Living at high altitude, I was wondering what air/fuel ratio gauges measure specifically. Is it the amount of combustible air vs. fuel? Basically, my question is, if I were shooting for a 14.7 ratio at sea level, would I be shooting for the same 14.7 up here at 6500 ft?

Get as technical as you like, I'm enjoy that stuff. :)

Thanks

ttt

I think the ratio goes up... dont remember the specifics and i dnt feel like looking it up, but i bet it has to do with atmospheric pressure. its probably closer to 16:1 ratio or higher..

and i think the ratio is how many parts air to parts fuel for complete combusion...

Kyle, I'm not trying to hijack your thread, but since you're talking about altitude, I figured I'll ask my question in here. Hope you don't mind:

Is it true at a high altitude like mine (6K) and Kyle's you can run 1 point higher in compression?

a/f remains the same, compression can be incresed because the air is less dense.

if there is NO air, no fuel flows through the carb...
light amount of air, light amount of air....
heavy dose of air, heavy dose of gas,,, until the jets become restrictive.

cool, thanks hhott. That's what I was after.

It's kind of tricky in how we word this. We have roughly 20.9% oxygen in our air, and 79% other gasses. That remains fairly constant as a percentage. Thing is, as we raise in altitude, there are less total gas molecules in a cubic foot of air as compared to lower elevations where they are tightly compacted. The ratio of oxygen and other gasses stays pretty much the same, so the ratio of air to fuel also has to stay the same. We still want around 12.8 for peak WOT power, and roughly 14.7 (stythmotric or however they spell it). The rub comes in because of this lack of gas molecules at high altitudes, and the mass compaction of gas moleculed at lower elevations. Just remember air is heavy and sinks to low elevations. But as it sinks and becomed compacted, it heats up 'all' of the gasses. And many of the other gasses that makes up the other 79% expands at a larger rate than oxygen when heated. The expandig gasses displace oxygen,, so do you have more or less oxygen??? Depends on what other gasses are present, and how much they displace oxygen. When your talking nearly 80% of the air is 'other' gasses, things become a guessing game at best.

Typically you have to jet down 1 jet size for every 2000' of elevation you raise. That's a very generalized statement. You throw humidity, temp and 'other gasses' in the mix and you add all sorts of variables.

Marv, where did you find out the percentage of oxygen in your air? I'm curious on what the difference is from back home (9 miles from Norwalk Raceway) to here in Cheyenne.

Nate, we'll probablky have to get smarter people in this than I to really set things exactly straight. But..... of the gasses in 'air' there is roughly 21% of it is oxygen. That's at sea level as well as Wyoming. The gasses all expand and there are less oxygen molecules (as well as less nitrogen molecules, less carbon dioxide, etc) But the percentage of the existing atoms or molecules ort whatever you describe them as, oxygen is still 21%. Having just spent 40 hours in a HAZWOPER class for hazmat work dealing with gasses,,,, here's is where things get squirley (and just as examples). Gasses are constantly mixing, tumbleing and co-existing,, as long as they are compatable gasses. EXCEPT, there are gasses that disperse oxygen. Like Halon,, it's function is to remove all the oxygen so the fire can't burn. In that air oxygen is 0%, not 21%, Similarly in the exhaust piule you have very little residual oxygen, but as it re-enters the atmosphere outside the exhaust pipe, it mixes with the other gasses ending up with about 21% oxygen. Naturally occouring gasses like Methane, hydrogen, all the other 'stuff' in our air can sway the 'percentages' slightly in 'pockets',, but you shake it up, throw the dice and 21% of the numbers on the dice is going to end up oxygen.

Make sense?

So why is sea level so much more oxygen rich,, because air is heavy and it runs to lower elevations. And because it's heavy and you pile gigagillions of atoms on top of each other, the lower layers become compressed. They are packed tighter together. Sitll the same percentages of gasses, just more of it in a tighter area.

just clarifying something:
oxygen is an atom
water, made up of 2 hydrogens(atoms) and 1 oxygen(atom), is a molecule.

a molecule is a substance composed of at least two atoms.

Marv - I see your point. Thanks for the clarification.

Most AF sensors compare to a calibrated compare exhaust to no burnt fuel air. Since the 14.7 is the "ideal" for complete emissions burn regardless of input O2. Less % O2 in the air because of altitude the less fuel need to burn to get to 14.7.

My LM-1 wideband has a calibration to set the sensor range before lighting the motor. I'd bet factory computors do the same.