Fuel requirements at altitude

[cru1s3r]

I live in Oregon at approximately 3,500 above sea level. Fuels available to me at home are regular (87 Octane), Plus (89 Octane), and Premium (91 Octane). Per my owner's manual, page 554, my LC requires 87 Octane or higher. Recently, while on a road trip to mountainous regions, I see that Regular is now 85 Octane, Plus is 87, and Premium is still 91. You can already guess my questions:

- does altitude change Toyota's requirements for the LC, or am I being scammed by fuel companies attempting to sell "lesser" fuel at a higher price point?
- what do those of you who live at high altitude run?

Thanks for educating me...

 

[mcgaskins]

I run 85 which is what I run in all of our vehicles that are naturally aspirated and run premium in anything turbocharged or supercharged.

 

[linuxgod]

Popular Mechanics says you're fine to use 85 while in the mountains, though you need to step back up to 87 before you get back down towards sea level. See the 3rd question:

30 Car Mysteries Solved: Low Octane Fuel at High Altitudes

Q:
I live in Arizona and use 87-octane regular. In Utah, Idaho and Nevada, stations were selling 85-octane as regular gas. This forced me to pay more for midgrade 87-octane. Is this the latest petroleum-industry scam to get more of our money? Will my car run okay on this bogus 85-octane regular?

A:
Octane is the ability of a fuel to resist knock, and high-compression engines tend to knock more. The obverse of that is that lower-compression engines can run on lower-octane gas. Air is thinner the higher above sea level you go. Less air going into the cylinders means less pressure at top dead center when things go bang. It's a lot like lowering the compression ratio in the engine, reducing the need for high octane. Cars will run just fine on lower-octane fuel when they're well above sea level--and all of those states are. Hopefully, by the time you get back down to denser air, you've burned off most of the low-octane stuff, and can refill the tank with higher-grade fuel.

There's a more recent study at https://crcao.org/reports/recentstudies2015/CRC 669/CRC No. 669 [E-108] Report Final.pdf which you can peruse at your leisure to decide if 85 octane is really OK for you

 

[M1911]

I learned something today!

 

[cru1s3r]

Thank you for the EXCELLENT replies. I've got plenty of reading to do...