Carburettor A/F ratio vs altitude

[Alaskaracer]

Too many people overthink this and get it wrong. I race here in Denver and have several friends that bring their cars up from somewhat lower altitudes to race. 99% of the time, a jet change is not needed. If your carb is properly tuned at lower elevations, it will be properly tuned here. fuel flow, metering, etc. are all functions of airflow through the carb. The air is thinner here, not uncommon to see it at 9500-10,000+ft of DA on an average day. As a result, less fuel will be pulled through the jets. Timing will make more of a difference than jetting, unless your stuff is way off to begin with.

[treyrags]

Too many people overthink this and get it wrong. I race here in Denver and have several friends that bring their cars up from somewhat lower altitudes to race. 99% of the time, a jet change is not needed. If your carb is properly tuned at lower elevations, it will be properly tuned here. fuel flow, metering, etc. are all functions of airflow through the carb. The air is thinner here, not uncommon to see it at 9500-10,000+ft of DA on an average day. As a result, less fuel will be pulled through the jets. Timing will make more of a difference than jetting, unless your stuff is way off to begin with.

This

[Circlotron]

The air is thinner here, not uncommon to see it at 9500-10,000+ft of DA on an average day. As a result, less fuel will be pulled through the jets.

So your saying it goes leaner at high altitude?
Earlier posts were saying it goes richer.
I’m lost.

[modok]

It usually goes richer at WOT, yes, but carburetors correct IMO.....50-60% naturally.
Drag racing is mostly concerned with full throttle operation in a fairly narrow RPM band, and there are big G-forces and changes in temp to contend with, rpm is changing quickly, the altitude is just one factor among many.
What kind of vehicle are you imagining?

[Truckedup]

Too many people overthink this and get it wrong. I race here in Denver and have several friends that bring their cars up from somewhat lower altitudes to race. 99% of the time, a jet change is not needed. If your carb is properly tuned at lower elevations, it will be properly tuned here. fuel flow, metering, etc. are all functions of airflow through the carb. The air is thinner here, not uncommon to see it at 9500-10,000+ft of DA on an average day. As a result, less fuel will be pulled through the jets. Timing will make more of a difference than jetting, unless your stuff is way off to begin with.

Then why do Bonneville racers constantly talk about changing air density and need to adjust air fuel ratio? are they all bad tuners?

[Circlotron]

I suppose adding blow-through carburettors to the conversation would complicate matters even further...

[Brian P]

In the idealized world ...

At a given RPM and (let's say) wide open throttle, the engine is a "volume flow rate" device.

The mass flow rate of air into the engine is fixed by volume but is in proportion to the density.

The pressure depression across fuel jets is in proportion to the density.

The fuel mass flow rate is in proportion to the square-root of the pressure difference across the fuel jet. But the fuel's density doesn't vary with pressure.

So, at a fixed volume flow rate (i.e. at a certain engine RPM and load condition), while the air mass flow rate is in direct proportion to the air density, the fuel mass flow rate will be in proportion to the square root of the air density.

If the density goes to 81% of what it was, it will still pull 90% as much fuel.

So, it goes rich. The fuel correction because of the reduced air density is about half of what it "should" be.

[Alaskaracer]

Too many people overthink this and get it wrong. I race here in Denver and have several friends that bring their cars up from somewhat lower altitudes to race. 99% of the time, a jet change is not needed. If your carb is properly tuned at lower elevations, it will be properly tuned here. fuel flow, metering, etc. are all functions of airflow through the carb. The air is thinner here, not uncommon to see it at 9500-10,000+ft of DA on an average day. As a result, less fuel will be pulled through the jets. Timing will make more of a difference than jetting, unless your stuff is way off to begin with.

Then why do Bonneville racers constantly talk about changing air density and need to adjust air fuel ratio? are they all bad tuners?

They do it for the same reason I'll make a jet change. To fine tune their setup for best performance. That's still a requirement regardless of elevation. Change the DA, a car will speed up or slow down depending on which way it swings as a result. For what I do, I'll generally just change the dial in on my car to allow for weather change. For them, they are after max performance every pass. Not so much for consistency as I am.

[Warp Speed]

Carbs will self adjust within their given jet size and calibration. Outside of that, an adjustment needs to be made to be spot on.
In our Cup carbs, it was about 1 jet per 800-900ft of DA.

[statsystems]

Carbs will self adjust within their given jet size and calibration. Outside of that, an adjustment needs to be made to be spot on.
In our Cup carbs, it was about 1 jet per 800-900ft of DA.

If you have double the jet area would that make the carb less sensitive to DA changes? As in a tunnel ram application over a single 4?

[David Redszus]

Yes, a carburetor is sensitive to air volume not mass, so it cannot react to the reduced air mass resulting from lower baro, higher temperature or to a lesser extent higher humidity, and thus runs progressively richer as altitude increases.

Exactly.

At sea level, baro = 29.92 in Hg, air density = 0.07358 lbs/ft^3, viscosity = 0.015883 cSt

At 4000 ft, baro = 25.14 in Hg, air density = 0.06183 lbs/ft^3, viscosity = 0.018903 cSt, fuel = -15.98%, power loss = -18.85%

At 8000 ft, baro = 21.12 in Hg, air density = 0.05194 lbs/ft^3, viscosity = 0.022500 cSt, fuel = -29.40%, power loss = -34.16%

At higher altitude, air density decreases requiring less fuel and will always result in a loss of power.
This assumes that temperature and humidity remain unchanged and does not account for temperature change due to fuel evaporation.

Note that while air density is reduced at higher altitudes, viscosity is increased causing an increased resistance
to inlet air flow.

[user-23911]

It's really a problem that's specific to make and model of carburettor.


One with tapered needles is going to work differently from one which uses emulsion tubes.