87 or 91+ octane?

[COJeeper]

Where I live at altitude, octanes available are 85, 87 and 91, so if Jeep says 87 is ok at lower elevations where octanes available are 87, 91, and 93, am I correct to assume 85 would work fine at higher altitudes?

I've been dumping 85 from Costco in my 2.0 for a year now, it hasn't exploded so that's good. I just went to 37s so for the extra performance, I think I'll start running Costco's 91. Gas prices are going back up too.

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[COJeeper]

Thanks for all your explanations.

Does that mean that in my 3.6 sometimes getting better octane fuel it will be better? even for "cleaning" purposes? I can get here even 98 octane fuel.


Thanks a lot

If you do a little research you'll find that there's no such thing as "better octane". A lot of people call fuel; low, mid, and premium grades. The quality of the fuel is not better or worse based on octane. Simply put (because I only understand it at a 3rd grade level), octane is a measure of resistance to igniting. A lower octane fuel has lower resistance to igniting, which means that it will ignite easier. A higher octane fuel has a higher resistance to igniting, which means it won't explode as easy.

Why does all this matter?

Well, think about it like this; what happens if you take a bunch of air and pressurize it and move it quickly? Have you ever aired down and then at the end of your trip, aired back up? Did you touch the air hose? It gets hot, right? Now think about that air mixed with fuel all under high pressure. The higher the pressure, the hotter it gets (plus all the residual heat from the engine). If it gets pressurized and hot enough, the fuel will ignite. Generally, in a high compression motor, they will recommend a higher octane fuel. This is because the fuel can get pressurized/hot enough to combust without the spark from the spark plug. This can happen too early in the piston cycle and can cause knocking which is bad. Knocking is bad because the motor is timed for a very specific ignite time where the explosion will push the piston back down. If the explosion happens too early, the piston is on the upstroke and just compressing the exploded gasses even more and fighting the explosion. The explosion wants to push the piston down but the crank shaft is still pushing it up. Who wins in this scenario, a semi or a train going head on? The train obviously because it has more force/energy and is built stronger but that train will still get some damage on the front of it. That's why knocking can damage an engine.

I've never heard what engine knock sounds like, maybe I have and just haven't noticed it though?

So, essentially, you can use the lowest octane fuel that doesn't cause your engine to knock, that's what you need to look out for. If you have engine knock, increase the octane. In the 3.6, it's not a high compression motor like the 2.0 so you should have no problems with lower octane. I would think at 98 octane, maybe that's too high and the fuel might not always ignite? That's a little above me but I would probably NOT recommend 98, that sounds like a racing fuel.

Anyway, I hope this MTM (as my friends like to call Mike's Teaching Moments) has been helpful... and accurate! Someone correct me if I'm off my rocker.

 

[jeepoch]

Thanks for all your explanations.

Does that mean that in my 3.6 sometimes getting better octane fuel it will be better? even for "cleaning" purposes? I can get here even 98 octane fuel.


Thanks a lot

Paul,

Injector cleaning isn't as much a function of octane but rather 'additives' that are also added to the fuel. Typically however, higher quality fuels (with detergent additives) are also the ones with the higher octane. Lower cost fuels are cheaper for a reason.

While running higher octane in your vehicle is generally better for higher performance and an overall lower probability of knock, cleaner injector operation comes from other additives blended in the fuel. For most fuels, even the cheaper ones, carbon build up only happens over time. So running a detergent based fuel, usually only offered with the higher octane fuels is never a bad idea, even if just periodically.

However, I'm not a fan of adding any type of additive directly in to your tank, either from a bottle, can or an 'on-pump' mechanism. Mixing in the exact right amount of solvents should be left to the refiners. Quality standards for fuel mixtures is a pretty important factor for optimal engine operation.

Also, as @COJeeper (Mike) points out, octane itself adds zero additional energy to the fuel. A cup of low octane petroleum has the exact same amount of chemical energy as a cup of high octane. The exact same amount! The difference is in it's 'flashpoint'. Mike called this "ignitability". Lower octane can ignite over a wider ranger of pressure and temperature thresholds. As pressure increases, so does temperature. This is due to a physical property described by the Universal Gas Law PV=nRT. Where P is pressure, V is volume, n is the number of molecules, R is a constant and T is temperature.

So in a cylinder as the piston reaches it's highest position (called Top-Dead-Center or TDC) volume V decreases to it lowest point but still a significant value. Now as the number of molecules in the fuel/air mix gets squeezed into a smaller and smaller space, pressure P goes up towards a maximum as designed by the cylinder's compression ratio. The higher the compression, the higher the pressure.

With both n and R now being fixed quantities, n the amount of fuel/air molecules injected at intake and R (Avogadro's number / Boltzmann's constant k), temperature rises proportionally with pressure.

Octane of the fuel will now determine the point where it will combust in this pressure and temperature environment within the cylinder. Generally, the fuel/air mixture will not combust until a spark is generated. At nominal operating temperatures this is true. However, at higher engine temperatures this may not always be the case. The fuel/air mix may inadvertantly ignite on it's own without a spark.

This is usually called 'pre-detonation'. And when it happens (before the piston reaches TDC) the explosive energy works against crankshaft rotation which generates violent mechanical forces that can be very detrimental to the cylinder. This is referred to as "knock". Piston Rings, can be damaged from the explosive blowback, causing abrasive scarring of the cylinder walls leading to loss of compression. This then of course leads to loss of power and overall engine failure.

Higher octane fuels promote a more precise 'flashpoint' where the likelihood of pre-detonation is greatly minimized. With higher octane, a tighter (smaller) margin of temperature and pressure thresholds need to be met before the fuel will actually ignite. Generally ensuring that combustion will only occur when a spark event is introduced.

Regardless, whenever the combustion occurs, whether exactly at the point the engine controller dictates (always after TDC) or inadvertantly during pre-detonation (before TDC), the amount of energy produced will be identically the same. So it doesn't take a rocket scientist to understand that you always (always) want each and every combustion event to occur AFTER Top-Dead-Center. Higher octane certainly promotes this. Lower octane does not.

So only you can determine what amount of paranoia needs to be considered. If you drive your rig like granny driving to church on Sundays, then the lower octane stuff is probably right for you. However, if you drive your Jeep like it's a fribben Ferrari, I'd recommend a higher octane offering.

As I've said on this forum (more than once) any knock event, yes even one, is detrimental to the longevity of your engine. You can take that advice or ignore it.

The manufacture recommends the lower octane just as a marketing ploy, promoting a lower cost of ownership.

Fill up with whatever makes you happy.

Jay

 

[COJeeper]

Paul,

Injector cleaning isn't as much a function of octane but rather 'additives' that are also added to the fuel. Typically however, higher quality fuels (with detergent additives) are also the ones with the higher octane. Lower cost fuels are cheaper for a reason.

While running higher octane in your vehicle is generally better for higher performance and an overall lower probability of knock, cleaner injector operation comes from other additives blended in the fuel. For most fuels, even the cheaper ones, carbon build up only happens over time. So running a detergent based fuel, usually only offered with the higher octane fuels is never a bad idea, even if just periodically.

However, I'm not a fan of adding any type of additive directly in to your tank, either from a bottle, can or an 'on-pump' mechanism. Mixing in the exact right amount of solvents should be left to the refiners. Quality standards for fuel mixtures is a pretty important factor for optimal engine operation.

Also, as @COJeeper (Mike) points out, octane itself adds zero additional energy to the fuel. A cup of low octane petroleum has the exact same amount of chemical energy as a cup of high octane. The exact same amount! The difference is in it's 'flashpoint'. Mike called this "ignitability". Lower octane can ignite over a wider ranger of pressure and temperature thresholds. As pressure increases, so does temperature. This is due to a physical property described by the Universal Gas Law PV=nRT. Where P is pressure, V is volume, n is the number of molecules, R is a constant and T is temperature.

So in a cylinder as the piston reaches it's highest position (called Top-Dead-Center or TDC) volume V decreases to it lowest point but still a significant value. Now as the number of molecules in the fuel/air mix gets squeezed into a smaller and smaller space, pressure P goes up towards a maximum as designed by the cylinder's compression ratio. The higher the compression, the higher the pressure.

With both n and R now being fixed quantities, n the amount of fuel/air molecules injected at intake and R (Avogadro's number / Boltzmann's constant k), temperature rises proportionally with pressure.

Octane of the fuel will now determine the point where it will combust in this pressure and temperature environment within the cylinder. Generally, the fuel/air mixture will not combust until a spark is generated. At nominal operating temperatures this is true. However, at higher engine temperatures this may not always be the case. The fuel/air mix may inadvertantly ignite on it's own without a spark.

This is usually called 'pre-detonation'. And when it happens (before the piston reaches TDC) the explosive energy works against crankshaft rotation which generates violent mechanical forces that can be very detrimental to the cylinder. This is referred to as "knock". Piston Rings, can be damaged from the explosive blowback, causing abrasive scarring of the cylinder walls leading to loss of compression. This then of course leads to loss of power and overall engine failure.

Higher octane fuels promote a more precise 'flashpoint' where the likelihood of pre-detonation is greatly minimized. With higher octane, a tighter (smaller) margin of temperature and pressure thresholds need to be met before the fuel will actually ignite. Generally ensuring that combustion will only occur when a spark event is introduced.

Regardless, whenever the combustion occurs, whether exactly at the point the engine controller dictates (always after TDC) or inadvertantly during pre-detonation (before TDC), the amount of energy produced will be identically the same. So it doesn't take a rocket scientist to understand that you always (always) want each and every combustion event to occur AFTER Top-Dead-Center. Higher octane certainly promotes this. Lower octane does not.

So only you can determine what amount of paranoia needs to be considered. If you drive your rig like granny driving to church on Sundays, then the lower octane stuff is probably right for you. However, if you drive your Jeep like it's a fribben Ferrari, I'd recommend a higher octane offering.

As I've said on this forum (more than once) any knock event, yes even one, is detrimental to the longevity of your engine. You can take that advice or ignore it.

The manufacture recommends the lower octane just as a marketing ploy, promoting a lower cost of ownership.

Fill up with whatever makes you happy.

Jay

Thanks for making me look like a moron!

But in all seriousness, thank you for this more technical post! I love learning nerdy things like this!! Great explanation and I'm bookmarking this post.

 

[jeepoch]

Thanks for making me look like a moron!

But in all seriousness, thank you for this more technical post! I love learning nerdy things like this!! Great explanation and I'm bookmarking this post.

Mike,

No you are not a moron, far from it. Maybe without stating the underlying physics your understanding and description was spot on. Originally I was only going to respond to Paul about detergent additives and not octane, is what helps clean injectors.

But your post provided more information to how octane contributes to the overall story. I just picked up and added just a little more detail.

Octane is something that just seems so confusing to people. Most everyone assumes higher octane means higher output energy. While it certainly may, it's only because of the advantages a modern engine controller can use to control spark at it's most optimum piston position.

With lower octane fuels, detonation can occur at piston positions not exactly specified by the optimal spark equation. So higher octane basically allows the engine controller to not only deliver the best spark timing for this combustion event but also better predict the optimal position for the next combustion event. The better these combustion events are delivered, the absolute best performance with lowest emissions are the result.

So it's not just the prevention of knock but also the absolute best engine operation is what higher octane can provide.

It amazes me how many people will find every reason under the sun to fill their precious Jeeps with the lowest octane offerings available.

Granted, dinosaur goo isn't cheap. But reduced engine performance, especially due to knock (even if imperceptible to the driver) is permanently damaging your engine a little bit more, at each and every pre-detonation event.

Why would you want to ever promote that? While the engine controller is great at detecting even imperceptible knock, it can only 'react' after it's happened. In other words, only after some damage has already been done.

Why not do what you can to minimize this to the highest degree possible? While no knock may occur by running a lower octane offering in most operational circumstances, the lower the octane the higher the probability knock may occur.

I choose to do what I can to get the best performance and longevity out of my engine. For a few dollars more per fill up, it seems (to me) to be a rather no-brainer.

Jeeping on in CO... Hope to run by you sometime. You can count on a wave from me.

Jay

 

[COJeeper]

Granted, dinosaur goo isn't cheap.

I was joking, I just really liked the technical explanation and seeing the mathematical formula. Nerdy stuff like that excites me. :D

I especially like the official scientific term for oil. :D

/Jeep Wave!

 

[Tredsdert]

The owners manual on my 20 states the engine is designed to run on 87 octane., with higher octane recommended for maximum performance.

Though mine is an EcoDiesel, my wife Subaru Forester takes 87 octane, but I like to throw in the 91 plus octane fuels every two to three fill ups because I find the higher octane fuels tend to help burn off a lot of the gunk in the engine.

There's a buc-ee's one city over from us, and I go there once a month and load her car up with the ethanol-free blue pump, it comes out to like 93 octane I believe, but with no corn in it. Amazing for the engine, sucks for the wallet, but it's still cheaper on average than diesel fuel around here. Her car doesn't seem to respond too much differently to it, but last time we took it to the dealership for a warranty repair on the engine, they said that the engine looks meticulously maintained. And even asked me what I have been putting in it. I thought they were being smart, but I've also heard that the higher octane helps burn off a lot of the gunk that the additives clean off, just add a more efficient level.

Better burn; cleaner engine, less corn; cleaner engine, cleaner engine; longer engine life, cleaner engine; less serious maintenance, less serious maintenance; lower maintenance cost.

You could dump any number of additives in there and probably get similar results, but a lot of the additives I've read on a older engineering forum years ago, basically just add a bunch of lubricants which the gasoline already has in it, and increases the octane so it gets a better burn. But you're adding like 16 oz to 18 to 23 gallons of gas depending what make and model you have. It's not going to make a huge difference, not quite like adding 18 to 23 gallons higher octane fuel.

If you want it save on cost, at minimum I would do every third or fourth tank with higher octane fuel. Just my personal preference, I'm no chemical engineer.

EDIT: I should probably note the Subaru Forester she has is a 2.5L non-turbo engine

 

[swedeviking]

I've been dumping 85 from Costco in my 2.0 for a year now, it hasn't exploded so that's good. I just went to 37s so for the extra performance, I think I'll start running Costco's 91. Gas prices are going back up too.

Been using 85 for past 3 years.. I get 27 MPG city and country No problem no loss of power and easier on the pocket book too now that we are creeping toward $4.00 + per gallon.. Knew it wouldn't last....

 

[swedeviking]

Where I live at altitude, octanes available are 85, 87 and 91, so if Jeep says 87 is ok at lower elevations where octanes available are 87, 91, and 93, am I correct to assume 85 would work fine at higher altitudes?

I live at 9500" elev and run 85 just fine

 

[Paulcardona]

Wow!!! You guys are amazing @jeepoch, @COJeeper and @Tredsdert, thanks a lot!!
Like we say in my country:

If you want it save on cost, at minimum I would do every third or fourth tank with higher octane fuel. Just my personal preference, I'm no chemical engineer.

EDIT: I should probably note the Subaru Forester she has is a 2.5L non-turbo engine

I´ll probably do this too, what do you guys think?

 

[jmr]

You can run 87 octane but the computer will give less boost and ignition timing so the performance will drop. Run premium to get the max boost and timing thereby improving performance as the manual states. Your mileage may improve as well.

 

[TopDown]

I'm going to run 87 octane.

FCA indicates the engine will be fine with 87 octane.

The difference in gas cost is tangible, not theoretical.

I didn't buy a Wrangler for engine performance.

 

[Billkowski]

I do use 93 octane in my 3.6, it's not much of a cost issue since I only put about 4K miles on it a year, but I do wonder how old the high test fuel is in the tanks at the station. As you can tell on this forum, not a lot of people buy it.

 

[jeepoch]

I'm going to run 87 octane.

FCA indicates the engine will be fine with 87 octane.

The difference in gas cost is tangible, not theoretical.

I didn't buy a Wrangler for engine performance.

Probably not emissions concerns as well.

No complaints. Really. Feed your Jeep grape juice if it helps you sleep well at night.

But also please (not you specifically) don't bemoan an ICE powerplant because everything is a compromise. With lower octane, this also results in potentially higher emissions. Anything (theoretically) decreasing the probability of the best combustion possible, results in more unwanted side effects including increased emission by-products.

Or go buy an EV with all the unintended environmental consequences that it generates, which everybody seems to be blissfully ignoring.

Mobilization of society comes with a cost. There are no great answers, only compromises. I choose to run higher octane to benefit not just my Jeep but also because I know I'm doing my part in helping everyone else. The few dollars more per fill up is inconsequential. I'm doing what I can control to save gas, improve performance, lower emissions, lower maintenance costs, increase engine longevity and even in the most minimal terms, help society in general. That to me matters.

Again, everyone grapples with the reasons they chose what they do. That's the really cool thing of having options.

Jay

 

[TopDown]

Probably not emissions concerns as well.

Let's be serious here. If a person has significant environmental concerns about the vehicle they drive, that person wouldn't be driving a Wrangler.

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