I was wondering if somebody could tell me how a car (electronic ignition) would perform at more than 10,000 feet altitude.
Fine. What happens at high altitude is that the air is less dense, less oxygen etc. per cubit. New fuel delivery systems adjust for this, but old carburated systems couldn’t.
Less density also means less compression. Less actual matter (oxygen, nitrogen, and argon mostly) is being drawn into the cylinders and therefore compression is less. At high altitides you may be able to use lower octane gas because the lower compression negates the tendency of high compression engines to preignite. Consult your owner’s manual.
Unfortunately, less compression also means that your quarter mile times from the local stoplight will suffer.
1 Like
From memory here, I think the general rule of thumb is a 3-4% power loss for each 1000 feet of altitude, figuring from sea level.
So a 300 Horsepower car at 10k feet would basically lose 30-40 HP at that altitude.
The exceptions would be on forced induction cars (turbocharged, supercharged) which can compensate for thinner air.
This is the reason many aircraft use forced induction; to offset that thin air and generate as much power as possible.
When I worked at a new car dealership in Colorado, the “accepted” rule of thumb was a 3% loss for every 1000 feet of altitude.
That was with carbureted engines. The default practice for every “new car prep” we did was to advance the ignition timing at least 5 degrees to try and compensate for some of that loss.
There will be a loss of performance unless a turbo or supercharged engine.
However if the car itself is not underpowered to begin with eg(V6, V8) it may not be really noticable. Its when you start with an adequate or underpowered vehicle that it really matters.
I think you mean 90-120HP loss at 10K ft. Thats 3-4%/1000 ft for a total of 30-40% loss for a normally aspirated engine.
If I’m calculating it right, you will 32% of your horsepower.
It will perform poorly. There are few roads in North America that will bring you to this altitude. The ones that do will bring you back down again fairly quickly. For every 1000 feet of elevation, reciprocating piston engines (normally aspirated) loose 4% of their rated power. The old piston aircraft engines used double turbo or supercharging to reach high altitudes
In addition the the prior advice the higher altitude will lower the need for high octane. If you are going to be there for a while you can pump in regular.
Actually when you purchase fuel in high altitude place the octane is typicaly lower. In denver CO for example regular fuel was 85 and premium I believe 89-90 where at sea level regular is 87 octane and 92-93 octane for premium.
Guess I should learn to do math before posting since 3-4% per 1000 feet does not equal 30-40HP on a 300 HP motor. Oops.
The thin area definitely kills performance. I’ve been to Colorado countless times on everything from a Harley motorcycle to a SAAB (non-turbo) to a Subaru and my Lincoln Mark and at 10-12k feet they’ve all been wheezing a bit; especially that SAAB and Subaru.
I had to join this thing just so I could straighten you people out. yes 3 to 4% per 1000 ft is correct. someone else straightened out the 4450 guy who couldn’t do math. Using 3.5%, a 300 horsepower engine loses about 105 horsepower at 10,000 ft. No offense but the person who said if your car is adequately powered to begin with you shouldn’t notice much difference? You don’t know what you’re talking about. Clearly you have never driven at altitude. I live in Denver and I’m up in the mountains all the time. If you think you can’t tell the difference between 300 horsepower effective and 195 horsepower effective you’re… what can I say and still be nice? The octane person?WRONG ALSO. Cars have more tendency to knock at high altitude due to the thin air. Using lower octane gas only makes that worse. Also you’re losing massive power already, lower octane gas just makes it worse. I think people professing to know something should state their qualifications. I renovate cars, build hot rod car engines & weld up race car frames for a hobby, have years of shop experience, a master’s in physics and as I stated above I live at altitude. Whatchyoo got?
Kellywood55, do you feel better now?
3 Likes
Incorrect. 85 is appropriate for 10,000 ft:
Q:
I live in Arizona and use 87-octane regular. In Utah, Idaho and Nevada, stations were selling 85-octane as regular gas. This forced me to pay more for midgrade 87-octane. Is this the latest petroleum-industry scam to get more of our money? Will my car run okay on this bogus 85-octane regular?
A:
Octane is the ability of a fuel to resist knock, and high-compression engines tend to knock more. The obverse of that is that lower-compression engines can run on lower-octane gas. Air is thinner the higher above sea level you go. Less air going into the cylinders means less pressure at top dead center when things go bang. It’s a lot like lowering the compression ratio in the engine, reducing the need for high octane. Cars will run just fine on lower-octane fuel when they’re well above sea level–and all of those states are. Hopefully, by the time you get back down to denser air, you’ve burned off most of the low-octane stuff, and can refill the tank with higher-grade fuel.
(source: 30 Car Mysteries Solved: Low Octane Fuel at High Altitudes)
edit: crap, didn’t see the thread was 9 years old…
Two questions for you.
Then why do they sell 85 and 86 octane regular in mile high states instead of 87 octane?
When I’m climbing a hill or otherwise putting a heavy load on an engine and it starts detonating, I can usually stop the detonation by closing the throttle a bit thus reducing intake manifold pressure. How does the engine know that the reduced intake manifold pressure was due to throttle restriction and not high altitude?
Since you know everything please resolve these apparent paradoxes for us.
2 Likes
@Kellywood55, thank you so much for joining and correcting something that was already corrected in November of 2007. And also spreading completely bogus info. For that I comend you. BRAVO !! 
2 Likes
@Kellywood55
This site is frequented with contributors who have an amazing depth of talent and skill. Bashing them or anyone is frowned upon, It also makes readers discount any of your input.
It’s not too late to go back and delete the words you wrote, and to start anew as a respected contributor to this board.
Yeah … it is. he/she has already made their first impression, and it aint flattering
4 Likes
In my defense I will say this.
It’s NOT that I can’t do math. On the contrary, I’m quite good with it. I simply made a mental error (a,k.a, typo…) for whatever reason and which I admitted to when corrected. I’m not that stupid believe it or not…
As for octane rating, I will say that I have run 85 octane during my many trips to the mountains of CO and NM. I have never experienced any pinging at all even which climbing a 12k foot pass.
That includes my Lincoln Mark which REQUIRES 91 octane according to the owners manual.
Even with the pseudo Cobra engine in the Mark I can feel some power loss at altitude. And I’m sure with your qualifications you won’t believe this either, BUT the gas mileage in my Mark actually improves about 1.5 MPG at altitude when compared to the flatlands here in OK where I live. And yes, it’s been checked, rechecked, and checked again for accuracy…
Surprising to me, I don’t remember having any concerns or troubles when driving my 71 nova with the 307, 200 hp up there, ignorance is bliss.