High altitude fuel-up — low altitude engine knock



By Russ and Tiña De Maris

knockingWe got a note from a reader who’d been on a road trip out West. Piloting his pickup with a V-10 engine, towing a travel trailer, he started to encounter severe engine knocking. He got seriously concerned, thinking he’d developed engine problems. Happily, “the light came on,” and it wasn’t that dratted “check engine light” either – it was the one upstairs between the ears.

Our reader encountered the problem while chasing around in Arizona in the relatively low, 2,000 foot altitude area. However, his last fuel fill-up had been in Albuquerque, New Mexico, elevation pushing nearly 6,000 feet. If you’re having an “Aha!” moment, good for you. Fuel stations in some high altitude areas (particularly Colorado) often sell “regular” grade gas with an octane rating of 85 – not what “‘most of us” buy, the 87 grade. What’s the deal?

Octane ratings are a measure of gasoline to resist engine knocking. The argument is made that at high altitude, where the air is “thinner,” there is less likelihood of engine knock. So, sell the customer 85 octane fuel (for the price equivalent of what you’d pay for 87 octane elsewhere) and let ’em go. Interestingly, Colorado’s own Legislative Council says that the “85 is OK at high altitude” argument is bogus, particularly for older (pre-1984) engines, but they’re still selling the lower-test gasoline throughout the state.

So where does that leave the “gasser” RVer who plans on checking out some of those high-altitude states? Well, if your engine is younger, while driving around the high altitude areas on the 85 octane level fuel, you “should be” good to go. Motorist group AAA recommends you ask your manufacturer or trusted mechanic, though. However, when you take your rig “down the hill” to a lower elevation, “thicker air” area, you may begin to notice what our reader did – serious engine knock.

What’s to be done? If you’re filling the tank and figuring you’ll be using it when you head out of the high country, you may want to spend extra bucks for the mid-grade, higher octane fuel. Burning higher grade than your engine specs require won’t hurt the power train, it’s just going to cost you more. Or you could try and skimp it, pump in enough of the 85 octane stuff to get you back down in altitude, then hit the first pumps you can find that sell the “real” 87 octane stuff.

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A vehicle will get better mileage at a higher elevation, at least in part, because the air is less dense and there will be less air resistance at a given speed.
Of course this assumes the engine is tuned for the particular elevation. With a properly designed and functioning modern fuel injection system, the engine should automatically adjust for changes in elevation, holding the air/fuel ratio close to 14.7 at any elevation. The old carburetor engines would get too rich a fuel mixture, and might even stop running completely at really high altitudes, like 10,000 feet or more. This is one of the big advantages of modern electronic fuel injection.
Of course, many RVs are not particularly aerodynamic, and many are basically barn doors from an air drag point of view. Air drag becomes a big factor in fuel economy at higher speeds. Even a 10 mph increase in cruising speed can cut an RV’s mileage considerably.


I used to take trips from East Texas to Colorado and Northern New Mexico and I always looked forward to the 85 octane gas at the higher elevations, because it always gave me better gas mileage. Even though I was climbing hills and mountains my gas mileage was a good bit better than when I was on the flat land at lower elevations with 87 octane gas.


I live in Colorado and have driven my fuel injected, multi-port 7.4L Chev class B all over the country to include sea level. I have had no knocking when going east down hill thousands of feet with the 85 octane Colorado gas. One thing not mentioned is the knock sensor device that all computer controlled fuel injection engines have had since the early 90’s. If the knock sensor is working properly, the engine’s computer “hears” the knock and adjusts the timing on the plug firing and erases the octane and knock issue = octane rating is not an issue [I have to assume the gas was not junk to begin with]. I would have the knock sensor on the engine block checked first, they are known to fail… there are 2, one on each bank on Ford V-10’s. Check the OBD-II code and see if it is tripped or in pre-trip for starters [a reader costs $50-100. or stop at Auto Zone, O’Reilly, NAPA, etc., and they will plug it into the blue plug under the dash for free]. I actually carry a code reader with me on trips to trouble shoot issues if they arise….has saved me money more than once.

Laura P. Schulman

For those of us who cut our teeth on old-fashioned carbureted engines, a gallon of “regular” might cost 30¢ and had a 90+ octane rating. Octanes are the type of petroleum hydrocarbons present in (in this case) vehicle fuel that resist explosion under extreme pressure, which we call “dieseling.” While explosion under pressure is exactly what we want from a diesel (compression) engine, it’s exactly what we DON’T want from an internal combustion engine. That thing we call “knocking” is actually the cylinders being banged around out of sequence by random diesel explosions. Not very good for the engine!

The best policy is to always put exactly what your engine owner’s manual specifies into your tank, and absolutely nothing else. If you get fooled by one of those pumps that says “regular,” and midway through fueling you notice it says “85,” it’s perfectly fine to shut that pump off and add some “89” to make up the difference. The vast majority of gasoline tow vehicles are going to specify 87 octane. 85 is just not going to run a heavy load for very long without dieseling. Fortunately, if you do notice your engine dieseling, you can simply add some higher test fuel to fix that. There are times when an engine will knock due to water or other fuel contaminants. While I’m very big on patronizing “mom-and-pop” businesses, I’m also careful to keep my tank topped up with high quality fuel so I’m not forced to buy from places that look like their last customer was in the year I was born (don’t ask).