

The car will take 5.8 hours to cover 58 miles.

Therefore, the car will take 5.8 hours to consume 1 gallon.

1/5.8=0.1724 or 0.18, rounded up.

This number depends on the accuracy of the IMPG readout.

Thats pretty close. I didnt think this was too accurate at first.
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Old 11-01-2012, 01:07 PM   #21
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Originally Posted by myrt1987 View Post
Idling in neutral or park?

That seems pretty good for an overestimation. Nice.
It was in park the entire time. I turned everything off that I could including the screen on the radio.

Yeah I was pleasantly surprised at the consumption rate. I always have at least 20 gallons in a monthly rotating stockpile so even if I came home on an empty tank I could power my sump-pump for more than 5 days straight.
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Old 11-01-2012, 02:27 PM   #22
myrt1987
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Quote:
Originally Posted by IveBeenRued View Post
It was in park the entire time. I turned everything off that I could including the screen on the radio.

Yeah I was pleasantly surprised at the consumption rate. I always have at least 20 gallons in a monthly rotating stockpile so even if I came home on an empty tank I could power my sump-pump for more than 5 days straight.
I only say overestimate because of the miles you drove to gas station. (we all know how great these cars are in short trips) Thanks for reporting.

This makes me wonder if shifting to neutral while stopped at a red light saves gas.
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Old 11-01-2012, 11:24 PM   #23
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I got around a litre per hour which is roughly 0.25 galons/hr.

Measured around 100 km/h coasting, 1l/100 km instantaneous consumption, so 1l/h.I have a MT, so for coasting I just depressed the cluch - which does reduce the drag with respect to being in neutral, but I don't believe it's significant. Might measure again to check...
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Old 11-01-2012, 11:54 PM   #24
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Coasting in gear uses zero fuel.
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Old 11-02-2012, 11:45 AM   #25
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Quote:
Originally Posted by Commander Keen
Coasting in gear uses zero fuel.
Really!? So that means I can stop shifting into neutral when coasting down hills? I thought I was saving gas but I guess all I was doing is wearing my breaks faster.
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