seamus

So I'm about 50 miles outside Colorado Springs and about 6000' elevation and I stop for gas. Normally, the fuel grades I'm used to seeing are 87 (low), 91 (mid) and 93 (high).

This station, and several I pass after that have 85, 87, 91 (and they're charging the same price as the lower altitudes for it).

Is there any scientific reasoning for this? And should I drop the grades I normally use in my bike and Jeep?

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Hammer

Buy the cheap shit as long as it does not ping.

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SheepOfBlue

I used 85 in the truck and 91 in the RR while there. Everything will seem like it is gasping for air though.

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seamus

Yeah, I understand the ping test and the lack of power from the altitude(severe for the Jeep pulling the trailer up the hills). I was more curious if there was a specific reason for the grade changes. I've never seen 85 other than here and thought there might be a engineering explanation for the lower grades. Especially with the number of big money sports cars and luxo vehicles that "absolutely must have higher grade fuel" rolling around here.

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The nation that makes a great distinction between its scholars and its warriors will have its thinking done by cowards and its fighting done by fools.
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G-ForceJunkie

I've seen 85 in the midwest before. And Premium I've seen range from 91 to 93. I dont think there is any standard, nor do I think it has anything to do with the gas stations altitude. Here in Cali we get 87/89/and 91.

Shenders1

Hmm - Pre-ignition, or 'pinging', occurs when the charge ignites due to pressure-related temperature increase (as opposed to spark). At high altitudes, the air is considerably less dense, so at equal compression ratio you'll have less pre-combustion cylinder pressure vs low altitude.

So yes - I think you could say that a given engine, naturally aspirated, would tolerate lower octane (which is just a rough measure of resistance to pre-ignition) at higher altitudes, simply because cylinder pressures are lower.

Can't say I'd ever thought of it this way before. I'll need to dust off my old engine-design textbooks to see if anything written there confirms the theory.

seamus

I think I'll test it out and see how my gas mileage and ping resistance fares. I'm interested to see what happens on a highly tortured, high mileage, high torque I6.

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Richard Herald, The Gentle Giant

The nation that makes a great distinction between its scholars and its warriors will have its thinking done by cowards and its fighting done by fools.
--- Thucydides

bda116

Air less dense = less compression = lower octane need.

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slickwill

BDA hit it right on the head. I had to research it years ago when I first moved from one side of Idaho to the other. Boise area is lower, 2600ish, so they get 87/89/91, Eastern Idaho is around 4500 ft so they sell 85/87/89/91

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Jetbdude

BDA hit it right on the head right after Shenders1 explained it in details.

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luvtolean

Yep, and UT was the first place I saw 85 monkey piss at the pump.

You cannot hear higher RPM detonation, so I wouldn't rely on your ears.

If your car is old, go up one grade from recommended to account for the carbon and muck in the combustion chambers. Whether at altitude or not.

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ONEvcs

Doesn't the Apache burn 110? Just use that....

seamus

Jet A. You can put it in a diesel engine if you like.

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Richard Herald, The Gentle Giant

The nation that makes a great distinction between its scholars and its warriors will have its thinking done by cowards and its fighting done by fools.
--- Thucydides

Baketech

Ya see, I knew you weren't ready for a VFR yet...worried about octane and all that...

That bike will take you places...and in those places, there will be gas...pump it in and go...

V....

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evl_twn

Or diesel...

Baketech

Only in a twin...

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RedRider

O...