AC question!!! Which saves more gas?!?!

Snakes709 · 08-25-2011, 07:11 AM

only time i use the a/c is if im on long road trips on the highway and if its really hot. The Z didnt have a huge issue buy my Evo does. Damn thing nearly stalls at when im at a light with the a/c on...thing sucks so much power from the car its unreal.

wstar · 08-25-2011, 07:35 AM

If cycling your AC on and off manually saves gas, it's just because you're making the average temperature higher than it would be if you left it on all the time at the coldest setting. In any case, even on older cars the A/C compressor cycles on and off while you have it "on". It's just part of how the whole system operates.

The A/C in the Z is thermostat controlled anyways. IMHO, set all the dials to "Auto" and set the temperature at your comfort vs cost limit. The colder you set the dial, the more gas you'll suck over time. It will be far less trouble and just about as effective.

A related issue that's been debunked before: rolling down the window vs running the A/C. Assuming you can bear the heat level of running with the windows down and no A/C to save gas, it actually does save gas at low speeds (city streets). However, on the highway, the parasitic drag on the car from having the windows open wastes more gas the running the A/C, so don't do that.

DaveyG · 08-25-2011, 08:12 AM

Quote:

Originally Posted by wstar

A related issue that's been debunked before: rolling down the window vs running the A/C. Assuming you can bear the heat level of running with the windows down and no A/C to save gas, it actually does save gas at low speeds (city streets). However, on the highway, the parasitic drag on the car from having the windows open wastes more gas the running the A/C, so don't do that.

So true, I wish people would understand this. I swear I always run into people saying ac uses more gas no matter what. Either you understand physics or you don't I guess.

Red__Zed · 08-25-2011, 08:52 AM

Quote:

Originally Posted by DaveyG

So true, I wish people would understand this. I swear I always run into people saying ac uses more gas no matter what. Either you understand physics or you don't I guess.

It's more of a fluids question...

...but at 70 or so, most cars use about 2.5% more gas with the windows down, and 5% more with AC on, so.....

wstar · 08-25-2011, 09:17 AM

Mythbusters tested this (measuring fuel consumption and efficiency in real cars), and then went back and revisited the issue after comments and concerns and re-tested. Wikipedia's final summary of their final summary:

The fundamental flaw in the MythBusters' test was that the point where the drag becomes powerful enough to inhibit a car's performance with windows down was inside their 45–55mph margin at 50mph. Going less than 50mph it is more efficient to leave the windows down, but going greater than 50mph it is more efficient to use the A/C.

ETA: It's probably a reasonable assumption that the 50mph figure will vary depending on the aerodynamic configuration of the car and the windows, as well as other differences affecting the A/C compressor's efficiency. But it's a reasonable ballpark, and reasonable cars' break-even point probably doesn't vary a whole lot.

Waiz · 08-25-2011, 09:48 AM

Quote:

Originally Posted by wstar

If cycling your AC on and off manually saves gas, it's just because you're making the average temperature higher than it would be if you left it on all the time at the coldest setting. In any case, even on older cars the A/C compressor cycles on and off while you have it "on". It's just part of how the whole system operates.

The A/C in the Z is thermostat controlled anyways. IMHO, set all the dials to "Auto" and set the temperature at your comfort vs cost limit. The colder you set the dial, the more gas you'll suck over time. It will be far less trouble and just about as effective.

A related issue that's been debunked before: rolling down the window vs running the A/C. Assuming you can bear the heat level of running with the windows down and no A/C to save gas, it actually does save gas at low speeds (city streets). However, on the highway, the parasitic drag on the car from having the windows open wastes more gas the running the A/C, so don't do that.

Great post, very informative!

OP, I do the same thing as you and the difference is barely noticeable.

08-25-2011, 07:11 AM	#1 (permalink)
Snakes709 A True Z Fanatic Join Date: Aug 2009 Location: Edmonton Posts: 1,205 Drives: 2011 Evo-X GSR Rep Power: 18	only time i use the a/c is if im on long road trips on the highway and if its really hot. The Z didnt have a huge issue buy my Evo does. Damn thing nearly stalls at when im at a light with the a/c on...thing sucks so much power from the car its unreal. __________________ 2011 Evo-GSR AMS 2.4 stroker/ bore, FP Black build in progress. Sponsor: DSG Performance

08-25-2011, 07:35 AM	#2 (permalink)
wstar A True Z Fanatic Join Date: Mar 2009 Location: Houston, TX Posts: 4,024 Drives: too slow Rep Power: 3595	If cycling your AC on and off manually saves gas, it's just because you're making the average temperature higher than it would be if you left it on all the time at the coldest setting. In any case, even on older cars the A/C compressor cycles on and off while you have it "on". It's just part of how the whole system operates. The A/C in the Z is thermostat controlled anyways. IMHO, set all the dials to "Auto" and set the temperature at your comfort vs cost limit. The colder you set the dial, the more gas you'll suck over time. It will be far less trouble and just about as effective. A related issue that's been debunked before: rolling down the window vs running the A/C. Assuming you can bear the heat level of running with the windows down and no A/C to save gas, it actually does save gas at low speeds (city streets). However, on the highway, the parasitic drag on the car from having the windows open wastes more gas the running the A/C, so don't do that. __________________ 7AT Track Car! Journal thread / Car setup details

08-25-2011, 09:17 AM	#5 (permalink)
wstar A True Z Fanatic Join Date: Mar 2009 Location: Houston, TX Posts: 4,024 Drives: too slow Rep Power: 3595	Mythbusters tested this (measuring fuel consumption and efficiency in real cars), and then went back and revisited the issue after comments and concerns and re-tested. Wikipedia's final summary of their final summary: The fundamental flaw in the MythBusters' test was that the point where the drag becomes powerful enough to inhibit a car's performance with windows down was inside their 45–55mph margin at 50mph. Going less than 50mph it is more efficient to leave the windows down, but going greater than 50mph it is more efficient to use the A/C. ETA: It's probably a reasonable assumption that the 50mph figure will vary depending on the aerodynamic configuration of the car and the windows, as well as other differences affecting the A/C compressor's efficiency. But it's a reasonable ballpark, and reasonable cars' break-even point probably doesn't vary a whole lot. __________________ 7AT Track Car! Journal thread / Car setup details

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