New tow vehicle!!

2001400ex wrote:

Grit dog wrote:

2001400ex wrote:
Fuel injected vehicles need 87 octane. The mountains of the west typically only have 85 or 85.5 for the regular gas. Must use mid grade in these areas.

Like all fuel injected vehicles, or just some? Which ones? Lol
Do you even know why lower octane is offered at high altitude?

My understanding is all fuel injected vehicles need at a minimum 87 octane, which should be in the manual. Of course some require 91.

If you know, then educate us I guess. My understanding is that lower octane cause the thinner air, but I'm not the expert there.

Apologies, yes 85 in the mountains (like CO not WA) burns similarly to 87 at low altitude.
The octane "requirement" is based on compression, cam and ignition timing etc and higher performance vehicles are built and timed for a slower (longer) burn, i.e. Higher octane. Higher octane burns slower producing more energy for the same amount of fuel. Back in the days of carbed vehicles without knock sensors, too low of octane resultedfor a given engine in detonation/pre ignition or "pinging".
Now all vehicles have knock sensors and the ecm will sense and adjust the ignition timing to combat detonation. Vehicle will sound like it's running fine but will be down on power a bit compared to running the higher reccomended octane. And at the same time likely has a bit worse fuel economy. Within reason of course.
There are other factors that affect this.
In simple terms, lower compression = lower octane requirements for naturally aspirated engines. Hence the reason less octane needed at altitude. With the thinner air, the dynamic compression ratio is slightly less due to less air being squeezed at top dead center. Same goes for temperature. Air gets thinner as it gets hotter. Hence the "cold air intakes".
Back in the day of carbed 2 stroke Sno machines, in the mountains I could end up re jetting the carbs 4x a day just due to altitude and air temp differences on some machines that were finicky.