87 to 85 octane-what altitude?

Octane rating refers to the resistance of the fuel to pre-ignition or knock and not to the BTUs of energy provided. If an engine is designed to use 89 octance gas and you put 87 octane in the gas tank then the engine computer will ****** the spark advance to compensate and prevent knocking and damage to the engine. This retarding of the ignition will result in less power, regardless of the elevation at which the engine is operating. If you put 93 octane gas in the tank you gain nothing over 89 octane with that engine.

With fuel injection there is far less of an elevation effect on engines. With carburators the jets were sized for sea level and when going up to higher elevations the mixture would be too rich and power was lost. With fuel injectors the mix is adjusted based on the air available which is why they improve fuel economy for engines.

Roger that, QC... in my experience, it's hard to beat a RAM, no matter the year, no matter the octane! Drive safe. - m

Thanks RimCountry. My old truck said 87 needed but it seems they didn't think people would go up in altitude and the truck ran quite a bit cooler at altitude with 85 and had no loss of power. Did research back then to see what reality was and doing it now for the same reason

QCMan wrote:

Thanks for the info. I am presuming you mean 85 octane when you refer to low test.

I should have been more specific... low-test here (in AZ) is 87. By the way, we're almost always hauling or pulling something, if not our Viking, then a ½-ton utility trailer for the business.

87 octane day in day out up or down. Modern engines have computers that adjust as necessary.

RimCountry wrote:
We run the same engine as yours on low-test, we live at 6200', make weekly runs down to 1400' and back, no knocks, no pings, no problem.

Thanks for the info. I am presuming you mean 85 octane when you refer to low test. That would be good info as the concern is about running too rich and cooking the exhaust system. We were behind a guy pulling a trailer about the same size as ours just west of Denver in 2005 and his exhaust got so hot the truck caught on fire and was totaled. We managed to get his trailer unhooked and gravity helped us roll it back and out of harms way. It was not a good day for him.
I guess I could also run it on E85 as it is capable of that but that has another set of drawbacks.

My E-450 (V-10) requires 87 octane minimum, even at higher altitudes. So you might check to see what your manufacturer recommends, especially if your vehicle is still under warranty.

Though Ford also recommends 91 or higher while towing, but I've never put anything other than 87 in it.

Your vehicle is designed to operate on regular unleaded gasoline with a minimum pump (R+M)/2 octane rating of 87.

Some fuel stations, particularly those in high altitude areas, offer fuels posted as regular unleaded gasoline with an octane rating below 87. The use of these fuels could result in engine damage that will not be covered by the vehicle warranty.

For best overall vehicle and engine performance, premium fuel with an octane rating of 91 or higher is recommended. The performance gained by using premium fuel is most noticeable in hot weather as well as other conditions, for example when towing a trailer. See Towing .

We run the same engine as yours on low-test, we live at 6200', make weekly runs down to 1400' and back, no knocks, no pings, no problem.

FWIW, I've camped at anywhere from sea level to 10,000 ft. with good ol' regular gas. I did have to adjust the carburetor on my generator for high elevation.