85 octane
While traveling out west this summer I noticed most of the gas stations offering 85 octane and 87 octane as their mid grade. Has anyone tried this in their gasser? I wasn't going to try it especially ...
Forum Discussion
As others noted 85 octane is fine at higher altitude for engines that normally recommend 87 or 89 octane because the lower atmospheric pressure and reduced oxygen content have the same impact as a lower compression ratio. This is the reason for the rule of thumb that naturally aspirated (non-turbo/supercharged) engines lose about 3% of horsepower for every 1,000 foot increase above sea level. We humans start feeling this same power reduction at high altitude :)
When the octane rating is too low for operating conditions the fuel/air mixture creates the characteristic knocking noise because it is igniting as an explosion instead of a relatively slow burn.
Prior to modern fuel injection systems the loss of power could approach 4.5% per 1,000 feet of altitude because the mixture would also be way off at higher altitude.
I would be somewhat careful in not filling up with lower octane fuel just before commencing a long drive at normal altitude with a heavily loaded engine. Engines are more prone to knock under heavy load and timing won't be dialed back until knock occurs. The knock sensor "listens" for the sound of the fuel/air mixture exploding rather than the desired controlled clean burn. Think of the firecrackers we (or at least some of us) played with as kids. Wrapped up tightly in its little cylinder it explodes with the satisfying bang but tear it open and spread out the contents where they aren't "compressed" as they burn and you get a slow burn instead of a bang. You don't want the fuel/air mixture going bang in your engine because this is hard on pistons, rings, connecting rods and mains.
Most current engines are far better at preventing knocking and resulting damage than vintage 1980s and most of the 1990 era production so be more careful if you are keeping a vintage rig on the road. And if your engine is prone to running hot the reduced timing that is used to reduce knock with lower octane fuel will make the engine run hotter since ignition is delayed and the burn is occurring later in the power stroke with more transfer of heat to the cylinder walls instead of in the combustion chamber. If a rig is already running hot you don't want to have timing further reduced by low octane fuel because it starts a vicious cycle with increased engine heat further causing "knock" resulting in even more timing reduction which further increases heat.
On edit: It is nice that the forum board software is friendly and politically correct but the classic term for ignition timing being "delayed" is the same as an unfortunate pejorative term for those of reduced cognitive capability. So the above was edited to remove the offending term and put in wording that conveys the ignition timing management without running afoul of the robotic peace keeper :)
When the octane rating is too low for operating conditions the fuel/air mixture creates the characteristic knocking noise because it is igniting as an explosion instead of a relatively slow burn.
Prior to modern fuel injection systems the loss of power could approach 4.5% per 1,000 feet of altitude because the mixture would also be way off at higher altitude.
I would be somewhat careful in not filling up with lower octane fuel just before commencing a long drive at normal altitude with a heavily loaded engine. Engines are more prone to knock under heavy load and timing won't be dialed back until knock occurs. The knock sensor "listens" for the sound of the fuel/air mixture exploding rather than the desired controlled clean burn. Think of the firecrackers we (or at least some of us) played with as kids. Wrapped up tightly in its little cylinder it explodes with the satisfying bang but tear it open and spread out the contents where they aren't "compressed" as they burn and you get a slow burn instead of a bang. You don't want the fuel/air mixture going bang in your engine because this is hard on pistons, rings, connecting rods and mains.
Most current engines are far better at preventing knocking and resulting damage than vintage 1980s and most of the 1990 era production so be more careful if you are keeping a vintage rig on the road. And if your engine is prone to running hot the reduced timing that is used to reduce knock with lower octane fuel will make the engine run hotter since ignition is delayed and the burn is occurring later in the power stroke with more transfer of heat to the cylinder walls instead of in the combustion chamber. If a rig is already running hot you don't want to have timing further reduced by low octane fuel because it starts a vicious cycle with increased engine heat further causing "knock" resulting in even more timing reduction which further increases heat.
On edit: It is nice that the forum board software is friendly and politically correct but the classic term for ignition timing being "delayed" is the same as an unfortunate pejorative term for those of reduced cognitive capability. So the above was edited to remove the offending term and put in wording that conveys the ignition timing management without running afoul of the robotic peace keeper :)
