No 87 Octane. Filled up with 85 Octane. Lower top speed?



Transporting blood platelets emergent to a hospital for a patient in trauma surgery.
75 mph Interstate speed limit. On upgrade, attained 85 mph.
Had I filled up with 87 octane as Ford owners manual instructs*, could the Expedition have gone faster?
Think I was holding up traffic behind me.
(*Ford instructs to NOT use lower octane at higher elevations.)


Maybe not. If you were driving in an area that sells 85 octane gas, you were also driving at a high elevation. Engines make less power at high elevations because the air is less dense, regardless of fuel octane rating. Also, at high elevations, because the air is less dense than sea level, the engine requires less octane, that’s why they sell 85 octane “regular” in places like Colorado and Utah. Engines that need 87 octane at sea level only need 85 octane in Denver or SLC.


The octane rating indicates resistance to spark knock and detonation, nothing else.
As long as the engine doesn’t have to retard ignition timing (to suppress knock) there’s no difference in power or speed.


An engine that can produce 100 horsepower at sea level will only make about 84 horsepower in Denver CO, 5208 ft above sea level.
On the top of Pikes Peak, 14,110 ft above sea level, it will only make about 61 horsepower.
On the top of Mt. Everest, it will only make about 33 horsepower, but will likely run just fine on white gas.

The declining horsepower with higher altitude is why airplanes have maximum altitude ceilings. With helicopters, since it takes more power to hover than to fly, the hover ceiling is lower than the flying ceiling. In other words, if a helicopter lands on top of a high mountain, it may not be able to take off, unless it faces downhill and uses its skids like skis and takes off like an airplane.


Thank you.
Yes, less O2 density as elevation increases. (In high school I used the excuse that as the air became less dense I became more.)
On previous emergent transports, thExpedition attained 95mph there. BUT, unknown wind direction/speed.
I remember going westbound on I-70 up the steep grade leaving Denver againstrong headwinds.

Some fixed wing aircraft which land in summer at high altitude airports (Aspen) have had to wait until winter to take off.
Can’t they take off with fuel 1/3 capacity at 0400 when desity altitude is lower and they have just enough dawn light to see the mountains?


Higher octane fuels have a higher flame propagation speed - meaning shorter time to burn the fuel from the plug to the edges of the piston.

So assume for the sake of this discussion, your knock sensor didn’t need to retard the spark because there was no knocking. Then yes - in theory - a higher octane fuel will marginally help. In practice however, you’ll never notice it, especially since you didn’t do anything to the engine to take advantage of that faster burn.


I think it’s just the opposite. It’s the high octane fuel that has a lower flame propagation rate. The reason lead in fuel raises the fuel’s octane rating is because lead is the opposite of a catylist, it retards combustion in stead of facilitating it.


We may be both be wrong.
Years ago in my internal combustion engines class, we learned that higher octane fuels reduce spark knock because of their higher flame speeds. Hence - what I stated in my earlier post.

However, I’ve since been trying to see if today’s information (albeit only on the web) still supports that assertion. Octane plays no role in flame front speed.

I’ll admit I’m not a chemist, and without going back to my old books, I’ll simply retract my above answer. I believe lead does affect anti-knock characteristics, but not by affecting flame front speed.

One of the better descriptions on octane and flame speed is at It also mentions lead’s role.