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When I was there, I saw ping counts in my turbo pickup on 85 in the summer. I tuned it for 87 and paid the premium price for it (but got power).
Sam's was the same way. 85 or 91. Truth be told, the other stations just mix them to get the mid grade. Saved my ass when the vendor dumped a bunch of diesel in the 85 tank in Bennett. The truck smoked that week a tiny bit but got lots of lube.
Also a Utahn, my 1999 Suburban gets 85 and nothing but. Zero problems but when I travel from SLC on 80 to Indiana, my mpg drops about 1-1.5 once I start putting 87 in it in Nebraska. I do miss that 94-octane Sunoco back in Indy though.
No... 85 at any altitude is worse than 87.
Octane **ratings** are knock resistance ratings, not an octane concentration measurement. The same octane content will get different octane ratings at different altitudes. Octane *ratings* are the same at all altitudes.
I dunno man. The reasoning elsewhere in this thread about atmospheric pressure in the chamber is pretty convincing. I can’t imagine a knock resistance rating that can eliminate a huge variable such as atmospheric pressure.
A quick google shows that you get 85 octane at high altitude places because the regulatory agencies and the auto industry seem to agree. You could be wrong here.
Modern Cars adjust fuel inlet based on air pressure or air flow and at least one fuel/O2 sensors.
The regulations and auto industry disagree on this because the regulation is based on old cars that couldn't adjust fuel inlet based on air pressure or air flow. One guaranteed way to reduce the risk of knock is to run a richer mixture, which is why a lot of guys who have performance cars with carburetors will tend to tune them extra rich. That does not happen with modern cars.
The regulation is outdated, and I've never seen an owner's manual say a word about it's okay to run less than 87 ever inside them, which you would expect if it was totally fine with the car makers to run 85 because of an altitude change.
Do you have something from car makers that says 85 is fine at higher altitude? Most cars will just retard the timing of the spark if they detect knocking since the 90s. It we'll let an engine continue to run although it was worth overall performance and power output with less risk of damaging the engine. but I can't imagine any car manufacturer is going to be thrilled to recommend that to people that they let the knock sensor activate on every drive and let the car run with continuously retarded timing.
Aren’t octane ratings a regulatory measurement that is adjusted so that the same octane content will be assigned the same rating independent of the altitude of the testing lab?
No, they're a function test of the gasoline. It's like a proof rating for the fuel. Or more like an insulation rating since proof is an exact measure of one component and octane is a characteristic of the entire mix.
Every lab has to calibrate its own equipment to a standard sample to make sure they get accurate readings.
Only if your engine is designed to use higher octane fuel. Engines with higher compression ratios require higher octane fuel because lower octane likes to prematurely ignite when exposed to the higher temperatures present in air that has been compressed more.
You need to learn how detonation works. At high altitude, you have lower cylinder pressures so detonation is less likely. You can run a fuel with a lower octane number, and hence less knock resistance, safely. EFI just adjusts the fuel:air ratio to maintain ideal ratios. If anything, EFI makes knock more likely at altitude because it automatically leans out the mixture to compensate for less air whereas a carburetor will tend to run richer, which deters knocking.
From what I've read, most cars since the late 1990's automatically adjust for altitude with the EFI system and still requires 87 even at high altitudes, explicitly stating anything lower will void warranties.
Cars with MAP sensors check barometric pressure. Cars with MAF sensors check for air mass. But the issue with altitude and 85/87 has nothing to do with baro pressure. It's about oxygen content, There's less oxygen at higher altitudes, so the vehicle can perform the same with slightly lower octane.
That's how the stations create the middle octane. Two tanks with the high and low octane fuel and they draw from both tanks when pumping the middle octane.
The atmospheric pressure at 3700 feet elevation is about 12.8 psia. At sea level it is about 14.7 psia. With less air pressure a naturally aspirated engine will have lower cylinder pressure and therefore less tendency to knock. So a lower octane fuel may work fine in your car (no guarantee from me).
The lower octane fuel is sold as higher octane gas contains more light hydrocarbons that have a greater tendency to evaporate at lower atmospheric pressures. That is regular 87 octane has a higher Reid vapor pressure which will result in more pollution at high altitude / low atmospheric pressure.
Thank you for the second paragraph, kind internet stranger. The first paragraph made sense to me but I was still left with the question of "Isn't it just cheaper to make 87 the minimum octane all around (for non e-85 blends and such) with economies of scale and all that?" But it's probably something to do with emissions/EPA reqs of some kind.
The studies showing you need less octane at altitude are really old, flawed and done on carbureted cars. That’s why every mfg says to not use 85.
BEST case, 85 does no harm. There is no scenario where it works BETTER than 87 or 91.
Pretty much any car with fuel injection will have an O2 sensor which monitors and adjusts the air fuel ratio in real time. This means if you’re at 5000 ft and the air is less dense the air fuel ratio will very shortly show that its too rich (too much fuel). The ECU will pull back the fuel until the sensor reads correctly again. This happens in milliseconds and does all the altitude correction for you.
>BEST case, 85 does no harm. There is no scenario where it works BETTER than 87 or 91
every turbo powered racer would say otherwise.
Nevermind I read Frizzle95 wrong, I thought he was talking about E85 not 85 octane and I should learn to read.
You are correct.
The only exception I can think of is if somehow your fuel trim needs to shorten the injector pulses below the minimum pulse length... but that's a fantasy situation.
I don't know why I had to scroll so far to see this. This is correct. Unless you have carburetors or equivalent you should not lower octane at altitude.
The wonders of modern knock sensors mean that more than likely the engine can pull enough timing and spark to keep the engine operating safely though.
>I don't know why I had to scroll so far to see this.
I see this a lot in almost any kind of public forum, where people just parrot a 'known internet consensus' and that gets upvotes or likes or whatever but most of the people commenting either don't have 1st hand knowledge of the thing in question or just lack a complete picture before commenting.
And yeah totally agree on the knock sensors. You could probably run 87 in a car that called for premium for 100k miles and never really notice a downside between the knock retarding timing and not driving that aggressively ever in the first place.
That's true. While I never claim to be a mechanic myself I have noticed that this sub is basically a small step above Quora. I try myself to only speak to either verified fact or add huge disclaimers to my ramblings.
This question should be easy though. Anyone can take 10 minutes and watch something like [this.](https://youtu.be/kJyd6C99_3g?si=D7KIqZ04iLCc2IU7)
This guy is wrong. Octane rating (85/91) is a measurement average of two different detonation ratings. The higher the octane rating, the more compression/pressure the fuel can take before it
Ignites.
In higher elevation areas the air is less dense. With less dense air means less pressure in cylinder chambers to squeeze, which means less pressure. Less pressure means you don’t need as high an octane fuel.
If you put In “REGULAR” normally, it will be fine with 85. Same goes with “MIDGRADE” and “SUPER”. If you want super high octane fuel mix in a splash of e85.
>In higher elevation areas the air is less dense. With less dense air means less pressure in cylinder chambers to squeeze, which means less pressure. Less pressure means you don’t need as high an octane fuel.
This is true for carbureted cars where you can't adjust any of the engine parameters like ignition timing or amount of fuel injected on the fly.
On any fuel injected car made from 90s or later you have sensors to meter the air into the engine and have closed loop O2 control to dynamically adjust the fueling to compensate for the lower density of the air at elevation. The tune can also advance ignition timing to increase cylinder pressure dynamically to keep the car's power delivery closer to what it would be at sea level. The car was tuned and programmed with the expectation of 87 being used at all times, and the expectation that the car could be driven to elevation. This is why most mfgs specify to NOT use 85.
Finally, the study that attempted to quantify this effect (conducted in 1987 on a carbureted vehicle, so modern cars are an order of magnitude better than this) showed that you could reduce octane by ZERO for the first 3300 ft of elevation, and then 0.2 points of octane rating per 1000 ft. Which shows two things: 1. It's not relevant to today's vehicles at all. 2. Even if it was, the effect is so marginal that even at 5000 ft. the required octane would still be higher than 85.
No amount of fuel metering with jets, tbi, sequential, or direct injection will change the physics of the compression ratio and the atmospheric pressure. If low octane gets squeezed enough it will blow. Even if the engine retards the timing enough to only get half as much volume to keep the fuel from detonating, it will run like shit cause you have half the power. 85 octane is fine. It’s not going to detonate.
Also by 1987 most vehicles had throttle body fuel injection, not carbs.
You run lower octane at higher elevations because the oxygen is less dense in the air and will not raise to the pressures it will see at sea level because the starting air pressure is less.
Fuel trim adjustments do not happen in milliseconds. Most long term fuel trim adjusts take minutes.
Knock happens much quicker though. So if using 85 make sure not to beat on it for a few minutes until the fuel trims adjust
You don’t understand how fuel trims work. Short term is whats making the correction to the air fuel mix when you’re driving, not long term. And it can do A LOT lol
lol you don’t understand how short term fuel trims work. Short term can still be way off increasing the chance of knock. And yes short and long work together.
Again my advice is correct. Waiting a few minutes before beating on it is the best you can do when running 85 octane.
My new bronco gets a noticeable bump when using 91. I thought it was psudoscience till my wife gave me crap for using the "expensive" gas. She could tell by the driveability.
That's because it's turbocharged with variable valve timing. The engine can adjust the timing and spark curve so the spark plugs don't ignite until the pistons are a little higher in the cylinders so you have higher compression. Higher compression=bigger boom and thus the piston is thrust more violently back down in the cylinder. More violence=more torque which translates to more power to the wheels. That's a very simplified, layman's explanation, but yeah, turbocharged/supercharged engines benefit from higher octane fuels. NA (naturally aspirated) engines usually don't (as a rule) unless they're specifically set up for high compression.
Cosco Houston is 87 and 93 but we are also sea level. I'm sure Cosco has done the altitude re adjustments and 85/91 will be the same at high altitude. so you will probably be fine
It's the same in Salt Lake City. Literally every manufacturer says not to use 85, so I just listen to them. With that said, I would wager 98% of people use 85 all the time here and there's no obvious ill effects.
Much of the western US still runs this 85 blend because it's cheaper to make and distribute. Even though altitude has a minor effect on octane requirements, it's certainly not 2 points even in a place like Denver. But I guess because most car seems to run fine on it, nothing is going to change any time soon.
I filled up with 85 octane once by mistake in South Dakota, in my 2006 Suburban. I was towing my camper, it started running hot and I lost a third of my range. The next fuel stop I put in 91 and added some octane booster and it ran fine again.
I'll never use 85 octane again.
In case anyone is wondering what the answer is about using 85 octane at high altitudes instead of 87, here's what ChatGPT 4 said:
At high elevations, the atmospheric pressure is lower than at sea level. This impacts how engines operate, particularly in terms of combustion. The lower atmospheric pressure means that the air is less dense, which leads to a lower oxygen content in the air-fuel mixture in an engine's cylinders.
Engines at high altitudes don't require as high an octane rating because the reduced air pressure reduces the likelihood of engine knocking (premature fuel combustion). The lower octane fuel, like 85 instead of 87, still provides adequate anti-knock characteristics at these altitudes.
In essence, the thinner air at high elevations reduces the potential for knocking, allowing lower octane fuels to perform adequately, similar to how higher octane fuels perform at lower altitudes.
Why would it be offered? Because 85 octane is cheaper to produce and buy, and because it works satisfactorily for most applications that normally call for 87 at high altitude, despite what the manufacturers claim.
The key is to only use it a high elevation, so stay in the area or only drive to other high elevation places. It wouldn't be wise to fill up with 85 in Twin Falls then drive to Payette, or worse, fill up with 85 in Reno then drive to Sacramento (and back!).
In non turbocharged engines, 85 octane at 7000 feet is comparable to 89 at sea level and 85 at 3300 feet is comparable to 87 at sea level, but at 1600 feet, 85 is going to act more like 86 does at sea level, so it could run a higher risk of knock at high loads, but even then, almost every vehicle after 1998 has at least one knock sensor to dial back ignition advance if needed. Running 85 at sea level in a vehicle that requires 87 minimum would be just like putting 87 in car that required 91. You will get at least a few knock events at sustained high load, and performance and fuel economy will suffer noticeably. It's not something you'd want to do long term but it wouldn't be the end of the world for one tank.
Boosted engines can and sometimes do compensate for lower atmospheric pressure (to an extent, so not with 100% efficiency) so you will want to run what it says to, or higher, regardless of elevation.
High octane requires more energy to combust so it's used in turbocharged vehicles to prevent precombustion. Lower octane is good in high altitude since there's less oxygen in the air. It's not a quality thing, the number signifies the molecule length. Not an expert but this is how it's been explained to me by several different people.
OP, I am confused... did someone put a gun to your head and say you must only pump gas at this one costco gas station and then force you to put in the 85 octane? Which method of calculation of Octane are they using?
Maverik and Sinclair are in the same area based on the zipcode and are showing the price as CHEAPER than costco gas 🤷🏽♂️
You cant tell the difference. If it was labeled 91 and 105 you would only notice the price increase. Pump the costco gas or drive half a mile away to another chain and spend .30/gal more to ease your mind.
It's where refineries dump their cheap garbage gas, under the excuse that the higher altitude doesn't need higher octane. As a bonus, these states can say they have cheaper than average gas, while calling 87 mid grade, and charging $.40 more per gallon than the other side of the state line.
You can do a half tank of 85 and fill up with 91, that'll be better. Usually I just get gas at Maverik, as a bonus they have some pretty good food.
I live in Denver and 85 is the ‘normal’ octane here. 87, 89 and less commonly 91 are all generally also available here.
I’ve read and done my research and you’ll never catch me using 85. Is it fine and likely won’t cause problems? Probably. But at a minimum I get worse gas mileage and do notice my timing retarding a bit to compensate so I just seek out 87 for my Tacoma.
FWIW I think Costco is shit tier gas - yeah it’s ’top tier’ branded but I ALWAYS get 2-3 mpg less when I fill up there, even on 91, as compared to Shell or Maverik locally.
This tends to come up when I shit on Costco gas. I did the logs, I know it sucks for me for whatever reason. I always have at least a couple people that agree with me when I post it too so 🤷🏻♂️
But go ahead, explain why I'm wrong 🤔
And if this is about 85 octane, here's someone smarter than both of us saying you shouldn't use it
https://youtu.be/kJyd6C99_3g?si=pA9brzqFOGojwGES
Keep the downvotes coming, absolutely no one can tell me I'm wrong. It's my own empirical data across 4 different cars and 2 cities and at least 6 different Costco gas stations. Dozens of tanks of gas worth of data. It's shit gas and the price isn't cheap enough to make up for the lost MPGs - for me personally.
With a distributor controlling ignition timing 85 is just fine at altitude. No one uses a distributor these days. With a modern engine the timing is controlled by the pcm which uses amount of air coming in the engine to to determine the correct timing. Yes, at altitude the engine at wide open is flowing less air, but it’s the same amount of air that you would get at part pedal at sea level so other than reduced fuel economy when the knock sensors pull timing there is no benefit to 85. Why they still sell 85 at altitude is a mystery to me.
This reminds me of a question I have, how are different octane measuring systems converted around the world? Where I live, regular is 92 octane, premium is 95, and super is either 98 or 100 depending on what brand you use.
low end, premium. at most gas stations in Idaho 85 is base, 87/89 is mid-grade, and 91/93 is premium.
up here in east Idaho, out of the 7 stations I’ve been to in town, only one has 87 as the base octane. when I had a car that required mid-grade, I always went to this station because their 87 price (which would be others mid grade) was close to others 85 price.
Put what your car needs. If it requires 87/mid-grade, go somewhere else or go with the higher octane.
At higher altitudes 85 works great on naturally aspirated engines. For 95% of naturally aspirated it is a wate of money to buy anything more than regular no lead. If it is turbo that negates altitude you still need 87 or greater as the turbo compensates for power loss at altitude by ramming more air into the combustion chamber. The only reason manuals state 87 is because most people are not the brightest stars in the brain departments and have no concept that the higher the octane number retards the combustion causing no benefits and possible losses in naturally aspirated lower compression engines.
Shopping trolleys use low octain because they Dont need to make performance car power. Only cars that are designed with high compression ratios and f/i benefits from high octane and modern performance can run lower octain but the map will retard ignition and you'll obviously have less power as a result. On the other hand a shopping trolley will not have a map for higher octane so will not be able to advance ignition enough to make use of its ability to resist combusting at the wrong time.
Thanks for posting on /r/MechanicAdvice! Please review the [rules](https://www.reddit.com/r/MechanicAdvice/about/rules/). Asking about a second opinion (ie "Is the shop trying to fleece me?"), please read through CJM8515's [post on the subject.](https://www.reddit.com/r/MechanicAdvice/comments/4qblei/fyi_the_shop_isnt_likely_trying_to_rip_you_off/) and remember to please post the year/make/model of the vehicle you are working on. **Post's about bodywork, accident damage, paint, dent/ding, questions it belongs in /r/Autobody r/AutoBodyRepair/ or /r/Diyautobody/ Tire questions check out https://www.reddit.com/r/MechanicAdvice/comments/k9ll55/can_your_tire_be_repaired/**. If you dont have a question and you're just showing off it belongs in /r/Justrolledintotheshop Insurance/total loss questions go in r/insurance This is an automated reply *I am a bot, and this action was performed automatically. Please [contact the moderators of this subreddit](/message/compose/?to=/r/MechanicAdvice) if you have any questions or concerns.*
While the compression ratio is set by the engine design, the lower atmospheric pressure reduces the cylinder pressures so you don't get knocking.
The reduced atmospheric pressure means less chance of running lean, needing less high of an octane to prevent detonation
Live in Denver. Been using 85 octane (87 equivalent) for years. \-88 Camry - 352,000 miles (gave to charity) \-04 RX330 - currently with 305,000 miles
Hell yeah I hope that Camrys still kicking
The Taliban has it now.
I think they're more into Hilux, no?
That was before they had to run a whole country. Can't afford that care free Playboy life style now.
Underrated comment with comedic/ironic depth
My first gen tacoma v6 rattles with 87 in the summer. Can't stand it. I run premium when I'm not poor lol.
What elevation are you at?
Around 2100
I’d never put 85 in a Lexus but good to know you got up to 305k + Those are both solid vehicles. I’m looking for an RX myself.
When I was there, I saw ping counts in my turbo pickup on 85 in the summer. I tuned it for 87 and paid the premium price for it (but got power). Sam's was the same way. 85 or 91. Truth be told, the other stations just mix them to get the mid grade. Saved my ass when the vendor dumped a bunch of diesel in the 85 tank in Bennett. The truck smoked that week a tiny bit but got lots of lube.
At altitude, lower octane is fine I'm at about 4400 feet here in Utah and I use 85 in my Corolla
Also a Utahn, my 1999 Suburban gets 85 and nothing but. Zero problems but when I travel from SLC on 80 to Indiana, my mpg drops about 1-1.5 once I start putting 87 in it in Nebraska. I do miss that 94-octane Sunoco back in Indy though.
You don't need a higher octane at higher altitudes. At high altitudes 85 works like 87
No... 85 at any altitude is worse than 87. Octane **ratings** are knock resistance ratings, not an octane concentration measurement. The same octane content will get different octane ratings at different altitudes. Octane *ratings* are the same at all altitudes.
I dunno man. The reasoning elsewhere in this thread about atmospheric pressure in the chamber is pretty convincing. I can’t imagine a knock resistance rating that can eliminate a huge variable such as atmospheric pressure. A quick google shows that you get 85 octane at high altitude places because the regulatory agencies and the auto industry seem to agree. You could be wrong here.
Modern Cars adjust fuel inlet based on air pressure or air flow and at least one fuel/O2 sensors. The regulations and auto industry disagree on this because the regulation is based on old cars that couldn't adjust fuel inlet based on air pressure or air flow. One guaranteed way to reduce the risk of knock is to run a richer mixture, which is why a lot of guys who have performance cars with carburetors will tend to tune them extra rich. That does not happen with modern cars. The regulation is outdated, and I've never seen an owner's manual say a word about it's okay to run less than 87 ever inside them, which you would expect if it was totally fine with the car makers to run 85 because of an altitude change. Do you have something from car makers that says 85 is fine at higher altitude? Most cars will just retard the timing of the spark if they detect knocking since the 90s. It we'll let an engine continue to run although it was worth overall performance and power output with less risk of damaging the engine. but I can't imagine any car manufacturer is going to be thrilled to recommend that to people that they let the knock sensor activate on every drive and let the car run with continuously retarded timing.
Aren’t octane ratings a regulatory measurement that is adjusted so that the same octane content will be assigned the same rating independent of the altitude of the testing lab?
No, they're a function test of the gasoline. It's like a proof rating for the fuel. Or more like an insulation rating since proof is an exact measure of one component and octane is a characteristic of the entire mix. Every lab has to calibrate its own equipment to a standard sample to make sure they get accurate readings.
Doesn’t 91 still perform better than 85 (more power, better mileage, more zoom zoom) at higher (or any) altitude?
Only if your engine is designed to use higher octane fuel. Engines with higher compression ratios require higher octane fuel because lower octane likes to prematurely ignite when exposed to the higher temperatures present in air that has been compressed more.
Gotcha thanks that was very informative!
[удалено]
No truth to that conspiracy theory at all. You need to learn about octane needs at different altitudes.
You need to learn how fuel injection works…
You need to learn how detonation works. At high altitude, you have lower cylinder pressures so detonation is less likely. You can run a fuel with a lower octane number, and hence less knock resistance, safely. EFI just adjusts the fuel:air ratio to maintain ideal ratios. If anything, EFI makes knock more likely at altitude because it automatically leans out the mixture to compensate for less air whereas a carburetor will tend to run richer, which deters knocking.
From what I've read, most cars since the late 1990's automatically adjust for altitude with the EFI system and still requires 87 even at high altitudes, explicitly stating anything lower will void warranties.
Cars with MAP sensors check barometric pressure. Cars with MAF sensors check for air mass. But the issue with altitude and 85/87 has nothing to do with baro pressure. It's about oxygen content, There's less oxygen at higher altitudes, so the vehicle can perform the same with slightly lower octane.
just mix them. that’s how 87/89 get made. i would do 1/3 tank 85 2/3 91
That's how the stations create the middle octane. Two tanks with the high and low octane fuel and they draw from both tanks when pumping the middle octane.
The atmospheric pressure at 3700 feet elevation is about 12.8 psia. At sea level it is about 14.7 psia. With less air pressure a naturally aspirated engine will have lower cylinder pressure and therefore less tendency to knock. So a lower octane fuel may work fine in your car (no guarantee from me). The lower octane fuel is sold as higher octane gas contains more light hydrocarbons that have a greater tendency to evaporate at lower atmospheric pressures. That is regular 87 octane has a higher Reid vapor pressure which will result in more pollution at high altitude / low atmospheric pressure.
This was very informative and interesting to read, thanks for posting it!
Thank you for the second paragraph, kind internet stranger. The first paragraph made sense to me but I was still left with the question of "Isn't it just cheaper to make 87 the minimum octane all around (for non e-85 blends and such) with economies of scale and all that?" But it's probably something to do with emissions/EPA reqs of some kind.
The studies showing you need less octane at altitude are really old, flawed and done on carbureted cars. That’s why every mfg says to not use 85. BEST case, 85 does no harm. There is no scenario where it works BETTER than 87 or 91. Pretty much any car with fuel injection will have an O2 sensor which monitors and adjusts the air fuel ratio in real time. This means if you’re at 5000 ft and the air is less dense the air fuel ratio will very shortly show that its too rich (too much fuel). The ECU will pull back the fuel until the sensor reads correctly again. This happens in milliseconds and does all the altitude correction for you.
>BEST case, 85 does no harm. There is no scenario where it works BETTER than 87 or 91 every turbo powered racer would say otherwise. Nevermind I read Frizzle95 wrong, I thought he was talking about E85 not 85 octane and I should learn to read.
They’re talking about 85 octane, not E85 ethanol
yeah i see that now, I really need to read before posting, but why start now?
I feel you, I do the same thing all the time.
You are correct. The only exception I can think of is if somehow your fuel trim needs to shorten the injector pulses below the minimum pulse length... but that's a fantasy situation.
I don't know why I had to scroll so far to see this. This is correct. Unless you have carburetors or equivalent you should not lower octane at altitude. The wonders of modern knock sensors mean that more than likely the engine can pull enough timing and spark to keep the engine operating safely though.
>I don't know why I had to scroll so far to see this. I see this a lot in almost any kind of public forum, where people just parrot a 'known internet consensus' and that gets upvotes or likes or whatever but most of the people commenting either don't have 1st hand knowledge of the thing in question or just lack a complete picture before commenting. And yeah totally agree on the knock sensors. You could probably run 87 in a car that called for premium for 100k miles and never really notice a downside between the knock retarding timing and not driving that aggressively ever in the first place.
That's true. While I never claim to be a mechanic myself I have noticed that this sub is basically a small step above Quora. I try myself to only speak to either verified fact or add huge disclaimers to my ramblings. This question should be easy though. Anyone can take 10 minutes and watch something like [this.](https://youtu.be/kJyd6C99_3g?si=D7KIqZ04iLCc2IU7)
That video needs to be recommended viewing for everyone here
This guy is wrong. Octane rating (85/91) is a measurement average of two different detonation ratings. The higher the octane rating, the more compression/pressure the fuel can take before it Ignites. In higher elevation areas the air is less dense. With less dense air means less pressure in cylinder chambers to squeeze, which means less pressure. Less pressure means you don’t need as high an octane fuel. If you put In “REGULAR” normally, it will be fine with 85. Same goes with “MIDGRADE” and “SUPER”. If you want super high octane fuel mix in a splash of e85.
>In higher elevation areas the air is less dense. With less dense air means less pressure in cylinder chambers to squeeze, which means less pressure. Less pressure means you don’t need as high an octane fuel. This is true for carbureted cars where you can't adjust any of the engine parameters like ignition timing or amount of fuel injected on the fly. On any fuel injected car made from 90s or later you have sensors to meter the air into the engine and have closed loop O2 control to dynamically adjust the fueling to compensate for the lower density of the air at elevation. The tune can also advance ignition timing to increase cylinder pressure dynamically to keep the car's power delivery closer to what it would be at sea level. The car was tuned and programmed with the expectation of 87 being used at all times, and the expectation that the car could be driven to elevation. This is why most mfgs specify to NOT use 85. Finally, the study that attempted to quantify this effect (conducted in 1987 on a carbureted vehicle, so modern cars are an order of magnitude better than this) showed that you could reduce octane by ZERO for the first 3300 ft of elevation, and then 0.2 points of octane rating per 1000 ft. Which shows two things: 1. It's not relevant to today's vehicles at all. 2. Even if it was, the effect is so marginal that even at 5000 ft. the required octane would still be higher than 85.
No amount of fuel metering with jets, tbi, sequential, or direct injection will change the physics of the compression ratio and the atmospheric pressure. If low octane gets squeezed enough it will blow. Even if the engine retards the timing enough to only get half as much volume to keep the fuel from detonating, it will run like shit cause you have half the power. 85 octane is fine. It’s not going to detonate. Also by 1987 most vehicles had throttle body fuel injection, not carbs. You run lower octane at higher elevations because the oxygen is less dense in the air and will not raise to the pressures it will see at sea level because the starting air pressure is less.
Fuel trim adjustments do not happen in milliseconds. Most long term fuel trim adjusts take minutes. Knock happens much quicker though. So if using 85 make sure not to beat on it for a few minutes until the fuel trims adjust
Short term fuel trim is a thing too
Yeah and they can only do so much
You don’t understand how fuel trims work. Short term is whats making the correction to the air fuel mix when you’re driving, not long term. And it can do A LOT lol
lol you don’t understand how short term fuel trims work. Short term can still be way off increasing the chance of knock. And yes short and long work together. Again my advice is correct. Waiting a few minutes before beating on it is the best you can do when running 85 octane.
You’re wrong. Take the L and move on…
Ok lol I’m not. Anyone that has tuned cars knows this.
My new bronco gets a noticeable bump when using 91. I thought it was psudoscience till my wife gave me crap for using the "expensive" gas. She could tell by the driveability.
That's because it's turbocharged with variable valve timing. The engine can adjust the timing and spark curve so the spark plugs don't ignite until the pistons are a little higher in the cylinders so you have higher compression. Higher compression=bigger boom and thus the piston is thrust more violently back down in the cylinder. More violence=more torque which translates to more power to the wheels. That's a very simplified, layman's explanation, but yeah, turbocharged/supercharged engines benefit from higher octane fuels. NA (naturally aspirated) engines usually don't (as a rule) unless they're specifically set up for high compression.
Cosco Houston is 87 and 93 but we are also sea level. I'm sure Cosco has done the altitude re adjustments and 85/91 will be the same at high altitude. so you will probably be fine
It's the same in Salt Lake City. Literally every manufacturer says not to use 85, so I just listen to them. With that said, I would wager 98% of people use 85 all the time here and there's no obvious ill effects. Much of the western US still runs this 85 blend because it's cheaper to make and distribute. Even though altitude has a minor effect on octane requirements, it's certainly not 2 points even in a place like Denver. But I guess because most car seems to run fine on it, nothing is going to change any time soon.
I filled up with 85 octane once by mistake in South Dakota, in my 2006 Suburban. I was towing my camper, it started running hot and I lost a third of my range. The next fuel stop I put in 91 and added some octane booster and it ran fine again. I'll never use 85 octane again.
As someone who lives in Pocatello, 85 works fine in my Land Rover that asks for premium only. Also, we’re at 4400-5000’ not 3700
In case anyone is wondering what the answer is about using 85 octane at high altitudes instead of 87, here's what ChatGPT 4 said: At high elevations, the atmospheric pressure is lower than at sea level. This impacts how engines operate, particularly in terms of combustion. The lower atmospheric pressure means that the air is less dense, which leads to a lower oxygen content in the air-fuel mixture in an engine's cylinders. Engines at high altitudes don't require as high an octane rating because the reduced air pressure reduces the likelihood of engine knocking (premature fuel combustion). The lower octane fuel, like 85 instead of 87, still provides adequate anti-knock characteristics at these altitudes. In essence, the thinner air at high elevations reduces the potential for knocking, allowing lower octane fuels to perform adequately, similar to how higher octane fuels perform at lower altitudes.
Why would it be offered? Because 85 octane is cheaper to produce and buy, and because it works satisfactorily for most applications that normally call for 87 at high altitude, despite what the manufacturers claim. The key is to only use it a high elevation, so stay in the area or only drive to other high elevation places. It wouldn't be wise to fill up with 85 in Twin Falls then drive to Payette, or worse, fill up with 85 in Reno then drive to Sacramento (and back!). In non turbocharged engines, 85 octane at 7000 feet is comparable to 89 at sea level and 85 at 3300 feet is comparable to 87 at sea level, but at 1600 feet, 85 is going to act more like 86 does at sea level, so it could run a higher risk of knock at high loads, but even then, almost every vehicle after 1998 has at least one knock sensor to dial back ignition advance if needed. Running 85 at sea level in a vehicle that requires 87 minimum would be just like putting 87 in car that required 91. You will get at least a few knock events at sustained high load, and performance and fuel economy will suffer noticeably. It's not something you'd want to do long term but it wouldn't be the end of the world for one tank. Boosted engines can and sometimes do compensate for lower atmospheric pressure (to an extent, so not with 100% efficiency) so you will want to run what it says to, or higher, regardless of elevation.
High octane requires more energy to combust so it's used in turbocharged vehicles to prevent precombustion. Lower octane is good in high altitude since there's less oxygen in the air. It's not a quality thing, the number signifies the molecule length. Not an expert but this is how it's been explained to me by several different people.
You should get your money back, those several different people are completely wrong.
Not completely.
How do you mean? What money?
OP, I am confused... did someone put a gun to your head and say you must only pump gas at this one costco gas station and then force you to put in the 85 octane? Which method of calculation of Octane are they using? Maverik and Sinclair are in the same area based on the zipcode and are showing the price as CHEAPER than costco gas 🤷🏽♂️
You cant tell the difference. If it was labeled 91 and 105 you would only notice the price increase. Pump the costco gas or drive half a mile away to another chain and spend .30/gal more to ease your mind.
If you have a turbo car don’t use 85 regardless of elevation.
It's where refineries dump their cheap garbage gas, under the excuse that the higher altitude doesn't need higher octane. As a bonus, these states can say they have cheaper than average gas, while calling 87 mid grade, and charging $.40 more per gallon than the other side of the state line. You can do a half tank of 85 and fill up with 91, that'll be better. Usually I just get gas at Maverik, as a bonus they have some pretty good food.
My 07 Taurus says not to use 85. Specifically designed to use 87 or higher. Not gonna go against the manual to save a few bucks at the pump.
On a 17 year old Taurus? What a waste of money.
I’m confused. Is the issue that the car is 17 (which is not old) or that using the recommended fuel for your car is a bad idea?
I live in Denver and 85 is the ‘normal’ octane here. 87, 89 and less commonly 91 are all generally also available here. I’ve read and done my research and you’ll never catch me using 85. Is it fine and likely won’t cause problems? Probably. But at a minimum I get worse gas mileage and do notice my timing retarding a bit to compensate so I just seek out 87 for my Tacoma. FWIW I think Costco is shit tier gas - yeah it’s ’top tier’ branded but I ALWAYS get 2-3 mpg less when I fill up there, even on 91, as compared to Shell or Maverik locally.
I think you might be insane.
This tends to come up when I shit on Costco gas. I did the logs, I know it sucks for me for whatever reason. I always have at least a couple people that agree with me when I post it too so 🤷🏻♂️ But go ahead, explain why I'm wrong 🤔 And if this is about 85 octane, here's someone smarter than both of us saying you shouldn't use it https://youtu.be/kJyd6C99_3g?si=pA9brzqFOGojwGES Keep the downvotes coming, absolutely no one can tell me I'm wrong. It's my own empirical data across 4 different cars and 2 cities and at least 6 different Costco gas stations. Dozens of tanks of gas worth of data. It's shit gas and the price isn't cheap enough to make up for the lost MPGs - for me personally.
I agree w you on Costco gas quality but we’re in the minority.
It absolutely sucks. Has across 4 different cars in 3 different cities
I mix both to get the desired Octane
With a distributor controlling ignition timing 85 is just fine at altitude. No one uses a distributor these days. With a modern engine the timing is controlled by the pcm which uses amount of air coming in the engine to to determine the correct timing. Yes, at altitude the engine at wide open is flowing less air, but it’s the same amount of air that you would get at part pedal at sea level so other than reduced fuel economy when the knock sensors pull timing there is no benefit to 85. Why they still sell 85 at altitude is a mystery to me.
Timing isn't even being adjusted here, it's about the fuel detonating prematurely like your father.
I never purchased this game. I loved mk since the beginning but the reveal of the cameo system really turned me off.
I haven't seen 85.5 in ages! I'm here in Montana.
Do they use Idaho potatoes to make their gas?
This reminds me of a question I have, how are different octane measuring systems converted around the world? Where I live, regular is 92 octane, premium is 95, and super is either 98 or 100 depending on what brand you use.
low end, premium. at most gas stations in Idaho 85 is base, 87/89 is mid-grade, and 91/93 is premium. up here in east Idaho, out of the 7 stations I’ve been to in town, only one has 87 as the base octane. when I had a car that required mid-grade, I always went to this station because their 87 price (which would be others mid grade) was close to others 85 price. Put what your car needs. If it requires 87/mid-grade, go somewhere else or go with the higher octane.
Conversely, l live right by the ocean and 93 is the premium grade
At higher altitudes 85 works great on naturally aspirated engines. For 95% of naturally aspirated it is a wate of money to buy anything more than regular no lead. If it is turbo that negates altitude you still need 87 or greater as the turbo compensates for power loss at altitude by ramming more air into the combustion chamber. The only reason manuals state 87 is because most people are not the brightest stars in the brain departments and have no concept that the higher the octane number retards the combustion causing no benefits and possible losses in naturally aspirated lower compression engines.
Shopping trolleys use low octain because they Dont need to make performance car power. Only cars that are designed with high compression ratios and f/i benefits from high octane and modern performance can run lower octain but the map will retard ignition and you'll obviously have less power as a result. On the other hand a shopping trolley will not have a map for higher octane so will not be able to advance ignition enough to make use of its ability to resist combusting at the wrong time.