Altitude: how it affects fuel/air ratio
- Thread starter Robbins
- Start date
Robbins
Team WTFO (Watch The Fun Occur)
for the sake keeping with units that we as aeromodelers are more familliar with, i translate that to;
235oz/in^2 ASL
VS
199 oz/in^2 At my elevation
this is a 36 oz/in^2 gradient which is considerable. Now if you want to factor in the composition ratio of O2 in the atmosphere and interpolate the resulting oxygen content (neglecting temp and humidity for now...) in that gradient, you can then actually show that fuel consumption will change. that can be measured in oz/minute.
the big thing that we must realize is that a properly tuned engine serial (number X) at one elevation will display the same characteristics if the engine is ran (serial number X) at the other elevation. Result: same plug color, same cylinder and piston condition, same happy user.
*Note performance data will be what changes.
can you shoot me an email at robbins540@gmail.com ? i saw something you were doing a long time ago and i have a few off topic questions. Thanks
Last edited:
Robbins
Team WTFO (Watch The Fun Occur)
if walbro increased the volume in that chamber there would no need for the hole. but this would take up toom and small engines like chainsaws, weedwhackers, airplane engines dont have much luxury for room back there. some twins might. @ghoffman should 3d print a couple "risers" and we could play with that.
what i highlighted in red i object very strongly to and here is why. There is no compensation taking place. the hole is to make sure that carburetor has an adequate fuel supply on the needle side of the diaphragm at all times: idle, transition, blips, and WOT. if the diaphragm has restriction, its efficiency to oscillate and do its job (pump fuel) will diminish.
i do agree with you on the green highlights, but this is so specific to many variables.
Terryscustom
640cc Uber Pimp
I'm not smart enough on engines to get that high tech, so I relate my knowledge from the continued use of myself as a crash test dummy
We travel around a lot everywhere from 600 to about 3000 feet elevation events, and I notice that my 100'ish cc engines run just fine, never really re-tune them for location for a weekend event because I know I'm going home in a couple days.
70cc is more noticeable but again tolerable / not worth tuning.
60cc and smaller can suffer pretty descent, I can remember going from 1400 to 3000 with a DLE55 and noticed a pretty fair difference.
I think exhaust can also make a difference as your low needle can be quite different from can's, pipes, stock muffs vs. aftermarket high flow like J&A's.
Living in the midwest we also have this friendly thing called humidity that also messes with your tuning because you have to compensate for all that water in the air that you are pumping through. Travel to 3000 feet and you probably lost that humidity, that again will effect your tuning depending on CC's and volume that it is pumping.
We travel around a lot everywhere from 600 to about 3000 feet elevation events, and I notice that my 100'ish cc engines run just fine, never really re-tune them for location for a weekend event because I know I'm going home in a couple days.
70cc is more noticeable but again tolerable / not worth tuning.
60cc and smaller can suffer pretty descent, I can remember going from 1400 to 3000 with a DLE55 and noticed a pretty fair difference.
I think exhaust can also make a difference as your low needle can be quite different from can's, pipes, stock muffs vs. aftermarket high flow like J&A's.
Living in the midwest we also have this friendly thing called humidity that also messes with your tuning because you have to compensate for all that water in the air that you are pumping through. Travel to 3000 feet and you probably lost that humidity, that again will effect your tuning depending on CC's and volume that it is pumping.
Bartman
Defender of the Noob!
DA makes a red anodized diaphragm cover that has two holes on the edges to improve the stability of air pressure entering the backside of the diaphragm chamber.if walbro increased the volume in that chamber there would no need for the hole. but this would take up toom and small engines like chainsaws, weedwhackers, airplane engines dont have much luxury for room back there. some twins might. @ghoffman should 3d print a couple "risers" and we could play with that.
what i highlighted in red i object very strongly to and here is why. There is no compensation taking place. the hole is to make sure that carburetor has an adequate fuel supply on the needle side of the diaphragm at all times: idle, transition, blips, and WOT. if the diaphragm has restriction, its efficiency to oscillate and do its job (pump fuel) will diminish.
i do agree with you on the green highlights, but this is so specific to many variables.
Robbins
Team WTFO (Watch The Fun Occur)
youre a whole lot smarter than you give yourself credit!I'm not smart enough on engines...myself...crash test dummy
and you arent a crash test dummy, cuz ive NEVER actually seen you crash (wad one up in a ball)!
the only crash i ever think you had was due to a problem that wasnt even your fault.
Robbins
Team WTFO (Watch The Fun Occur)
yes i am aware of that. they are very pretty too!
It is another way to skin the cat though isnt it!
now were a little off topic but can you tell me with certainty that there is a hole in every walbro carb diaphragm cover?
Alky6
150cc
Agreed: pressure also reduces thereby reducing the pressure on the fuel to push it through the needles (jets) of the carb... I'll throw another wrinkle to this though. In actual application of racing outboards, I would "simply" add nitromethane to compensate for power loss in altitude conditions. No change in jetting (adding nitro leans the mixture). Ran great.
Sent you an email as well.
Alky6
150cc
Considering this some more. Pressure differential across venturi will remain the same. Pressure available to push fuel through the needles is also reduced proportionally to the pressure (hence your column of air). However, if we assume flow of a liquid through an orifice, that is on the square root of the pressure change. Therefore there continues to be an increase (more fuel to air) in stoichiometric ratio, though much smaller. The "perfect" carb would compensate for this, but carbs are not perfect.
Another issue to consider - a portion of the fuel added is used for cooling. Going to higher altitude (less power) produces less heat. Thereby the small component of fuel used for cooling (a portion of which is not needed) adds to the "richness" experienced. Could one compensate for this by achieving better cooling to reduce the need to use excess fuel for cooling?
Another issue to consider - a portion of the fuel added is used for cooling. Going to higher altitude (less power) produces less heat. Thereby the small component of fuel used for cooling (a portion of which is not needed) adds to the "richness" experienced. Could one compensate for this by achieving better cooling to reduce the need to use excess fuel for cooling?
TazmanianDevil
Xtreme by DeFinition !
Hi guys
I'm flying here in Israel from about -1200' to +3000' and you can feel that there is a different but it never got me to change the needles.
The times I do change it's in very hot days (105) and only then I've opened the low end a smidge.
I did try plying with the needle a bit when I was in those specific heights but I didn't feel much of a difference.
I'm flying here in Israel from about -1200' to +3000' and you can feel that there is a different but it never got me to change the needles.
The times I do change it's in very hot days (105) and only then I've opened the low end a smidge.
I did try plying with the needle a bit when I was in those specific heights but I didn't feel much of a difference.