Altitude: how it affects fuel/air ratio

[Robbins]

Lets say i am at or near sea level. the column of air on top of my airplane is very tall.

then i travel to my place at 1350" ASL

I would probably need to adjust the amount of fuel at the needles, correct?

second, i f i were to fly at the "near sea level" setting at the new home (1350" ASL) WITHOUT adjusting needles, would my engine act fat or lean?

 

[ericb]

..

 

[Bartman]

at higher altitudes, using needle settings from sea level, because there is less oxygen the motor will run rich. but the altitude difference you're talking about probably isn't enough to be noticed in how the engine will run on any given day. a cold day at sea level vs a hot day at the higher elevation might be enough to notice.

 

[Bunky.F.Knuckle]

Just fly it!

Steve Stricker said it best in 1996 at the TOC. Never touched the needles on his 3W 164 4 holer. Practicing from MD then going to Tuscon to compete.

 

[Robbins]

well thanks for the input thus far. however we all know how finicky the smaller 2 stroke engines can be, agreed?

im talking 40cc twin

does anyone know what the column of air on the venturi of the carb weighs near sea level versus south dakota at 1350ft (lets assume the RH and Temperature are the same)?

 

[Alky6]

I am guessing 3-5% less. I don't have that info in front of me, but I could look it up later. VVRC twin on your EF?

 

[Alky6]

OK. 7% less is the change from SL to 15oo' ASL.
air density:
0.0765 lbm/cuft at SL
.071+- lbm/cuft at 1500' ASL

you will see larger fluctuations in atmospheric, RH, and temp at the same elevation throughout the year than you will changing from 0 to 1500' ASL, IMHO. That's based on results I have seen to correct dynamometer engine HP to SAE standard temperature and pressure. Of course you will exacerbate the problem if you go from low, dry and cool, to high, wet and hot.

 

[john_M]

I believe on the gasser walbro type carbs, there is an atmospheric vent on the cover of the fuel regulator diaphragm, which should compensate as the atmosphere pressure changes with altitude once you set the air/furl ratio at ground level... granted, I couldn't tell to what degree it will compensate... but I do know that air pressure build up inside the cowling can have an affect on the fuel regulator diaphragm, and as a result can have an affect on the fuel mixture... that's why some extend the fuel regulator diaphragm vent to a more suitable location where there is no pressure build up from the propeller and or forward flight.


John M,

 

[dhal22]

My head hurts.

 

[Bunky.F.Knuckle]

I'm a "what"?!?!?!