Octane and Altitude

You will need the required octane for the elevation where you are operating the vehicle. If you will still have the low octane in the tank at lower elevations, then you should fuel up with higher octane first.

The engine may not ping like the old cars did but the knock sensor will retard the timing which will increase fuel use and reduce power somewhat.

If I'm towing, I make sure I fill up with 87 before heading back downhill.

I am on my 2nd Chevy Silverado, 5.3 L. V-8 and neither manual says a thing about lowering octane for altitude. I've tried it both ways and once my "service engine" light came on with the 85 octane. My theory is that the idea of lower octane is outdated with the newer motors that adjust to various conditions so one should stick with the 87. But I'm no engineer. Problem is that 87 is no longer the lowest price in those places.

What vehicle/motor are we talking about? Most if not all modern engines have knock sensors that will retard timing if knocking occurs.

carringb wrote:
If I'm towing, I make sure I fill up with 87 before heading back downhill.

You don't need 87 octane going down hill. You barely need gas. :)

87 octane is mid grade here whereas 85 is regular. When we first moved to Colorado, the vehicle we had took some time getting used to 85 as it had been run on 87 for the first 4 years of its life while at sea level. Our new vehicle runs just fine on 85 as it hasn't had any higher octane fuel for the ECM to calibrate to.

The octane rating on automotive gasoline is "at mean sea level".

The octane increases with altitude.

Aviation gasoline has two ratings: one is the rating for "MSL" and the other is the rating for 10,000 feet above MSL.

Two common fuels are 80/87 and 100/115. the first number is at sea level and the second number is at 10,000 feet above MSL.

In your car or truck you don't need 85 octane (@ MSL) when you are at 5000 feet; you can get by with it because the engine performs as it were 89 octane.

A fuels octane does not change or increase with altitude however a motors octane requirements drop with increasing altitude. This is due to thinner air resulting is lower cylinder pressure which means a motor is less suseptible to detonation at a given octane level

camperpaul wrote:
The octane rating on automotive gasoline is "at mean sea level".

The octane increases with altitude.

Aviation gasoline has two ratings: one is the rating for "MSL" and the other is the rating for 10,000 feet above MSL.

Two common fuels are 80/87 and 100/115. the first number is at sea level and the second number is at 10,000 feet above MSL.

In your car or truck you don't need 85 octane (@ MSL) when you are at 5000 feet; you can get by with it because the engine performs as it were 89 octane.

Not quite a correct description of Avgas.
This is what Wikipedia has to say:
"Many grades of avgas are identified by two numbers associated with its Motor Octane Number. The first number indicates the octane rating of the fuel tested to "aviation lean" standards, which is similar to the anti-knock index or "pump rating" given to automotive gasoline in the US. The second number indicates the octane rating of the fuel tested to the "aviation rich" standard, which tries to simulate a supercharged condition with a rich mixture, elevated temperatures, and a high manifold pressure. For example, 100/130 avgas has an octane rating of 100 at the lean settings usually used for cruising and 130 at the rich settings used for take-off and other full-power conditions."

This is in line with what the Navy taught me in 1960, and what I re-learned after getting my A&P license.
Actually, these days it is very difficult to find any Avgas other than 100LL (Low Lead).

Octane is added to gas to slow the burning for higher compression engines so it doesn't pre-ignite and cause spark knock. It does not add power to an engine rated for 87 or 85 octane. That grade actually burns faster that 91 octane. If your TV is rated for 87 octane you don't have to do anything. The ECU does it all.