Tangent on fuel octane.

2 years ago i was driving through Montana and my little Prius was having a hell of a time driving at elevations above 7000 ft. My friend who was with me suggested putting higher octane fuel in. So we did…

The thing is we didnt reach elevations that high for the rest of trip.

So did his suggestion help? What should we have done to help our ailing car.

Unless your engine was pinging, high octane would not have done any good.

It wasn’t a question of fuel, but not enough air (specifically, oxygen).

For the same reason that climbers take supplemental oxygen when climing Mt. Everest - air pressure decreases at higher elevations.

Your car’s engine is tuned for a specific ratio of air to fuel. Deviating from that ratio (in either direction) reduces the engine output power and can damage the engine. Modern cars with pollution control and engine management computers use one or more sensors in the exhaust to monitor this. Assuming a normally-aspirated engine (I doubt the Prius is supercharged or turbocharged) there’s no way to force more air into the engine.

I’ve run my race car at 14,000+ feet and it is “interesting”.

Top Gear went to Bolivia and got up to 17,200 feet before both the people and the cars gave out.

It is possible to build a vehicle that can operate at that altitude (as evidenced by there being a road that continued upward from where Top Gear turned back).

As Terry Kennedy said, your problem wasn’t about octane, it was about a lack of air. At higher elevations, you can actually put in a lower octane fuel and still not have it knock, simply because the lesser amount of air results in less compression inside the engine.

Octane rating is only about how much you can compress the fuel before it spontaneously combusts, and if you have too low of an octane the fuel will combust before it is supposed to (it’s supposed to wait for the spark), resulting in knocking and poor performance. As long as you’ve got a high enough octane to avoid knocking, any higher octane doesn’t help. Filling the tank with high octane fuel didn’t do anything useful. In fact, you probably could have saved some money and gone with a lower octane rating than normal due to the high elevation.

Older carburetor style cars couldn’t adjust the air/fuel mixture on the fly. The mixture was controlled by a set screw on the carburetor. As a result, a car tuned to low elevation would run very poorly if it was brought up to a higher elevation. You could make it run better by adjusting the fuel to air mixture, but then it would run poorly once you got back down to low elevation and you’d have to adjust it again. A modern computer controlled car makes these alterations automatically, but it does have limits. If you go up high enough, there simply isn’t enough air. The engine sucks in as much air as it can, but it still loses performance and may start to run very poorly.

Top Gear Bolivia was one of my favorite episodes ever, by the way.

Nitpick: You have the same *compression *at higher altitudes as at sea level. The engine is still built for 8:1 or 9.5:1 or whatever amount of compression. But you do end up with less absolute *pressure *in the combustion chamber, since you’re applying the same compression *ratio *to a lower ambient pressure value.

And as you say, octane rating is a measure of how much *pressure *the gasoline vapor can be subjected to before it spontaneously ignites.

'nudder nitpick- higher octane rating means the fuel has a higher autoignition temperature; pressure isn’t the direct actor. the temperature rise in the fuel-air charge resulting from the rapid increase in pressure is what can cause the fuel to auto-ignite.

You could easily get around the problem by having a manual mixture control like those fitted to aeroplane piston engines (which are very basic engines). you can also get better high altitude performance by using a turbo charger.

Octane is LESS required at high altitude, and the suppliers partially take this into account. In Denver or Albuquerque, the premium at the pump will only be 91 or 92 octane, and regular will be 87 and I have sometimes seen as low as 84.

This can cause problems with supercharged engines (turbo or mechanically powered) because the forced induction compensates for the altitude related loss in density.

I don’t recall seeing a rule of thumb, but based on what I have seen at the gas pump, it looks like you can drop about 1 octane point for every 2500’ of elevation, and it is also not linear. I have never seen a clean normally aspirated engine that will knock at 8 or 9,000’ even on low octane gas.

It used to be that you could hear knocking on acceleration and know if you were running the octane too low. Now cars have more sound proofing, and the electronic engine controls back off the ignition timing at the first hint of knocking. So running too low an octane rating will hurt power and fuel milage a little bit. Some BMW motorcycles don’t do this automatically, but have a jumper you move to tell the computer you couldn’t buy premium on the last fill.

Your owners manual will tell you the minimum octane needed for your engine design. Beware that American pumps show R+M/2 derived Octane ratings while some European marques may use a different based rating that gives a number a point or two higher. (I know this is the case with BMW motorcycles). This results in high compression engines with a specified octane requirement that will never be seen on an American gas pump.

On one occasion I ended up late at the airport and didn’t want to make a fuel stop to fill my motorcycle, so I filled it with 100 octane low-lead avgas (no cat on that bike) and it ran exactly the same as on pump premium.