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Author Topic: Octane at higher elevations  (Read 738 times)

John Stephens

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Octane at higher elevations
« on: November 01, 2018, 08:13:23 AM »
Here's an argument going on between two people who own Workhorse chassis.

One guy says the Workhorse owner's club has told him to use a higher octane gas when traveling through the West with higher elevations due to the decrease in oxygen in the atmosphere.

Another guy says that is absolutely false and every gas engine requires a lower octane for the exact same reason.

Which is correct?
John
Cape Coral, Fl.
2005 Winnebago Adventurer 38J
2018 Chevy Equinox

Back2PA

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Re: Octane at higher elevations
« Reply #1 on: November 01, 2018, 08:27:56 AM »
Here's an argument going on between two people who own Workhorse chassis.

One guy says the Workhorse owner's club has told him to use a higher octane gas when traveling through the West with higher elevations due to the decrease in oxygen in the atmosphere.

Another guy says that is absolutely false and every gas engine requires a lower octane for the exact same reason.

Which is correct?
Thinner air at higher elevation reduces the chance of detonation, therefore lower octane at higher elevation
Scott
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youracman

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Re: Octane at higher elevations
« Reply #2 on: November 01, 2018, 08:40:54 AM »
Thinner air at higher elevation reduces the chance of detonation, therefore lower octane at higher elevation

Yup.   That's why "regular" at the pump is 87 octane at all Houston gas stations and 85 here in Denver.
Ed Sievers/Denver, CO
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sightseers

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Re: Octane at higher elevations
« Reply #3 on: November 01, 2018, 08:47:21 AM »
 Octane is Knock-reducer.

 The lower octane fuel you put in the more knock (pre-ignition) occurs.   But... you will never hear a knock because to prevent this highly destructive pre-ignition explosion, your engine control unit (ECU) will reduce the timing advance.

 and with this lower ignition timing, your engine will now run less efficiently and make less power.  So you end stepping on the gas harder to accomplish the same work.....and you end up using more fuel.

IMO....Always get the best fuel you can buy,  and let the engine run the best it can.
« Last Edit: November 01, 2018, 09:13:14 AM by sightseers »

Gary RV_Wizard

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Re: Octane at higher elevations
« Reply #4 on: November 01, 2018, 11:09:56 AM »
The octane sold as "regular gas" at most name brand fuel stations will vary by the altitude. If your coach runs on "regular unleaded gas", buy that wherever you go. As youracman says, it will be a lower octane number (usually 85) in high altitude regions.  You probably need to be upwards of 3000 ft before you see any changes, and more likely 4000+.
Gary
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Arch Hoagland

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Re: Octane at higher elevations
« Reply #5 on: November 01, 2018, 01:14:41 PM »
I've put 90,000 miles on my 8.1 mostly in the mountains of the west and have never used anything but 87 octane.

Runs fine.
2004 Monaco La Palma 36 DBD
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2000 Lexus RX300, 4020lb
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Isaac-1

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Re: Octane at higher elevations
« Reply #6 on: November 01, 2018, 01:22:34 PM »
I had some slight issue with knocking with my 8.1L while in Wyoming last year when running 85 octane, but not when using 86 or 87
2002 Safari Trek 2830

SpencerPJ

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Re: Octane at higher elevations
« Reply #7 on: November 01, 2018, 01:50:18 PM »
Quote: "Although higher octane can cost substantially more per gallon, it does not necessarily mean it is better for your car. Higher octane gas is processed through additional steps that further refine the blend and cause it to burn more slowly than lower octanes. These additional processes are what contribute to the higher pricing, but that does not mean the higher octane will offer any advantage over other blends in many engines. Octane does not offer any better fuel mileage, increase engine horsepower, or make the engine start quicker. Higher octane only reduces the likelihood of engine knock or ping."


kdbgoat

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Re: Octane at higher elevations
« Reply #8 on: November 01, 2018, 02:52:19 PM »
Quote: "Although higher octane can cost substantially more per gallon, it does not necessarily mean it is better for your car. Higher octane gas is processed through additional steps that further refine the blend and cause it to burn more slowly than lower octanes. These additional processes are what contribute to the higher pricing, but that does not mean the higher octane will offer any advantage over other blends in many engines. Octane does not offer any better fuel mileage, increase engine horsepower, or make the engine start quicker. Higher octane only reduces the likelihood of engine knock or ping."



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sightseers

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Re: Octane at higher elevations
« Reply #9 on: November 01, 2018, 04:11:55 PM »
you can buy the lowest octane, worst quality gas made in Mexico.... and chances are you will think your engine is running just fine, because your ECU will adjust.

Anyone that has ever done a valve job.... knows the affects of cheap gas.

It's not uncommon to hear people say that the car ran fine on regular for many years... then, they had to start running premium fuel because it pinged all the time.

When you take the heads off,  you will see all the deposits causing the pre-ignition problems.   


do you trust what these guys say ?   :D..... https://www.kbb.com/car-advice/articles/premium-gas-when-and-why/
« Last Edit: November 01, 2018, 04:36:47 PM by sightseers »

Lou Schneider

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Re: Octane at higher elevations
« Reply #10 on: November 01, 2018, 04:52:50 PM »
Air pressure at 6,000 ft. is about 80% of what it is at sea level.   At 10,000 ft. it's about 70% of the sea level pressure.

Since the engine's compression ratio remains constant, this means the cylinder's compression pressure will be reduced by a similar amount as you gain altitude.  An engine with a 10:1 compression ratio produces the same compression pressure at 6,000 ft. as an engine with a 8:1 compression ratio does at sea level.   At 10,000 ft. that drops to an effective 7:1 sea level compression ratio.

Since the absolute pressure inside the cylinders reduces with altitude, so does the octane level needed to prevent pre-ignition.

In other words, use the same grade of gasoline at elevation as you would at sea level, even though the octane rating is lower on the pump.  If your engine runs fine on regular at low elevations, it will continue to run fine on regular at elevation.  If you need premium gas at sea level, use the same at high elevations. 

But you won't gain any advantage switching an engine that runs fine on regular gas to premium at high elevations.
« Last Edit: November 01, 2018, 06:31:50 PM by Lou Schneider »

kdbgoat

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Re: Octane at higher elevations
« Reply #11 on: November 01, 2018, 05:33:15 PM »
Deposits are not caused by lack of octane. Cheap gas that contains little detergent is what causes deposits. That and not giving the engine a "blowout" once in a while. And yes, I've tore many an engine apart. As yes, I put most back together.
I know you believe you understand what you think I said,
But I am not sure you realize what you heard is not what I meant

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Gary RV_Wizard

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Re: Octane at higher elevations
« Reply #12 on: November 01, 2018, 06:10:13 PM »
People get misled because higher compression engines generally produce more power but require higher octane fuel to prevent misfires (knocking).  So they think that higher octane = more power.  Not true, of course, but the myth prevails.
Gary
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Gary Brinck
Summers: Black Mountain, NC
Home: Ocala National Forest, FL

sightseers

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Re: Octane at higher elevations
« Reply #13 on: November 01, 2018, 07:04:10 PM »
People get misled because higher compression engines generally produce more power but require higher octane fuel to prevent misfires (knocking).  So they think that higher octane = more power.  Not true, of course, but the myth prevails.

I don't believe Higher octane will ever get any more horsepower than the engine was designed to produce.... (as tested under optimum conditions  (i.e.), octane, air density, and temps.)

But I do believe lower octane fuel will lower it's performance,  if not immediately by the ECU (when the anti-knox device tells the computer to lower the ignition advance),  but defiantly over time because lower octane fuel does not usually have all the detergents.
« Last Edit: November 01, 2018, 07:10:06 PM by sightseers »

Larry N.

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Re: Octane at higher elevations
« Reply #14 on: November 02, 2018, 09:48:28 AM »
I've put 90,000 miles on my 8.1 mostly in the mountains of the west and have never used anything but 87 octane.

Runs fine.
Interesting. Did you buy the mid-level (or premium) gas then when refueling at high altitude stations (4,000' and above) in order to still get 87 octane? Everywhere I've been in Colorado and Wyoming (and New Mexico) regular is 85 octane, so I'm curious how you maintained that 87. And, as Lou mentions, lower octane works fine at higher altitudes to give the same anti-knock properties in a normally aspirated engine.
Larry and Mary Ann N.
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Larry N.

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Re: Octane at higher elevations
« Reply #15 on: November 02, 2018, 09:53:00 AM »
Quote
The lower octane fuel you put in the more knock (pre-ignition) occurs.   But... you will never hear a knock because to prevent this highly destructive pre-ignition explosion, your engine control unit (ECU) will reduce the timing advance.

In the days before computer controlled engines, when I came to the high country (5K or more) I'd have the timing readjusted (advanced, I think, but it's been a long time) to regain some of the sea level performance (no where near all, but it ran better). Then at lower altitudes I needed to change the timing back. Computers deal with all that, and more, these days.
Larry and Mary Ann N.
2016 Newmar Ventana 3709 -ISB6.7 XT 360HP
2015 Wrangler Sahara Unlimited toad
Formerly: Trailmanor 2720SL, Bounder, Beaver
  de N8GGG

lynnmor

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Re: Octane at higher elevations
« Reply #16 on: November 02, 2018, 10:24:48 AM »
If an engine has a turbocharger, there is little change in pressures at higher elevations, so the same octane should be used at all times in those engines.  The idea that the ECU can adjust for low octane is not entirely true, because detonation from pressure has nothing to do with ignition timing.  The adjustment in timing will decrease power and mileage and is done in an effort to protect the engine from poor choices of fuel.  If all else fails read the manual, there is a lot of incorrect information in this thread.

kp1xx1

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Re: Octane at higher elevations
« Reply #17 on: November 02, 2018, 11:23:27 AM »
These types of threads get wild and I am reluctant to chime in because there are always many, many experts on these topics.  With that said, take my simple statements at your own risk.  I will state that my profession is the design of engines that many of you probably own.

Knock and detonation are a result of cylinder temperature.  Temperature is defined by a number of things including ambient air temperature, geometric compression ratio, engine load, etc.  Temperature in a IC engine changes as the volume of the cylinder changes and as thermodynamic processes (combustion) take place.  Octane (although not really a great indicator) is a rating of a fuel's ability to resist combustion.   There is a direct relationship between the Octane number and the energy required to start combustion.

Its important to understand there is a difference between knock and pre-detonation.  An ECU can pick-up pinging (knock) and make adjustments to attempt to deal with that.  The quickest method is to pull out timing.  On most modern engines it works pretty well.  If an engine experiences true pre-detonation, which is uncontrolled combustion as the result of high pressure waves that snowball on top of one another, there is nothing to be done, it runs away in fractions of a second and will destroy your engine.  No amount of timing adjustment will fix that.

Something one may find interesting is that many of today's turbocharged gasoline engines run on the knock line constantly.  They only make rated power on premium fuel and under WOT conditions are constantly mitigating knock.

All of that said, the general theme in this thread is correct; that at higher altitudes, cylinder temperatures (and hence pressure) will be lower and the required octane to prevent engine damage is reduced.  It is also true premium fuel is called premium not really due to its octane rating, but because it is of much higher quality.  As some mentioned in here, buy the best fuel you can afford.  It doesn't hurt!  I've ran Tecumseh snow blower engines on race gas left over from the summer.  No difference in operation other than the exhaust had an awesome aroma!

Sprucegum

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Re: Octane at higher elevations
« Reply #18 on: November 02, 2018, 11:51:51 AM »
My 2017 ford operator’s manual says I can use regular fuel and recommends I buy a tank of pr emium occasionally. How often is that? Are they making this recommendation to clean out any building up?
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jackiemac

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Re: Octane at higher elevations
« Reply #19 on: November 02, 2018, 03:54:14 PM »
Can't comment on the motorhome scenario but we had problems starting our truck after refuelling and we looked at the user manual, excerpt below regarding gas. which we were surprised to even be in the manual  ...  problem turned out to be a purge solenoid.


This engines is designed to meet all emissions regulations and provide satisfactory fuel economy and performance when using high quality unleaded gasoline having a posted octane number range of 87 to 89 as  #specified by the (R+M)/2 method. The use of 89 octane “Plus” gasoline is recommended for optimum performance and fuel economy. While operating on gasoline with an octane number of
87, hearing a light knocking sound from the engine is not a cause for concern. However, if the engine is heard making a heavy knocking sound, see your dealer immediately.

Use of gasoline with an octane number lower than 87 can cause engine failure and may void or not be
covered by the New Vehicle Limited Warranty.

Poor quality gasoline can cause problems such as hard starting, stalling, and hesitations. If you experience these symptoms, try another brand of gasoline before considering service for the vehicle.


I suggest checking your manual as it might give guidance in there..
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sightseers

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Re: Octane at higher elevations
« Reply #20 on: November 02, 2018, 03:59:27 PM »
These types of threads get wild and I am reluctant to chime in because there are always many, many experts on these topics.  With that said, take my simple statements at your own risk.  I will state that my profession is the design of engines that many of you probably own.

Knock and detonation are a result of cylinder temperature.  Temperature is defined by a number of things including ambient air temperature, geometric compression ratio, engine load, etc.  Temperature in a IC engine changes as the volume of the cylinder changes and as thermodynamic processes (combustion) take place.  Octane (although not really a great indicator) is a rating of a fuel's ability to resist combustion.   There is a direct relationship between the Octane number and the energy required to start combustion.

Its important to understand there is a difference between knock and pre-detonation.  An ECU can pick-up pinging (knock) and make adjustments to attempt to deal with that.  The quickest method is to pull out timing.  On most modern engines it works pretty well.  If an engine experiences true pre-detonation, which is uncontrolled combustion as the result of high pressure waves that snowball on top of one another, there is nothing to be done, it runs away in fractions of a second and will destroy your engine.  No amount of timing adjustment will fix that.

Something one may find interesting is that many of today's turbocharged gasoline engines run on the knock line constantly.  They only make rated power on premium fuel and under WOT conditions are constantly mitigating knock.

All of that said, the general theme in this thread is correct; that at higher altitudes, cylinder temperatures (and hence pressure) will be lower and the required octane to prevent engine damage is reduced.  It is also true premium fuel is called premium not really due to its octane rating, but because it is of much higher quality.  As some mentioned in here, buy the best fuel you can afford.  It doesn't hurt!  I've ran Tecumseh snow blower engines on race gas left over from the summer.  No difference in operation other than the exhaust had an awesome aroma!

 :))

lynnmor

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Re: Octane at higher elevations
« Reply #21 on: November 02, 2018, 05:11:22 PM »
My 2017 ford operator’s manual says I can use regular fuel and recommends I buy a tank of pr emium occasionally. How often is that? Are they making this recommendation to clean out any building up?

I believe your manual says this:

Your vehicle is designed to operate on
regular unleaded gasoline with a minimum
pump (R+M)/2 octane rating of 87.
Some fuel stations, particularly those in
high altitude areas, offer fuels posted as
regular unleaded gasoline with an octane
rating below 87. We do not recommend
these fuels.
For best overall vehicle and engine
performance, premium fuel with an octane
rating of 91 or higher is recommended. The
performance gained by using premium fuel
is most noticeable in hot weather as well
as other conditions, for example when
towing a trailer.  See
Towing (page 257).


John From Detroit

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Re: Octane at higher elevations
« Reply #22 on: November 02, 2018, 05:40:34 PM »
The Theory they tell me is the "Thinner" air at Altitude does nto need the same level of Octane

My Engine said "OH YES I DO".
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Arch Hoagland

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Re: Octane at higher elevations
« Reply #23 on: November 02, 2018, 06:43:08 PM »
Interesting. Did you buy the mid-level (or premium) gas then when refueling at high altitude stations (4,000' and above) in order to still get 87 octane? Everywhere I've been in Colorado and Wyoming (and New Mexico) regular is 85 octane, so I'm curious how you maintained that 87. And, as Lou mentions, lower octane works fine at higher altitudes to give the same anti-knock properties in a normally aspirated engine.

In the midwest and places where I run into 85 octane I run 85 octane.  Whatever is the lowest.
2004 Monaco La Palma 36 DBD
W22, 8.1 gas,  Allison 1000 Transmission
7.1 MPG over 90,000 miles

2000 Lexus RX300, 4020lb
U.S. Gear Braking System

sightseers

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Re: Octane at higher elevations
« Reply #24 on: November 02, 2018, 06:55:38 PM »
I believe your manual says this:

Your vehicle is designed to operate on
regular unleaded gasoline with a minimum
pump (R+M)/2 octane rating of 87.
Some fuel stations, particularly those in
high altitude areas, offer fuels posted as
regular unleaded gasoline with an octane
rating below 87. We do not recommend
these fuels.

For best overall vehicle and engine
performance, premium fuel with an octane
rating of 91 or higher is recommended. The
performance gained by using premium fuel
is most noticeable in hot weather as well
as other conditions, for example when
towing a trailer.
  See
Towing (page 257).

and,  I said the same thing as the Ford manual ...  ;)

Premium fuel does make a difference.
« Last Edit: November 02, 2018, 06:59:31 PM by sightseers »

Arch Hoagland

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Re: Octane at higher elevations
« Reply #25 on: November 02, 2018, 07:34:42 PM »
Question.....How do they make different levels of octane gas?

I know a truck driver who transports gasoline and he says it all comes out of the same tank and they add things to it when they fill his tanks..

So is more of something added to the basic gasoline to raise the octane level?
2004 Monaco La Palma 36 DBD
W22, 8.1 gas,  Allison 1000 Transmission
7.1 MPG over 90,000 miles

2000 Lexus RX300, 4020lb
U.S. Gear Braking System

Larry N.

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Re: Octane at higher elevations
« Reply #26 on: November 02, 2018, 08:38:08 PM »
If an engine has a turbocharger, there is little change in pressures at higher elevations, so the same octane should be used at all times in those engines.  The idea that the ECU can adjust for low octane is not entirely true, because detonation from pressure has nothing to do with ignition timing.  The adjustment in timing will decrease power and mileage and is done in an effort to protect the engine from poor choices of fuel.  If all else fails read the manual, there is a lot of incorrect information in this thread.
Which is why I said, "lower octane works fine at higher altitudes to give the same anti-knock properties in a normally aspirated engine." (emphasis added)

Larry and Mary Ann N.
2016 Newmar Ventana 3709 -ISB6.7 XT 360HP
2015 Wrangler Sahara Unlimited toad
Formerly: Trailmanor 2720SL, Bounder, Beaver
  de N8GGG

Isaac-1

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Re: Octane at higher elevations
« Reply #27 on: November 02, 2018, 09:28:55 PM »
Interesting. Did you buy the mid-level (or premium) gas then when refueling at high altitude stations (4,000' and above) in order to still get 87 octane? Everywhere I've been in Colorado and Wyoming (and New Mexico) regular is 85 octane, so I'm curious how you maintained that 87. And, as Lou mentions, lower octane works fine at higher altitudes to give the same anti-knock properties in a normally aspirated engine.

Larry, yes that is basically what I did after discovering the issue I added some premium fuel to bring up the average octane of the remaining fuel in the tank, then until heading back home to sea level (sea level-ish it is 240 ft above sea level at my house) opted for the mid grade if the base grade was 85 octane, or used the base grade if it were 86 octane.

Ike

p.s. that should be retard timing for altitude, as advancing timing is used to get more horse power, but requires higher octane.
« Last Edit: November 02, 2018, 09:31:00 PM by Isaac-1 »
2002 Safari Trek 2830

Isaac-1

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Re: Octane at higher elevations
« Reply #28 on: November 02, 2018, 11:41:22 PM »
Question.....How do they make different levels of octane gas?

I know a truck driver who transports gasoline and he says it all comes out of the same tank and they add things to it when they fill his tanks..

So is more of something added to the basic gasoline to raise the octane level?

The octane level of the gasoline is established in the blending at the refinery, they blend for a number of different traits, often re-blending if a batch is off on one thing or another.  I live in SW Louisiana where there are many refineries, and have an old friend that worked in the lab at one of them back in the 90's, he tells stories of hating the then newer low emissions gasoline blends like RFG because the blend tolerances were so much tighter than the old stuff, so batches would often have to be reblended multiple times to meet specs.   What your driver friend was likely talking about was the detergent packs for each name brand of gasoline, which get added in when the trucks are loaded at the depot.

So in other words you may have an area where all the gasoline comes from the nearby Shell fuel depot, regardless if it is going to an Exxon station, or a Citgo Station.  It just gets Exxons proprietary detergent blend pack added in when it gets loaded into the truck to be hauled to the Exxon station.
2002 Safari Trek 2830

Arch Hoagland

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Re: Octane at higher elevations
« Reply #29 on: November 03, 2018, 12:16:13 AM »
The octane level of the gasoline is established in the blending at the refinery, they blend for a number of different traits, often re-blending if a batch is off on one thing or another.  I live in SW Louisiana where there are many refineries, and have an old friend that worked in the lab at one of them back in the 90's, he tells stories of hating the then newer low emissions gasoline blends like RFG because the blend tolerances were so much tighter than the old stuff, so batches would often have to be reblended multiple times to meet specs.   What your driver friend was likely talking about was the detergent packs for each name brand of gasoline, which get added in when the trucks are loaded at the depot.

So in other words you may have an area where all the gasoline comes from the nearby Shell fuel depot, regardless if it is going to an Exxon station, or a Citgo Station.  It just gets Exxons proprietary detergent blend pack added in when it gets loaded into the truck to be hauled to the Exxon station.

I'm trying to understand the difference in 87, 89 and 91 octane and how they achieve it.

So are you saying they make a batch  87 octane and place it in a ginormous tank labeled 87 octane and then make another batch and put it in another ginormous tank label 89 octane and so on? 

And so then the tanker truck would fill up a specified octane at a specific  tank, add whatever miracle cleaners that the receiving  gas station wanted  and deliver that load.

Is that correct?
2004 Monaco La Palma 36 DBD
W22, 8.1 gas,  Allison 1000 Transmission
7.1 MPG over 90,000 miles

2000 Lexus RX300, 4020lb
U.S. Gear Braking System