Bings and elevation...

[brentde3]

Hey folks,

Well, it appears that I have lost my job in upstate New York, so we'll be heading back home to New Mexico. Thing is, my r90 has never left New York, where the elevation stands at about ~1300 feet. Back home in Las Cruces we're talking closer to 4000.

Has anyone made a move like this? Is there anyone in the southwest that can help with jet sizes and whatnot? Is there anything else to consider?

[20774]

Bing has a manual (costs about $10-12) which discusses the approach to adjusting for altitude. If you were just making a temporary trip, you probably wouldn't change a thing. For a permanent change, you should likely adjust the carb. You'll be running rich at 4000 feet because there's less air molecules in Las Cruces.

The book seems to indicate that you'll need to drop the main jet size by about 5%. Beyond that, you should really just redo a carb synch which will adjust the mixture and idle speeds. That should get you close. Check plug color and monitor performance and gas mileage.

[PHMarvin]

Hi, Brentde3,
I bought my '76 R90/6, new, in NC exactly 32 years ago - 1 April 1977. After spending the next 4 years in NC (6 mo.), Europe (a year), MS and NC (another year) and storage (year and a half), I moved to El Paso. It seemed to run rich and the mileage was a little low. I dropped the needles one notch and, from there on, it ran much better with mileage approaching 50mpg. REMEMBER, this was in the 55mph era AND premium leaded fuel was still available. I seldom rode at 55, but seldom rode consistently at the 75+ speeds now common on well-traveled roads in the West. I've been told that dropping the needle was too much of a change. However, after the change the plugs (Bosch W6D) ran a nice tan color and I had no problems indicative if running lean. Good Luck!

BTW, I still live in El Paso. E-mail me when you get to Las Cruces. I just lost my job at the copper mine in Morenci, AZ. I was born in upstate NY. We may have some things to talk about.

[brentde3]

I'll do that, Phil. Be nice to meet some other Southwestern airheads.

[535is]

Bing has a manual (costs about $10-12) which discusses the approach to adjusting for altitude. If you were just making a temporary trip, you probably wouldn't change a thing. For a permanent change, you should likely adjust the carb. You'll be running rich at 4000 feet because there's less air molecules in Las Cruces.

The book seems to indicate that you'll need to drop the main jet size by about 5%.
That doesn't make sense to me. The Bings are vacuum-operated slide type carburetors that should automatically adjust to elevation changes. If there is less dense air, then the vacuum is not as great and the metering needle is therefore not lifted as much. This is also part of the reason your engine is down on power at high altitudes. Less air & less fuel for a given throttle setting = less power (and this would extend throughout the range, even at redline). In older (downdraft, automotive) carburetors where the fuel metering was simply a hole in the side of the throat, then you would get too much fuel dumped into the engine. You could even flood it with too much fuel, but I don't see how this would happen on a Bing. I'll be delighted to hear a coherent explanation why. :ear

[lkchris]

That doesn't make sense to me. The Bings are vacuum-operated slide type carburetors that should automatically adjust to elevation changes. If there is less dense air, then the vacuum is not as great and the metering needle is therefore not lifted as much. This is also part of the reason your engine is down on power at high altitudes. Less air & less fuel for a given throttle setting = less power (and this would extend throughout the range, even at redline). In older (downdraft, automotive) carburetors where the fuel metering was simply a hole in the side of the throat, then you would get too much fuel dumped into the engine. You could even flood it with too much fuel, but I don't see how this would happen on a Bing. I'll be delighted to hear a coherent explanation why. :ear

I believe you have it correct--changing Bing jets for altitude is overrrated.

Higher altitude WILL give you better fuel economy--for reasons you've detailed plus less aero drag.

[20774]

That doesn't make sense to me. The Bings are vacuum-operated slide type carburetors that should automatically adjust to elevation changes. If there is less dense air, then the vacuum is not as great and the metering needle is therefore not lifted as much.

But with the slide not raising as much, the velocity of the air through the throat is going to go up. Higher speeds of air results in more gas being sucked up the jet stack. Is the slide lowered enough to compensate for the increased velocity? It's hard to say.

I think the CV carbs work OK...to a point...with altitude change. That's why they say to just ride through it if you're transitioning a change in altitude, either up or down. But eventually, it becomes a point of diminishing returns and if the bike is relocated permanently, permanent adjustments are recommended.

Bing calls for a jet size decrease...must be because it would be too rich at higher altitude. I believe they also talk about dropping the needle, as one poster already mentioned. Needle changes are very coarse jumps...a main jet change is a finer change and typically affects about 1/2 throttle out to WOT. A needle change would affect the mid-range operation.

[crazydrummerdude]

I believe you have it correct--changing Bing jets for altitude is overrrated.

Higher altitude WILL give you better fuel economy--for reasons you've detailed plus less aero drag.

I don't think anyone would notice the difference in drag due to elevation. Altitude, on the other hand, would provide a noticeable difference, but unfortunately our airheads don't actually fly.

[20774]

I'll be delighted to hear a coherent explanation why. :ear

Further to the point, Bing's manual says that the engine and carb only react to flowing of the same amount of fuel and that a good mixture at sea level becomes rich at higher elevations. (I believe that means that the engine is going to demand the same amount of air to fill the vacuum created by the falling piston. If the density of the air is less, then more of it at a higher speed is needed to fill the combustion chamber). The manual goes on to say, as you suggested, that the carb is less affected to changes in elevation than other carbs, and does not typically require jetting changes for idle and lower throttle settings...the mixture helps a lot at these conditions. The upper throttle range requires lower of the needle and continuous operation at higher elevations needs a change to a smaller main jet.

[widebmw]

Remember that if you change the mixture you will have to change it back if you go down in elevation.

[lkchris]

I don't think anyone would notice the difference in drag due to elevation.

Been discussed lots of times and it is noticeable.

Just last week drove my diesel Mercedes from NM to zero elevation in S. Texas and fuel mileage was up to 4 mpg less at lower altitude. Have seen mileage in the mid 50s with my RS in Colo mountains.

[Guenther]

I live at 7,000 ft and I have done zero adjustments (standard jet sizes and needle position) with my R100GS. I know at 14,000 ft the engine stalls at idle and I have a "feeling" there is more power at lower elevations (Texas, Baja).

I can't tell a difference in mpg because it varies between 40-50 mpg anyway depending on wind, temperature and luggage (=my wife, but don't tell her).

So my 2 cents, I would keep the recommended standard setting of the carbs.

/Guenther (in Colorado)

[72828]

Hi all,

My R75/6 originally came from New Orleans and the jetting was never changed. Here in Albuquerque where the bike now resides, I was getting 30 mpg, noticeable fuel on every intake system internal surface and running VERY rich.

Made a call to Bing and they recommended re-jetting for all fuel circuits along with new needles and rebuild kits.

http://www.bingcarburetor.com/bmw/bmw.html

It's not a bad idea to give the OEM a call and see what they think.

Now to get the top end back together and get this thing running! Perhaps my 750cc engine can now get better mileage than a Gold Wing!

Marley

[alkoivisto]

On 5000 mile trip '07 with my R100RT elevations ranged from about 200' to nearly 12,000' above sea level. I can't say rideability changed at all that I recall. Adjusted idle mixture once or twice. Slide/metering needle height is controlled by air passage from top of diaphram compartment to port located between throttle butterfly and slide. More throttle equals more intake vacuum at this port causing slide to be "sucked" up. At equal throttle position, higher altitude equals less dense air equals less pull on slide and leaner mixture? I wouldn't touch jetting it if it's running fine now. Besides it's a PITB to adjust metering needles in CV carbs. That's my take anyway.

My mileage was not out of the ordinary entire trip either.

BTW, a condom can be used to temporarily repair cracked CV diaphram!! A co-rider on trip last year did fix on his R100. Cheap truckstop brand OK. No need for anything exotic. Hide it somewhere on bike where your wife won't find it and come to incorrect assumption!!

[jforgo]

I have brought 2 R100's from Bay Area and brought them here. I ive at 5200'. The only problem I had was soemone had put #135 mains in a 1982, and it starved at full throttle. Put back to stock, no other issues. Mileage also improved. Plugs look good, etc etc with both.

I think these 40mm Bings work very well unmodified at changing altitudes. Thin air is thin air irregardless, unless you want a computer controlled blower to compenstae (!)

I suspect the 40mm are a bit more flexible with altitude than the 32's, as they have more flow to work with. Just an idea, anyway...

[brentde3]

Thank you all so much for these replies. Lots of stuff to chew over. I've ordered the Bing book.

By the way, the condom thing was absolutely hilarious. :laugh

Thanks again,
Brent

[crazydrummerdude]

Been discussed lots of times and it is noticeable.

Just last week drove my diesel Mercedes from NM to zero elevation in S. Texas and fuel mileage was up to 4 mpg less at lower altitude. Have seen mileage in the mid 50s with my RS in Colo mountains.

Consider me a skeptic.

I think there are far too many variables, and although it is a factual thing that does exist, I think the difference is so small to be negligible.. for regular Joes riding regular Airheads around the regular world.

[bobh41]

Skepticism is good...

But 4 MPG, as reported by Chris, is not negligible within a measurement range of 30 - 50 MPG. Negligible is when your instrumentation cannot resolve an output difference with varying inputs - or when you just don't care.

[bikerfish1100]

Skepticism is good...

But 4 MPG, as reported by Chris, is not negligible within a measurement range of 30 - 50 MPG. Negligible is when your instrumentation cannot resolve an output difference with varying inputs - or when you just don't care.

the 4mpg difference is real. attributing it to decreased drag of thinner hi altitude air (as Chris did) is likely fantasy.

[bikerfish1100]

Hi all,

My R75/6 originally came from New Orleans and the jetting was never changed. Here in Albuquerque where the bike now resides, I was getting 30 mpg, noticeable fuel on every intake system internal surface and running VERY rich.

Made a call to Bing and they recommended re-jetting for all fuel circuits along with new needles and rebuild kits.

http://www.bingcarburetor.com/bmw/bmw.html

It's not a bad idea to give the OEM a call and see what they think.

Now to get the top end back together and get this thing running! Perhaps my 750cc engine can now get better mileage than a Gold Wing!

Marley

your situation had nothing to do with the altitude change. mpg will improve at altitude, and 30 mpg is well out from normal. your carbs needed the new parts because the old ones were badly worn. fwiw- i would expect in the vicinity of 50 mpg from a 75/6 (my old R90 got a best of 62, but that was on good hi-test RED gas, leaded, back in the day).

[lkchris]

the 4mpg difference is real. attributing it to decreased drag of thinner hi altitude air (as Chris did) is likely fantasy.

Your solution?

[widebmw]

Once you get to your highest altitude you mileage will improve. :scratch


Its all down hill from there.:doh

[crazydrummerdude]

Your solution?

I believe there are a seemingly infinite amount of variables in a road test, many of which have a greater impact on fuel economy than the density of the air.

For instance; headwinds/tailwinds, hills, mph, drafting other vehicles, the temper of your foot/hand (mashing the gas, or gently accelerating), etc. These seem to be the heavy hitters.

-In a truck with a camper shell, I think I can feel tailwinds.
-I also know I save gas by going slower up hills than I do down hills.
-If I travel from A to B at 80, then B to A at 40, I will notice a decrease in fuel consumption. One time I drove to my farm at 60 mph and used 1/8 of a tank. One time, I drove at 80-85 mph and used more than half a tank. I've been told that you use twice as much fuel at 80 than you do at 55. I'm not sure if that's accurate, but I'd tend to lean towards it being so.
-In my brothers truck (lifted old Chevy on big tires) that gets roughly 8mpg, I drove 110 miles using my regular driving style. I used a full tank. On the way back, I drafted (and went slightly slower because of it), and used less than 1/4 of a tank. Same route.
-My other brother slams on the gas, and slams on the brakes. I know he consumes much more fuel than I do, and I'm easy on acceleration, and am always optimizing my coasting time.

I think any one person can have different moods or styles of driving at any two moments. I think these make more of a significant impact than the density of the air. So much so, that I think density-induced fuel economy is negligible as part of the whole. As I originally said, it does obviously contribute, but when we're talking about not using significant portions of your gas tank, the other factors seem to overpower it. I would say that it is indeed negligible for regular Joes riding regular Airheads around the regular world.

This is all my opinion, but I am surrounded by people much smarter than myself every day at school, so I might bring it up sometime to see if anyone agrees.

[bobh41]

Your opinions are okay - and are very regular. But I take exception with your opinion that altitude, thus air density is a negligible factor. I've troubled with this far more than it's worth, but it's interesting...

This is far away from my own training, but it doesn't seem too complicated on it's merits: This is a calculation of the effect of air density on air drag of a 2001 Honda Civic running 50 MPH at sea level and at 10,000 feet. The data is public domain.

Definition: Drag Force = ½(ρACD v2 )

Given: ACD for a 2001 Honda Civic = 7.34 (That is Drag Coefficient * Frontal Area)

Given: Velocity set to 50MPH

Given: Density of air at sea level = 1.2 kg/m3
Given: Density of air at 10,000 feet = 0.8 kg/m3

Calculating:

Drag Force at sea level = ½(1.2 * 7.34 * 2500) = 11010 Newtons = 2475.14 Pounds

Drag Force at 10,000ft = ½(0.8 * 7.34 * 2500) = 7340 Newtons = 1650.01 Pounds

My apology - I haven't found a reference that would relate Drag Force to MPG, but I'm still looking. I'm very sure it would easily exceed the 4MPG that lkchris referred to. The drag associated with the 10,000 feet difference in altitude is very significant, certainly not negligible.

[20774]

My apology - I haven't found a reference that would relate Drag Force to MPG, but I'm still looking. I'm very sure it would easily exceed the 4MPG that lkchris referred to. The drag associated with the 10,000 feet difference in altitude is very significant, certainly not negligible.

Basically your equation boils down to the ratio of density at sea level versus 10000 feet, or 1.2 versus 0.8. That's a 2/3 decrease which translates to a 2/3 decrease in drag. However, the flight manual for any military aircraft has a table in the appendix which shows the density ratio for any altitude...this is the density at any altitude divided by the density at sea level. According to this table, the density ratio at 10000 feet is 0.7385, not the 0.6667 you're suggesting. Possibly you're not assuming standard day-standard temperature for these calculations.

I suspect that the effect of drag on mpg for a car versus a motorcycle is going to be different, also when considering that the mpg of the car is likely 50% that of the motorcycle.

[crazydrummerdude]

Your opinions are okay - and are very regular.

Ha.

This is far away from my own training, but it doesn't seem too complicated on it's merits

I've read it once, and haven't spent a ton of time thinking about it before this response, but that looks to me like some fuzzy math.

The premise is there, but mixing MPH^2 with meters and outputting lb-f. I just see the possibility of some error.

Also, I originally worded my previous responses "around the regular US" before I changed it "regular world." I've wondered how many Airheads live and operate at 10,000ft.. It seems like that's the realm of the tips of mountains (http://simple.wikipedia.org/wiki/List_of_U.S._states_by_elevation), with mean elevation ranging from 6800 to 60 ft. Not quite 10,000.

I am willing to change my opinion if convinced otherwise, but I'm still sticking by it; Compared to other effects, elevation-related density-induced fuel economy on a regular airhead is negligible.

...

Rather than editing my previous post, I forgot to mention that temperature is another heavy-hitter in this category.

[20774]

I've read it once, and haven't spent a ton of time thinking about it before this response, but that looks to me like some fuzzy math.

In the end it doesn't really matter. The equation is more or less right... I recall the equation for lift of an aircraft as being Lift = Coefficient of lift times angle of attack times wing area. The coefficient is expressed as 0.5*density*Vel*Vel. The equation for drag is very similar.

But the only thing that changes between the two equations presented above is the value of density at the two different altitudes. Thus, the relationship falls out as being the ratio between these two values.

And yes, I agree, the temperature has something to do with it...

[bobh41]

I've found the issue very interesting. My searching has revealed a lot about aerodynamics that I don't usually think about - for example, the effect of turbulence resulting from surface topology.

There are very many factors affecting mileage, as pointed out; not quite infinite, but many. My take on the discussion revolved around Kent's experience with air density driving a Merc being "non-negligible". The equation and data are not mine, but I encountered the same in several different fora. I feel the conclusion from the calculation is clear - air density is a valid real factor in fuel mileage. Anything else thrown in the discussion is fine, but doesn't negate the point.

As to "real world" gas mileage going up and down mountains I'm sure we would agree that there's a lot of stuff going on with far more impact than air density but for engineers and engineering students the obvious can be left to the casual observer. The details are where you find the devils.

[535is]

Your solution?
I think he implied you should lay off the weed. But I could be wrong ... :whistle

[crazydrummerdude]

I'm sure we would agree that there's a lot of stuff going on with far more impact than air density

Agreed.

[bikerfish1100]

i'll certainly go with air density being a primary factor in the internal combustion equation, and that air resistance is a major factor in the entire equation of efficiency- but i've yet to find anything that adequately addresses density in the resistance formula. even the numbers that show a big differential between sea level and 10,000' (and Albequerque is closer to half of that 10thou, so even that number is not very valuable) air density still don't address it as a factor in resistance.

[lkchris]

Yet your jetliners fly at 40K feet instead of 10K and find it economical to make the climb.

[Guenther]

Yet your jetliners fly at 40K feet instead of 10K and find it economical to make the climb.

Yeah, but they are not propelled by an airhead engine. Propulsion systems like to "throw out" lightweight stuff FAST. That's where the higher altitude comes in.

Guenther
(remember...I am not a real doctor)

[Guenther]

Coming back to original question:

The CV Bings are not Constant "Vacuum" carbs. It's Constant "Velocity" (Gleichstromvergaser in German). According to that old guy Venturi the vacuum in for example a pipe is proportional to the speed of the gas in the pipe. Something hard to maintain in a carburetor for idle speed. This is where the CV comes in. At idle speed the air volume is low and so is the airspeed. The CV carb with the piston inside reduces the opening over the needle jet thus increasing the air speed and thus the vacuum to suck gasonline out of the needle jet. With increasing volume (opening throttle) the airspeed increases and the little hole at the bottom of that piston generates more vacuum in the dome of the carb thus raising the piston...to a point where we get the constant air velocity again. But now it has also pulled the needle higher up and we get more gasoline in the mixture. This way the designers of the CV carb had high hopes that it mostly maintains the right mix between air and gasoline.

Now I am not sure how this nicely created carb copes with thinner air = less oxygen to burn and lower ambient air pressure. This goes beyond my education level. :scratch

Guenther
(remeber...I am not a real doctor)

[bikerfish1100]

Yet your jetliners fly at 40K feet instead of 10K and find it economical to make the climb.

...and the difference between 6K and 10K elevation is not nearly the difference between 10K and 40K. factor in that the drop in air pressure is relatively linear from 0 to about 10K feet, and then goes thru a very rapid rate of decrease from 11K to about 70k, when it drops off to nearly zero.

bottom line- comparing cruising altitude to internal combustion dynamics thru air is an apples to oranges comparison.