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Air Density and Fuel Mixture
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Author:  Minimadmarty [ Tue Mar 11, 2014 8:57 am ]
Post subject:  Air Density and Fuel Mixture
How much does air density affect fuel/air mixture in every day running? Is it significant?

If say I have tuned my SU's on a Sunday morning, my house is at sea level, humidity and barometric pressure remains constant all day it was 20 degree C when I tuned the carburettors. By midday the temperature is hitting 40 degree (I live in Perth) obviously air density will decrease and my mixture will be running richer but is the difference significant or negligible? Will the richer running help in engine cooling or will it just fowl the plugs and start running rough?
Author:  FNQ [ Tue Mar 11, 2014 11:13 am ]
Post subject:  Re: Air Density and Fuel Mixture
Basically in both the scenarios you provide the engine compartment stats will be pretty similar. ie the 180-190 degree F internal temp , but engines do love cool air and it can be felt a little. ( i had an intercooled turbo disco that used to run a touch less revs at 100km on a cold dense air night, and fuel consumption was tinily better. ..

If you drove 2000 metres up the mountain things change, but mostly the mini likes running a tad richer than stoich(14.7) anyway
Author:  Irish Yobbo [ Tue Mar 11, 2014 12:52 pm ]
Post subject:  Re: Air Density and Fuel Mixture
FNQ wrote:
Basically in both the scenarios you provide the engine compartment stats will be pretty similar. ie the 180-190 degree F internal temp , but engines do love cool air and it can be felt a little. ( i had an intercooled turbo disco that used to run a touch less revs at 100km on a cold dense air night, and fuel consumption was tinily better. ..

If you drove 2000 metres up the mountain things change, but mostly the mini likes running a tad richer than stoich(14.7) anyway


You wouldn't have been using less revs at 100km/hr, unless you had something to change your gear ratio ;)

Altitude will make a difference, but not so much in Australia, sice we don't have high altittudes. I've heard 2% per 1500 ft before, but don't quote me on that. Temperature will make a difference too, but I don't know how much.

The stoichiometric air-fuel ratio is about 14.7:1, but it's very rare for any engine to run at that. It's usually a bit richer, because the mixture is never perfectly mixed, and there is little time for combustion. A modern EFI car will run at higher air-fuel ratios on the highway, because it's better for fuel economy. But around town they will still run a little rich, just like a carburetted car - when you put your fut down, you want it to go.
Author:  Dan the Mini Man [ Tue Mar 11, 2014 1:05 pm ]
Post subject:  Re: Air Density and Fuel Mixture
No Marty you can't install a mixture knob in your car.

Well you could but I'm just saying would it be right without a manifold pressure gauge?? :twisted:
Author:  Minimadmarty [ Tue Mar 11, 2014 5:20 pm ]
Post subject:  Re: Air Density and Fuel Mixture
Dan the Mini Man wrote:
No Marty you can't install a mixture knob in your car.

Well you could but I'm just saying would it be right without a manifold pressure gauge?? :twisted:


Mmmm..... Now that's a great idea, a mixture knob, EGT and MP gauge! You got any supercharged Lycomings sat in the back of your garage Dan?
Author:  Mick [ Tue Mar 11, 2014 5:23 pm ]
Post subject:  Re: Air Density and Fuel Mixture
Air density at sea level will vary by about 10 percent at the temperature range you have spoken of. Whether that warrants a change in mixture I don't know. http://www.engineeringtoolbox.com/air-t ... d_771.html

If you play with a gunson colortune, you will notice however that there is a plateau the carb tune reaches as it passes from rich (orange) to stoichiometric (bunson blue). It will stay there for a few flats (four on my car) if you continue to turn before it starts to go lean (powder blue).

To me, in my opinion, this means that there is a small range of mixture through which the mixture will remain correct despite a change in conditions. This may be due to carb design or otherwise, I'm unsure, but it does give the impression the carb will be be fine with a small change in mixture.

This was relevant to a 1275 with standard cam and single 1.5" SU.
Author:  simon k [ Tue Mar 11, 2014 5:27 pm ]
Post subject:  Re: Air Density and Fuel Mixture
*cough* fuel injection *cough cough*
Author:  Dan the Mini Man [ Tue Mar 11, 2014 6:58 pm ]
Post subject:  Re: Air Density and Fuel Mixture
Maybe just a turbo supercharged Conti but stop mucking about and turbine in there Marty.
Author:  FNQ [ Tue Mar 11, 2014 7:06 pm ]
Post subject:  Re: Air Density and Fuel Mixture
doooh revs At a specified speed with same gear ratios must be the same....... sorry irish ... homer moment
Author:  Mick [ Tue Mar 11, 2014 7:19 pm ]
Post subject:  Re: Air Density and Fuel Mixture
simon k wrote:
*cough* fuel injection *cough cough*


Sounds like its missing under load there Simon.
Author:  Minimadmarty [ Tue Mar 11, 2014 9:24 pm ]
Post subject:  Re: Air Density and Fuel Mixture
Mick wrote:
Air density at sea level will vary by about 10 percent at the temperature range you have spoken of. Whether that warrants a change in mixture I don't know.



Based on temperature change only at sea level, standard barometric pressure of 1013 and zero humidity at 20 degree Celsius air density is 1.204 kg/m3 which equates to estimated AWOS 600 ft.
At 40 degree Celsius air density is 1.135 kg/m3 and equates to an estimated AWOS 2600 ft. The 2000 ft different will make a difference but will it be noticeable? Probably not! Back to the drawing board!
Author:  Mick [ Tue Mar 11, 2014 9:30 pm ]
Post subject:  Re: Air Density and Fuel Mixture
The linked chart made that easier, but it is impressive all the same :P
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