87 or 91+ octane?

[Initial-Jeep]

#non392folkproblems

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[Zandcwhite]

#diesel/392nooptionsforcheapfuel

 

[SQTowelie]

#EnergyisEnergyforanEV
still got the 2.0L though as well

 

[RoadiJeff]

@RoadiJeff,

It appears that you haven't read this thread. If you have you would have run across the phenomenon called 'knock'. Each and every knock event lowers performance and damages your rings and cylinder walls to some extent. Over time this will result in a gradual loss of compression.

Higher octane fuels significantly lower the probability of producing knock. Enjoy the explanation from post #53 (and others) from within this very thread.

https://www.jlwranglerforums.com/forum/threads/87-or-91-octane.89931/page-4#post-1898618

Cheers.
Jay

Yes, I read the entire thread. Did you?

If My Car Recommends Regular Gas, Is It Good to Use Premium Occasionally?

If you burn premium because you think it makes the engine peppier, that is probably psychological: “I’m paying more for gasoline, so I must be getting more.” Some motorists claim they get better fuel economy with premium, but some of that could be due to favorable weather conditions (such as warm weather instead of cold) or other factors.

If you use premium fuel because your engine knocks on regular, you are treating the symptom, not the cause. Something else might be causing the knock, such as carbon deposits or hot spots that should be diagnosed and treated by a mechanic.

 

[Trini]

Usually I cruise at 70-75. This week I tried 65-70 and the difference was about 20 miles (almost a gallon of gas). I did take off the front doors for two of the days, so that may have affected it, but I didn't think it would be that much. I've also only used 91 premium in my engine since I got it.

 

[Initial-Jeep]

If My Car Recommends Regular Gas, Is It Good to Use Premium Occasionally?

The owner's manual to the engine in question here recommends premium gas.

 

[dougywarren]

I usually run 87 in all my vehicles, but when I run premium from a place like Shell, in every vehicle I can tell the engine runs slightly smoother and has a slight bit more power. Not worth it to me for the slight difference, but when I do it I enjoy it. I remember reading it’s good to run premium every now and then in every vehicle to clean some deposits that build up. Not sure if that’s relevant or not anymore.

I just got a 4Xe and so far been putting premium in it during the break in, and it runs and sounds pretty smooth. I’ll probably switch to the 87 when I go on my road trips this summer

 

[Speed331]

The owner's manual to the engine in question here recommends premium gas.

You might want to read it again....

 

[jeepoch]

Yes, I read the entire thread. Did you?

If My Car Recommends Regular Gas, Is It Good to Use Premium Occasionally?

If you burn premium because you think it makes the engine peppier, that is probably psychological: “I’m paying more for gasoline, so I must be getting more.” Some motorists claim they get better fuel economy with premium, but some of that could be due to favorable weather conditions (such as warm weather instead of cold) or other factors.

If you use premium fuel because your engine knocks on regular, you are treating the symptom, not the cause. Something else might be causing the knock, such as carbon deposits or hot spots that should be diagnosed and treated by a mechanic.

Wow, sometimes it's just not possible to get through to some people.

Please by all means run whatever fuel you care to. It's your engine treat it however you damn well want.

I spent a good part of my career as a Powertrain Software Engineer with Chrysler. For over a decade and only moved on to another great software development opportunity near Denver CO. Do I miss Michigan? Not in the slightest. Do I miss developing and calibrating Engine Controllers? Probably by far the most interesting and rewarding product development program I've ever worked on.

I've implemented many fuel delivery, spark timing, EGR, Auto Idle and a boatload of federally mandated OBDII emissions monitors. One of the primary concerns that is always evaluated is the quality of the fuel used. There used to be at one time a long thin stainless steel tube and other sensors to help determine gasoline viscosity, octane and ethanol content in order to select the best 'tuning' parameters for any sensed fuel condition.

However, that was both expensive and sometimes unreliable. It's remarkable the differences in fuel quality from different grades, seasonal additives, detergents, ethanol content, and yes, even octane from all the different suppliers, distributors and retailers. If you think fuel is the same everywhere, you're pretty foolishly naive.

Yet, your engine has to adapt and be able to cope with any of it...

In more modern engine designs, those fuel quality sensors have been thrifted away with more sophisticated algorithms which monitor engine metrics, such as power, temperature, various air pressures, knock but mostly emissions by-products via both the upstream and downstream O2 sensors.

While many particulates like Carbon Monoxide (CO), Nitrous Oxide (NO2) and others aren't directly detected, the overall amount of O2 or lack thereof, can infer quite a lot of information of the quality of the combustion events.

In these more sophisticated schemes, a reference standard is measured from all these various engine metrics in order to produce a specified performance goal. But this is based upon using a very high quality fuel (that you typically can't buy on the consumer market) as a baseline. This is the top of the line quality fuel that all others are compared to.

As these performance metrics degrade with the introduction of lower quality fuels, the engine controller tries to continue to achieve the very lowest amount of emissions typically sacrificing even more performance. It does this by 'de-tuning' things such as fuel-to-air ratio, spark timing, idle speed setpoints, exhaust gas recirculation among others. Recall, the EPA rules.

Typically, the lower the fuel quality the lower the performance in order to achieve a federally regulated emissions output goal. This should be obvious.

While octane itself, is a small factor to the overall quality of the gasoline purchased, the lower the octane, the less reliable the ignition flashpoint becomes. The spark timing equations become less 'predictable'. A natural by-product of this is pre-detonation which causes knock. Any knock, of any amount is DETRIMENTAL to the well being of your piston rings and cylinder wall surfaces. Knock creates violent vibrations that cause excessive wear that will lower compression over time.

Higher octane fuels dramatically lowers the potential of knock because it's flashpoint is much more precise for a given temperature and pressure threshold within the spark chamber. The engine controller can therefore better predict what the optimal ignition point is going to be and adjust the spark timing to it's most optimal position of the piston after Top Dead Center (not before). Hence a much lower probability of pre-detonation (knock).

So while lower octane by itself doesn't necessarily lower the overall fuel's quality, it doesn't help. The only factor you as a consumer can control at the pump is octane. However, the lower the octane, typically also the lower the grade (overall quality) of the gasoline offered. The higher priced grades of fuels are not just due to octane alone, but they're marketed that way as an easy number to indicate at the pump, besides just price.

Furthermore, all auto manufactures selling 'afforfable' vehicles for the masses specify the use of lower octane fuels as a way to advertise a lower cost of ownership. Nothing more. No science whatsoever to what's printed in the owners manual. Pure propaganda. The science all boils down to each and every combustion event in real time.

So go ahead, continue to put grape juice in your jalopy. Psychologically feel good at pressing the button for the cheapest, lowest quality fuel at the pump. I'll however follow the science and get another 350K+ miles from my current engine.

I'll bet you didn't read this post in it's entirety. Tl;Dr. Too long; Didn't read. Well pal, not only did I read them, I've also wrote a few of them and better yet, I've lived and helped developed lots of the engine controllers on the road.

There's a pretty decent chance that either you or someone you know has driven a vehicle running with some of my Engine Controller software. I'm pretty proud of that, whether or not anyone reads anything on this forum.

Jay

 

[Zandcwhite]

You might want to read it again....

Reading comprehension is hard, for "optimal performance" run 91 octane or higher period. It's in the highlighted section you quoted in plan English. The engine will run fine on 87, but suboptimaally.

 

[Initial-Jeep]

You might want to read it again....

Right. "satisfactory" vs. "optimal" (which isn't applicable to the 3.6)

Would "performance is limited" fall under "satisfactory" or "optimal"?

 

[58Willys]

Wow, sometimes it's just not possible to get through to some people.

Please by all means run whatever fuel you care to. It's your engine treat it however you damn well want.

I spent a good part of my career as a Powertrain Software Engineer with Chrysler. For over a decade and only moved on to another great software development opportunity near Denver CO. Do I miss Michigan? Not in the slightest. Do I miss developing and calibrating Engine Controllers? Probably by far the most interesting and rewarding product development program I've ever worked on.

I've implemented many fuel delivery, spark timing, EGR, Auto Idle and a boatload of federally mandated OBDII emissions monitors. One of the primary concerns that is always evaluated is the quality of the fuel used. There used to be at one time a long thin stainless steel tube and other sensors to help determine gasoline viscosity, octane and ethanol content in order to select the best 'tuning' parameters for any sensed fuel condition.

However, that was both expensive and sometimes unreliable. It's remarkable the differences in fuel quality from different grades, seasonal additives, detergents, ethanol content, and yes, even octane from all the different suppliers, distributors and retailers. If you think fuel is the same everywhere, you're pretty foolishly naive.

Yet, your engine has to adapt and be able to cope with any of it...

In more modern engine designs, those fuel quality sensors have been thrifted away with more sophisticated algorithms which monitor engine metrics, such as power, temperature, various air pressures, knock but mostly emissions by-products via both the upstream and downstream O2 sensors.

While many particulates like Carbon Monoxide (CO), Nitrous Oxide (NO2) and others aren't directly detected, the overall amount of O2 or lack thereof, can infer quite a lot of information of the quality of the combustion events.

In these more sophisticated schemes, a reference standard is measured from all these various engine metrics in order to produce a specified performance goal. But this is based upon using a very high quality fuel (that you typically can't buy on the consumer market) as a baseline. This is the top of the line quality fuel that all others are compared to.

As these performance metrics degrade with the introduction of lower quality fuels, the engine controller tries to continue to achieve the very lowest amount of emissions typically sacrificing even more performance. It does this by 'de-tuning' things such as fuel-to-air ratio, spark timing, idle speed setpoints, exhaust gas recirculation among others. Recall, the EPA rules.

Typically, the lower the fuel quality the lower the performance in order to achieve a federally regulated emissions output goal. This should be obvious.

While octane itself, is a small factor to the overall quality of the gasoline purchased, the lower the octane, the less reliable the ignition flashpoint becomes. The spark timing equations become less 'predictable'. A natural by-product of this is pre-detonation which causes knock. Any knock, of any amount is DETRIMENTAL to the well being of your piston rings and cylinder wall surfaces. Knock creates violent vibrations that cause excessive wear that will lower compression over time.

Higher octane fuels dramatically lowers the potential of knock because it's flashpoint is much more precise for a given temperature and pressure threshold within the spark chamber. The engine controller can therefore better predict what the optimal ignition point is going to be and adjust the spark timing to it's most optimal position of the piston after Top Dead Center (not before). Hence a much lower probability of pre-detonation (knock).

So while lower octane by itself doesn't necessarily lower the overall fuel's quality, it doesn't help. The only factor you as a consumer can control at the pump is octane. However, the lower the octane, typically also the lower the grade (overall quality) of the gasoline offered. The higher priced grades of fuels are not just due to octane alone, but they're marketed that way as an easy number to indicate at the pump, besides just price.

Furthermore, all auto manufactures selling 'afforfable' vehicles for the masses specify the use of lower octane fuels as a way to advertise a lower cost of ownership. Nothing more. No science whatsoever to what's printed in the owners manual. Pure propaganda. The science all boils down to each and every combustion event in real time.

So go ahead, continue to put grape juice in your jalopy. Psychologically feel good at pressing the button for the cheapest, lowest quality fuel at the pump. I'll however follow the science and get another 350K+ miles from my current engine.

I'll bet you didn't read this post in it's entirety. Tl;Dr. Too long; Didn't read. Well pal, not only did I read them, I've also wrote a few of them and better yet, I've lived and helped developed lots of the engine controllers on the road.

There's a pretty decent chance that either you or someone you know has driven a vehicle running with some of my Engine Controller software. I'm pretty proud of that, whether or not anyone reads anything on this forum.

Jay

An excellent post by a very knowledgeable person. Thank you!

 

[stumblinhorse]

@jeepoch great post. Can you explain what impacts the need for octane is impacted by environmental factors, like atmospheric pressure, air density, outside air temperature. dynamic compression ratio is impacted a lot by elevation. How does an ECU compensate for that variable? If an 11:1 compression engine detonates on 87 at sea level will it detonate at 5k or 10k on 87? What will an ecu do to compensate for 20-40% less O2 on a mountain pass? Also if outside air is 0* or 100* does that have an impact? Does lower temp give better combustion ?

appreciate your knowledge here.

 

[Deleted member 59498]

@jeepoch great post. Can you explain what impacts the need for octane is impacted by environmental factors, like atmospheric pressure, air density, outside air temperature. dynamic compression ratio is impacted a lot by elevation. How does an ECU compensate for that variable? If an 11:1 compression engine detonates on 87 at sea level will it detonate at 5k or 10k on 87? What will an ecu do to compensate for 20-40% less O2 on a mountain pass? Also if outside air is 0* or 100* does that have an impact? Does lower temp give better combustion ?

appreciate your knowledge here.

No one cares?

Don't think we even have 91.

 

[jeepoch]

@jeepoch great post. Can you explain what impacts the need for octane is impacted by environmental factors, like atmospheric pressure, air density, outside air temperature. dynamic compression ratio is impacted a lot by elevation. How does an ECU compensate for that variable? If an 11:1 compression engine detonates on 87 at sea level will it detonate at 5k or 10k on 87? What will an ecu do to compensate for 20-40% less O2 on a mountain pass? Also if outside air is 0* or 100* does that have an impact? Does lower temp give better combustion ?

appreciate your knowledge here.

Very good questions. These are the sorts of things that most people never consider. To the vast majority of drivers they never think about even the most fundamental physics that go into any type of machine. Or anything technical for that matter. In terms of their vehicles, they simply hit the starter, put it in gear and stomp on the accelerator pedal, blissfully unaware of all the science and engineering effort that was achieved to make this all happen. Or worse yet, the incredible amount of energy in a moving mass. If they did, nobody would tailgate. But I diverge, that's a different subject.

Back on topic, we embedded software developers have a mantra, "if no one has to think about or is aware of the functionality you're implementing, only then have you truly succeeded."

For most, like the previous poster states, "who cares?"

How many people other than yourself have even considered these types of questions as they drive? So kudos to you.

Let's see here where to start. I'll try my best to be concise but it will be a challenge. First the primary focus is to achieve something called the 'Stochiometric Ratio". From a simple Google search:

"The Stochiometric Ratio is the exact ratio between air and flammable gas or vapor at which complete combustion takes place. The stoichiometric ratio of combustion varies for various fuels and oxidizers. If the engine has less air than the stoichiometric ratio, you have a rich mixture, because it is rich in gasoline."

One of the primary goals of any engine controller is to try and achieve this very difficult condition in order to achieve complete and total combustion. When this occurs, maximum power and minimum emissions are produced. Unfortunately however, the Stochiometric Ratio is always rather elusive.

It seems simple enough, in theory just deliver the proper amount of fuel for the available intake air in order to perfectly combust within the piston's ignition chamber. Well in practice, almost everything is working against this lofty and noble goal.

Barometric (atmospheric) pressures vary based not only on altitude but also the weather. Typically, the lower the air pressure the less available oxygen (O2) there is to combust.

Innocently enough your vehicle will produce less power during a thunderstorm as compared to a bright sunny day. Just due to a lower barometric pressure. That's also exactly why your engine struggles more at higher altitudes. This is also why turbo chargers are so attractive for increased performance. It's nothing more than a high speed fan increasing the intake manifold air pressure (MAP) which increase more available O2 (higher concentration of O2 molecules per unit volume of air).

Next, fuel vapor must be mixed in at just the proper amount in order to combust cleanly. If there is to much air as compared to fuel (a lean mixture), power is reduced and emission by products increase due to a weak (incomplete) combustion. Contrarily, two much fuel as compared to the amount of O2 (a rich mixture) wastes fuel and also increases emissions again due to incomplete combustion. The amount of pollutant particulates is also dramatically worse when too much fuel is introduced.

Another factor is compression. Each engine is designed to compress air and fuel vapor to the point where the pressure increase causes a rise in cylinder temperature where only a minimal spark is required to ignite the mix causing explosive 'combustion'. This is the actual source of power. I know call me Capt. Obvious. But what may not be obvious is that this is based on the physics equation of the Universal Gas Law PV=nRT which indicates that pressure (P) and temperature (T) are proportional by it's total volume (V), the amount of substance (n) and the Ideal Gas Constant (R).

So as pressure increases, so does the temperature as air is compressed in a smaller and smaller volume.

With diesel engines, the compression ratio is high enough that a 'glow' rather than a 'spark' plug is only required in order to ignite the mixture. At those pressures and temperatures a less refined fuel is then needed.

Now let's again talk about octane. All similar quality fuels but differing only in octane rating has the exact same amount of chemical energy per unt volume. Let that sink in. A gallon of low octane gasoline has the exact same amount of energy as a gallon of high octane fuel. The exact same. So many (many) people don't understand this.

Octane is a measure of a fuel's 'flashpoint'. That is at what temperature and pressure that fuel (in a vapor) will ignite. With lower octane fuel there is a wider range of pressures and temperatures where the fuel will ignite. This allows for a much less predictable piston location where the fuel will actually start to combust. In high load or very high internal cylinder temperatures or pressures, the fuel may ignite on its own (without a spark). This can then clearly happen outside of the Engine Control Modules (ECMs) direct control.

If this ignition point happens too early (pre-detonation) before the piston achieves it's Top-Dead-Center (TDC) position then the explosive energy is working against positive crankshaft rotation. This creates potentially violent vibrations because the crankshaft is normally spinning in one direction but any pre-detonation is forcing the piston to now try and rotate in the opposite direction. But the powertrain's load and inertia are pretty huge as compared to one cylinder's energy output.

Clearly though, mechanically something has to give. These opposing forces are focusing its energy on the weakest component, generally the piston's rings. As the rings absorb this blowback pressure it creates very high frictional stresses that cause additional rubbing and abrasion between the rings and all sliding surfaces along the cylinder walls. The overall outcome of all this internal chaos is simply called 'knock'.

Any knock of any amount is causing either piston, camshaft or crankshaft wear and tear if not outright damage. Given enough knock over time, the piston ring seal will typically deteriorate to the point of compression loss. With a severe enough drop in compression, proper combustion in that cylinder will no longer be possible, thus leading to catastrophic engine failure.

Fortunately, higher octane fuel narrows the range of temperatures and pressures where it's vapor will actually ignite. Thereby decreasing the likelihood that the fuel will pre-detonate and only combust at the point where a spark is actually delivered. Under ECM control the explosive combustion will only occur after TDC and all energy will contribute, rather than retard, forward crankshaft rotation. Thus greatly minimizing any piston ring, camshaft or crankshaft stresses. Everything continues to operate as intended.

Also note that the only thing the ECM can do is to either retard spark, or alter the fuel/air mix to try and reduce the possibility of pre-detonation. BUT it can only do this 'after' knock has already been detected. In other words, some potential damage has unfortunately already occurred.

By using higher octane fuel gives the ECM better control on not only this combustion event, but more confidence in optimizing the prediction of the overall ignition solution for the next combustion event. This naturally improves performance because as long as it is able to manage ignition events more precisely and accurately, the engine will naturally operate better. Dah...

Also, colder air is naturally denser, meaning that for any fixed volume there will be more O2 molecules present when colder. So the cooler the intake air, the more potential power is available assuming that the stochiometric ratio can still be achieved in the face of everything else working against it.

Regardless, a higher octane fuel will always benefit the ECM's ability to more accurately manage and predict the ignition equation, leading to better overall engine performance, reliability and longevity.

Just because the manual states that using lower octane fuel is possible, it's certainly not the most beneficial. But as always, people will still argue why lower octane is the best thing since sliced bread.

Hopefully, my diatribe gives you a better sense of both the risks and rewards of choosing any particular gasoline. You are what you eat and that goes for your Jeep as well.

Best wishes, and thanks for being inquisitive.

The lemmings who blindly go through life without any care to how the world works around them are absolutely free to do so. But I've never subscribed to that philosophy. Clearly, neither do you.

Jay

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