# How did automotive carburetors compensate for altitude? **URL:** \ **Category:** Factual Questions\ **Created:** [April 11, 2025, 6:02pm UTC](https://boards.straightdope.com/t/how-did-automotive-carburetors-compensate-for-altitude/1016922 "2025-04-11T18:02:38Z")\ **Posts on this page:** 9\ **Page:** 2
**Author:** ![hajario](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/hajario/32/171_2.png) [@hajario](https://boards.straightdope.com/u/hajario)\ **Post date:** [April 12, 2025, 1:34pm UTC](https://boards.straightdope.com/t/how-did-automotive-carburetors-compensate-for-altitude/1016922/21 "2025-04-12T13:34:14Z")
Yes .. ---
**Author:** ![Machine\_Elf](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)\ **Post date:** [April 12, 2025, 2:25pm UTC](https://boards.straightdope.com/t/how-did-automotive-carburetors-compensate-for-altitude/1016922/22 "2025-04-12T14:25:16Z")
> [@echoreply](#): > > YouTube tells me the 85 is a hold over from carbureted engines, and should not be used in modern fuel injected engines. The computer with all of its sensors can compensate for the altitude and doesn’t need the lower octane to run well. Of course that means the computer ends up compensating even more for the too low octane. > > I imagine 90% of the gas cars I see are running 85 octane, and there isn’t an epidemic of destroyed spark plugs in Denver, so it probably isn’t that big of a deal. No car _needs_ low-octane fuel to run well, but some cars do need high-octane fuel in order to avoid knock. In general, a car’s octane requirement is lower when operated at higher altitudes. Gas stations at higher altitudes generally offer all of their fuel blends with slightly lower octane ratings. Here in Michigan, most stations offer octanes ranging from 87 to 93; I’m guessing that there in Denver, you see octanes ranging from 85 to 91. If your car needs 93 octane at low altitude, it’ll do fine with 91 octane at high altitude; if your car tolerates 87 octane down here, then it’ll do fine up there with 85. Also, severe knock doesn’t ruin spark plugs - it ruins engines. It can erode head gaskets, or in worse cases, dump so much heat into the piston that it melts. > [@md-2000](#): > > I’ve never hear d of this before. I remember driving over the mountains in Montana on a Honda 500 in the late 1980’s, and realizing the altitude issue when I opened the throttle going uphill and response was noticeably lackluster. it worked, but I wouldn’t call it performance. After that, I still managed to get a speeding ticket in Wyoming on the interstate. All naturally-aspirated engines will have reduced power output at higher altitude due to reduced ambient air density; each mixture intake event brings in less air and fuel than it would at lower altitude. If you’re at the top of Pikes Peak (14,100 feet above sea level), ambient air density is about 60% of what you get at sea level - and so the max power available is only about 60% of what the engine could produce at sea level. > [@TriPolar](#): > > Don’t modern fuel injected engines handle these conditions without manual adjustment? Yes. The engine control unit has a barometer and built-in lookup tables so it knows how long it should hold each fuel injector opening when the ambient pressure is X psi and you’re running the engine with Y RPM and Z% throttle opening. It further trims the fuel quantity based on near-constant monitoring of an exhaust oxygen sensor: generally speaking, if there is unused O2 in the exhaust, it gradually pares the injection quantity down, and if there’s no O2 in the exhaust, then it gradually amps up the injection quantity. This makes best use of the catalytic converter in the exhaust: it banks O2 during lean operation, and then uses that O2 to oxidize unburned hydrocarbon and carbon monoxide during rich operation. ---
**Author:** ![SmartAleq](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/smartaleq/32/163_2.png) [@SmartAleq](https://boards.straightdope.com/u/SmartAleq)\ **Post date:** [April 12, 2025, 2:34pm UTC](https://boards.straightdope.com/t/how-did-automotive-carburetors-compensate-for-altitude/1016922/23 "2025-04-12T14:34:11Z")
I went over Tioga Pass in the Sierra Nevadas, heading west to Yosemite, just under 10K ft elevation. '67 Mercury Cougar with a two barrel Hearst carb on it. That car could smoke most vehicles at sea level (I lived in Sacramento at the time, elevation about 17 feet) but by the time I got to the top of that pass the car could barely make it to 35mph. Once it was headed downhill things got easier though. Carbureted vehicles generally speaking did not appreciate serious elevation. ---
**Author:** ![mixdenny](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mixdenny/32/2962_2.png) [@mixdenny](https://boards.straightdope.com/u/mixdenny)\ **Post date:** [April 13, 2025, 5:47pm UTC](https://boards.straightdope.com/t/how-did-automotive-carburetors-compensate-for-altitude/1016922/24 "2025-04-13T17:47:39Z")
> [@Kedikat](#): > > Did the Stromberg carbs with those diaphragm mounted long variable diameter needles compensate for air pressure? My old MGB was a finicky tune. It had those before I went to dual Webbers. That’s a good question. The purpose of the rising piston on the SU and Stromberg carbs is to keep constant air flow through the carb body by maintaining a constant pressure drop across the main venturi. The needle gets thinner as the piston rises to add more fuel so the fuel ratio stays the same. I don’t know if that by itself would self correct for thinner air but certainly changing the needle to a different profile would do it. And they do offer them. Of course the best answer is to supercharge the engine. > **[Metering Needle, High Altitude](https://mossmotors.com/uc-metering-needle-high-altitude)** > > Metering Needle, High Altitude ---
**Author:** ![Happy\_Fun\_Ball](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/happy_fun_ball/32/17664_2.png) [@Happy\_Fun\_Ball](https://boards.straightdope.com/u/Happy_Fun_Ball)\ **Post date:** [April 14, 2025, 4:46am UTC](https://boards.straightdope.com/t/how-did-automotive-carburetors-compensate-for-altitude/1016922/25 "2025-04-14T04:46:24Z")
This is what I remember. I bought a 1970 vw bug in Colorado at 15 years old and drove it on several engines coast to coast over the next 6 years. It ran fine in Denver and on the front range where I went to high school, but I would definitely have to do adjustments at lower altitude or it would knock. Or maybe it was vice-versa. It was 35+ years ago now. But I remember trying to adjust the octane by buying higher octane fuels or by using additives; but the only thing that would really work was adjusting the carburetor and replacing the jets. I rebuilt the carburetor on that car at least 6-8 times and the engine twice. ---
**Author:** ![Machine\_Elf](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)\ **Post date:** [April 17, 2025, 11:31am UTC](https://boards.straightdope.com/t/how-did-automotive-carburetors-compensate-for-altitude/1016922/26 "2025-04-17T11:31:04Z")
Digging into this further, I discovered that basic carburetors are also sensitive to ambient temperature. At colder temps, the air mass flow increases, but the fuel flow does not, resulting in leaner mixtures; things move in the opposite direction at warmer temps. For piston-engine aircraft that use carburetors (and I think this is most piston-engine aircraft), pilots have the ability to adjust the mixture ratio to compensate for altitude effects, which includes the effects of pressure _and_ temperature - but this article advises pilots to make bulk changes to mixture twice a year to compensate for seasonal changes in air temperature: > **[Seasons Change and So Should Your Mixture | American Flyers](https://americanflyers.com/seasons-change-and-so-should-your-mixture/)** > > As we transition from the cold, snowy winters to the sweltering summer heat, apart of answering the question, “how to become a commercial pilot”... So, adding to my OP question: How did automotive carburetors automatically compensate for ambient temperature changes? When I lived in the upper midwest, we could see summer temps as high as the mid 90s and winter temps as low as -10F. With no compensation, that’ s enough of a change in air density to bump the A/F ratio up or down by 10%. ---
**Author:** ![Just\_Asking\_Questions](https://avatars.discourse-cdn.com/v4/letter/j/ba9def/32.png) [@Just\_Asking\_Questions](https://boards.straightdope.com/u/Just_Asking_Questions)\ **Post date:** [April 17, 2025, 2:36pm UTC](https://boards.straightdope.com/t/how-did-automotive-carburetors-compensate-for-altitude/1016922/27 "2025-04-17T14:36:01Z")
When i took my 74 Dodge, set up to run in AZ, up to Colorado I was wondering why it was so under powered. It had no acceleration, and was having trouble even reaching the speed limit. Then I saw a sign go by, saying 11000 feet. oh! The fact that all the trees were leaning relative to the road should have clued me in as well! So, how do they compensate? Not very well! ---
**Author:** ![xtenkfarpl](https://avatars.discourse-cdn.com/v4/letter/x/c2a13f/32.png) [@xtenkfarpl](https://boards.straightdope.com/u/xtenkfarpl)\ **Post date:** [April 17, 2025, 3:03pm UTC](https://boards.straightdope.com/t/how-did-automotive-carburetors-compensate-for-altitude/1016922/28 "2025-04-17T15:03:30Z")
> [@Machine\_Elf](#): > > For piston-engine aircraft that use carburetors (and I think this is most piston-engine aircraft), Slightly off topic, but is that correct? I recall reading that for a while a key advantage the ME109 had over the Spitfire in WW2 was that the ME used fuel injection while the Spit had a carburetor. So the Spit engine would stall out if the plane got into certain flight angles. Apparently a modification was devised for the Spit carburetor which somewhat alleviated this. Don’t immediately have a cite for this, but I’m pretty sure it’s factual. As for modern piston-engine aircraft, I don’t know… ---
**Author:** ![yendis](https://avatars.discourse-cdn.com/v4/letter/y/ed8c4c/32.png) [@yendis](https://boards.straightdope.com/u/yendis)\ **Post date:** [April 17, 2025, 4:02pm UTC](https://boards.straightdope.com/t/how-did-automotive-carburetors-compensate-for-altitude/1016922/29 "2025-04-17T16:02:33Z")
> [@xtenkfarpl](#): > > Slightly off topic, but is that correct? > I recall reading that for a while a key advantage the ME109 had over the Spitfire in WW2 was that the ME used fuel injection while the Spit had a carburetor. So the Spit engine would stall out if the plane got into certain flight angles. The main way that Spitfires compensated for altitude was not by adjusting mixture but by adjusting the supercharger gear on the engine. The problem of the engine cutting out was because the carburetor on the Merlin engine was designed to work only in a vertical position. So if a pilot flew inverted the engine would be starved of fuel after a short while. The bigger issue is that the same thing would happen if the engine experienced negative g-forces. i.e. the pilot pushed the nose hard down which was the equivalent of being upside down from the point of view of the carburetor. So Luftwaffe pilots would evade by ‘bunting’, pushing their noses hard down when a Spitfire or Hurricane was on their tail. The chasing aircraft would have to roll inverted and then follow in the dive, wasting time. So why didn’t the engines cut out when performing _this_ inverted maneuver? Because the pilots would pull back on the stick after rolling upside down, which would cause a positive g force on the engine and keeping the carburetor working. The fix for the carburetors on Merlin engines was rather cheekily called ‘Miss Shilling’s orifice’, after its inventor. [Previous page](https://boards.straightdope.com/t/how-did-automotive-carburetors-compensate-for-altitude/1016922.md?page=1)