Octane at higher elevations

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John Stephens

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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?
 

Back2PA

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John Stephens said:
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
 

youracman

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Back2PA said:
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.
 
S

sightseers

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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.
 

Gary RV_Wizard

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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+.
 

Isaac-1

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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
 

SpencerPJ

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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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spencerpj said:
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."

:))
 
S

sightseers

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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/
 

Lou Schneider

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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.
 

kdbgoat

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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.
 

Gary RV_Wizard

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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.
 
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sightseers

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Gary RV_Wizard said:
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.
 

Larry N.

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Arch Hoagland said:
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 N.

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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.
 

lynnmor

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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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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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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?
 

jackiemac

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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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