Air/Fuel Mixture at Altitude for Bing carburetors, as fitted to BMW
From the Bing
Agency (the best place to get Bing parts in North America) Manual:
"From sea-level to about 3,000 feet / 914 meters elevation, atmospheric pressure decreases about one-inch-per-thousand feet / 0.1 bars per 1000 meters of altitude-therefore our 29.92 inches of Mercury / 1.013 bars barometer reading at sea-level now becomes about 27 inches of Mercury / 0.914 bars. At 5,000 feet / 1500 meters, about 25 inches of Mercury / 0.84 bars , and 20.6 inches of Mercury / 0.69 bars at 10,000 feet / 3000 meters.
Our engine (and carburetor) only react to air velocity-not weight of air. Not knowing this, the carburetor continues flowing the same amount (by weight) of fuel, and all of a sudden our "ideal mixture" at sea-level becomes extremely rich at higher elevations.
The Bing carburetor is less susceptible to changes in elevation than other carburettors in use, and in general does not require jetting changes for the idle and lower part-throttle settings. These settings can be compensated for by mixture screw adjustment. However, the upper part-throttle range will require the lowering of the jet needle to lean out mixture and continuous operation at higher elevation certainly necessitates the changing of the main jet. The table provides all information necessary to determine jetting changes in relation to altitude changes..
|Altitude in feet:||0||1,600||3,300||5,000||6,500||8,200||10,000||11,500||13,000|
|Altitude in meters:||0||500||1000||1500||2000||2500||3000||3500||4000|
|Temp (F)||Temp (Celsius)||A2 or A1:|
|-22 F||-30 C||104||103||101||100||98||97||95||94||93|
|- 4 F||-20 C||103||102||100||99||97||96||95||93||92|
|+14 F||-10 C||102||101||99||98||96||95||94||92||91|
EXAMPLE: You're going to do a lot of touring in Colorado (Elevation about 5,000 feet / 1500 meters). Your machine is performing just fine in Galveston, Texas (Elevation 7 feet / 2 meters). Your main jet is size 150. Temperature in Galveston (86 degrees F / 30 degrees C)-Denver (50 degrees F/ 10 degrees C). The new jet required to obtain the same mixture ratio as was provided by the size 150 is determined from the equation:
J2= A2/A1 X J1
A2 = the second altitude (Colorado)
A1 = the first altitude (Galveston)
J1 = original main jet (150)
A2 = 96 (alt 5,000-temp. 50 deg.)
A1 - 99 (alt "0"-temp 86 deg.)
J1 - 150 (original main jet)
J2 =96/99 x 150 or, .97 x 150 = 145
Therefore, the same mixture at sea-level with a 150 main jet will be achieved at 5,000 feet / 1500 meters with a 145 main jet (AT ABOVE TEMPERATURES ONLY)
If temperature in Colorado was 86 degrees, we would have:
J2 = 94/99 x 150 = .95 x 150 = 142
When changing jets for altitude, always make correction for temperature as well."
by Tom Vervaeke
(Doing some research) ..."on the effects of elevation on available horsepower. I did a quick web search and found the following HP calculator.
I did a few runs using the factors below. I only changed the altitude after each calculation.
Constants I used:
Air Temp: 80 F / 27 C (median for us in summer)
Barometric Pressure: 29.96 / 1,013 bar (standard)
Dew Point: 50 F / 10 C (about normal for Colorado)
So, on average, around town in Denver I've got about 81.4% of the HP someone at sea level would have. On top of Mt. Evans (14,000 feet / 4267 meters) you only have about 54.7% of the available HP. Typical mountain riding in the 8000-10,000 feet / 2400 – 3050 meters range shows us with a 30-35% loss. About what I'd guess. I have no idea on how accurate this calculator is but at a first glance it seems about right.
Tom Vervaeke, Colorado, USA"
Editors note - I think this is roughly valid, ASSUMING that the jetting is corrected for each altitude as well. If you don't change anything and go from properly jetted at sea level straight to 14,000 feet / 4267 meters, the loss will be even greater. EFI bikes should be pretty close to this as the injection system should compensate.
The R1100GS for instance works quite well at altitude without any changes. My R80G/S at 10,000 feet / 3048 meters without jetting changes is truly pathetic. Don't make the mistake one traveller made, and set the jetting for altitude then drop to sea level, as you can easily do in Peru in a few hours, and not change the jets - you'll melt the engine.
Metric conversion thanks to Daan Stehouwer, the Netherlands
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