Why is 85 octane a thing? | Poal: Say what you want.

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[–] • 0 pt

Shit burns different up above 4000 feet. I've had issues with chainsaws and lawn mowers needing major choke adjustment from where it was at lower elevations. I haven't noticed any difference in my car using 85, but have been told if I go back towards sea level I will notice a knock develop

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[–] • 0 pt

Complete BS (RE: knock if you change elevation)

I was born and raised in Wyoming, folks are still in Montana while I'm suck with my wifes lunatic family along a coast. Twice a year I drive back and forth, and have done so since 1999. For the decade before that I was single and traveled the country in a carbbed 1986 Jeep that was bought new in Wyoming (and lived on a steady diet of 85 for over 100k miles). No knock developed on the jeep, and have no issues with my current vehicles (modern EFI or the '66 Bug that we tow occasionally). Even monitoring my engine run conditions (Torque Pro app and OBDII) shows no real deviation in operating conditions between near sea level and upwards of 8000' in elevation while running the cheapest gas I can find (well, not e85).

The issue you've found with your small engines is true for big vehicles too, there's less oxygen density in the atmosphere and you have to change the carb jetting/settings to compensate and get back to a stoichiometric (efficient) setting. Track down any old VW guy who's traveled to elevation and ask if they rejet at elevation (or carry spare carb jets). Closed loop EFI (the norm today) handles this without issue... Open loop could deal with some elevation change but generally had different "maps" for elevation ranges.

As for the original question, I only have my personal understanding of it as explained by an oil engineer I know at one time and no hard facts, but RON+MON is derived by running fuel through a CFR engine (calibrated test engine designed with this specific task in mind). Take the same fuel and test it at 0/sea level and 5250 (1 mile) above sea level and your output number is different. The fuel is the same, test equipment is the same, operator is the same... the only change is the air density. Nothing "different" about the fuel other than where it's tested.

As for those who've read their owners manuals about using a minimum 87, most either have small print right next to that stating "or equivalent" or have a separate section for operating at elevation where it covers the alternatives.

Complete BS (RE: knock if you change elevation) I was born and raised in Wyoming, folks are still in Montana while I'm suck with my wifes lunatic family along a coast. Twice a year I drive back and forth, and have done so since 1999. For the decade before that I was single and traveled the country in a carbbed 1986 Jeep that was bought new in Wyoming (and lived on a steady diet of 85 for over 100k miles). No knock developed on the jeep, and have no issues with my current vehicles (modern EFI or the '66 Bug that we tow occasionally). Even monitoring my engine run conditions (Torque Pro app and OBDII) shows no real deviation in operating conditions between near sea level and upwards of 8000' in elevation while running the cheapest gas I can find (well, not e85). The issue you've found with your small engines is true for big vehicles too, there's less oxygen density in the atmosphere and you have to change the carb jetting/settings to compensate and get back to a stoichiometric (efficient) setting. Track down any old VW guy who's traveled to elevation and ask if they rejet at elevation (or carry spare carb jets). Closed loop EFI (the norm today) handles this without issue... Open loop could deal with some elevation change but generally had different "maps" for elevation ranges. As for the original question, I only have my personal understanding of it as explained by an oil engineer I know at one time and no hard facts, but RON+MON is derived by running fuel through a CFR engine (calibrated test engine designed with this specific task in mind). Take the same fuel and test it at 0/sea level and 5250 (1 mile) above sea level and your output number is different. The fuel is the same, test equipment is the same, operator is the same... the only change is the air density. Nothing "different" about the fuel other than where it's tested. As for those who've read their owners manuals about using a minimum 87, most either have small print right next to that stating "or equivalent" or have a separate section for operating at elevation where it covers the alternatives.

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