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Old 04-16-2019, 02:02 AM   #17
Senior Member
Brand: Thor Motor Coach
Model: A.C.E. 29.2
State: Oklahoma
Posts: 115
THOR #2542

Originally Posted by MJC62 View Post
On the same topic of gasoline. Most mfgrs state in the owners manual that a minimum Octane rating of 87 should be used in their engines. So why do some states eg. (Wyoming and Montana) advertise and sell regular unleaded at 85 octane that the mfgr. says don't burn. The midgrade is 87 octane which in my area is regular. I won't burn 85 octane in anything I drive and am forced to buy the higher priced mid grade. How is it oil companies in these states can sell a product that car mfgrs say not to use? Am I missing something?
Is this a reasonable answer from the web? Higher altitude has lower air density so it actually lowers your compression because the engine draws less air, so you don't need as high octane to stop detonation more specifically: If the manufacturer of a naturally aspirated engine specifies that 87 octane fuel is required, it is assumed that the engine is running at sea level. As both cars and people go up in elevation, such as New Mexico or Colorado, it becomes more difficult to breathe. The reason for this, is the mass of oxygen in a given volume of air is less.
If you check the barometric pressure in Colorado vs Florida, you will see the pressure is much lower in Colorado due to a lesser mass of oxygen. A modern, electronically fuel injected vehicle will be able to calculate the quantity of oxygen available in the air.
As an example, a two liter engine will displace 2 L of air whether it is in Florida or Colorado. At wide open throttle, the barometric pressure in Florida of around 100 kPa will be inside the intake manifold and available to fill the cylinders. In Colorado, the same engine at wide open throttle will still displace 2 L of air, but as the barometric pressure available to fill the cylinders may only be 85 kPa, it will produce less horsepower due to having less oxygen available in that 2 L of air.
The computer system will deliver the correct amount of fuel for however much oxygen is available, regardless of barometric pressure. Unfortunately, as the quantity of oxygen and fuel are reduced with higher elevations, the engine will make less power. (Many piston aircraft engines are turbocharged so as to maintain sea level horsepower at elevation, not to necessarily make more horsepower.) An engineer would say that the peak cylinder pressure is less at altitude than at sea level. As the cylinder pressure is less, the speed of combustion is slower. The speed of combustion is what drives the octane requirements of an engine. (If the speed of combustion is too fast, the cylinder pressure will rise too quickly causing the unburnt fuel to quickly burn from the high temperatures, as a diesel engine would do, instead of the typical method from the spark plug and radiating out to the cylinder wall.) This is the reason 85 octane fuel is what's typical in much of Colorado. So, as engines make less horsepower due to the lower air pressure at high altitude, the octane requirement will go down. As higher octane fuel is more costly to manufacturer and the engine does not require it, it is not typically found at higher elevations.
All of the above was from the perspective of taking an 87 octane vehicle up into the mountains, but what happens if you fill up your car with 85 octane fuel and drive it back to a lower elevation? As the elevation goes down the air pressure goes up and the engine can once again produce it's rated power. This additional power, compared to how it was running at altitude, will drive the octane requirements back to 87. If you're running an 87 octane engine hard, at sea level, with 85 octane fuel, it is possible to damage the engine. Thankfully, most modern engines have a very good sensor for detecting if the engine combustion process is poor due to having low octane fuel. This device, called a knock sensor, will tell the computer that the engine is not running as it should and adjustments will be made to stop the poor combustion. In so doing, you can expect the efficiency of the engine to go down and the fuel consumption to go up.
If, on the other hand, you fill up with 85 octane fuel and drive it back down the side of the mountain, but your engine is of an older design, it is possible to damage the engine due to poor combustion quality.
By the way, poor combustion quality is what used to be known a few years ago in America as detonation, pre-ignition, knocking, pinging, and as the British would say, pinking. Today, if the fuel does not burn properly in the cylinder for any reason, it is called poor combustion.

I didn't know that!
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