87 or 91+ octane?

[stumblinhorse]

@jeepoch last question. Does an ecm usually start with a preset optimal amount of timing and then only retard based on pre-detonation? Or does it have a starting point and then add if AFR is rich? I know not all are the same, but generally speaking?

thanks again!

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[jmccorm]

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. [ ... ] Fortunately, higher octane fuel narrows the range of temperatures and pressures where it's vapor will actually ignite.

This is the paragraph that meant the most to me.
It's about the size of the envelope and predictability. Not "a flashpoint number." Thank you!

 

[jeepoch]

@stumblinhorse,

Keep being curious, there should never be a 'last' question. That's how science works (really). Question everything all the time. At some point as your knowledge grows, this curiosity becomes more than just that. You develop an interest or a passion and deeper understanding then only expands exponentially. The human brain is a remarkable thing. Always seek answers to whatever questions you can dream up.

I always say that education isn't formal it's inherent. College very rarely teaches you many answers, but it does teach you where to go look them up. You don't need specialized degrees to truly understand.

As for your continued curiosity in all things engine related, this answer is tricky because there are several approaches based on what you have to work with. Not all engine controllers are created equally. It generally depends on the 'horsepower' of the CPU and it's available memory and overall hardware architecture.

Since sophistication requires cost, the cheaper the vehicle, typically the more simple the controller. In fact in many low-end automobiles, the engine, transmission and body management are all performed in a single Powertrain Control Module (PCM). In more higher-end products the control is more distributed and each component has it's own dedicated computer. Each implementation has it's particular pros and cons. Such as simple information coordination vs complicated communication signalling, typically over several Controller Area Network (CAN) buses throughout the entire vehicle.

Given a powerful enough processing platform where lots of control logic is built into the controller along with high bandwidth CPU's, the spark timing equations are many orders of magnitude more sophisticated. The lower bandwidth engine control schemes offer a wholly different spark timing approach.

Since our JL's have a really decent engine control solution, I'll just briefly explain the more fundamental, (simpler) approach first.

On controllers with only the most marginal amount of information available, spark timing is more 'reactionary' rather than 'predictive'. In this scenario, spark is incrementally advanced until 'badness' starts to happen such as lower performance, more emissions or even knock. At which point the controller reverses this advance and begins to then retard it. It does this until once again 'badness' begins to happen yet again. This time, it will be almost totally dependent on performance and emission monitors since pre-detonation is more unlikely.

This ping-pong of 'too advanced' or 'too retarded' is always going on to some degree. The computer software algorithms are generally designed to learn a sweet spot' where the engine is running optimally. However, there are of course many factors that will alter this sweet spot. Things like engine load, stochiometric fuel/air dynamics, emissions, fuel quality, octane value and others. So the spark equation is continually hunting for and ever changing this sweet spot. Of course there are maximum and minimum spark timing thresholds which are constrained to certain absolute piston positions in relation to Top-Dead-Center (TDC) in order to prevent engine damage. But, this solution typically ping pongs back and forth as engine dynamics change, which they do all the time.

Now moving on to the more sophisticated approach. This invests in more of a predictive model. This requires a lot more monitoring of just about anything the auto company is willing to invest in. Typically more pressure, emissions and knock sensors or at least a way to derive them through statical analysis performed in real time.

Within this environment, the Calibration Team (the true engine scientists) perform all the math and physics modeling to monitor, track and predict what the most optimal spark timing needs to be. Rather than the trial and error (ping pong) approach, spark timing is always advanced just enough to provide the most optimum combustion, but no more. This timing should (in theory) prevent or minimize knock. However, we know from a previous post, that with low octane, the probability increases to where the fuel/air mix may pre-detonate on it's own before the optimal ignition point within the piston's cycle, outside of software control.

If knock happens, as within the more simple algorithm, the spark timing will be retarded along with more aggressive fuel/air mixture adjustments.

The benefit of this sophistication however is to greatly minimize knock and undesired emissions generation while improving performance. Modern Engine Controllers excel at this. Essentially because the ever increasing CAFE and emmision regulatory mandates provide the auto companies with essentially no other options.

So the Calibrators are hired and tasked with solving all the very complicated calculus and statistical math in real time. They do this though the use of various data structures defining one dimensional vectors, two dimensional surfaces, three dimensional maps and higher dimensional abstractions. The differential and matrix algebra equations are necessary to solve the physics of predicting not only the fuel/air mix but the most optimal piston position in order to deliver the most beneficial ignition point with the never ending goal of producing Stochiometry.

Without this sophistication, all ICE vehicles would immediately fail the latest emission regulations. The lower-end products would still have to pay the 'carbon' fines, but the car companies get somewhat of a pass as long as their fleet averages satisfy EPA thresholds. So over time even more sophistication will be required until every vehicle running on dinosaur goo is phased out.

Unfortunately (IMHO), the battery tech that is being offered is still not a practical viable alternative. We need something better, not worse before we eliminate our current 'horse' power.

I know I really touched on a lot of things here. I really hope it helps you towards a better understanding.

Stay inquisitive.
Jay

 

[RoadiJeff]

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

Once again, using an octane higher than what is recommended in the owner's manual does absolutely nothing to increase performance if it was not tuned for it. Perhaps you think you know more than the engineers who designed your Jeep. The placebo effect is strong with this one, Yoda.

Also, using a higher octane than what is recommended to reduce knock is just putting a Band-aide on a larger problem that should be corrected.

I could post more links to official studies but I doubt it would make any difference to you.

 

[Initial-Jeep]

Perhaps you think you know more than the engineers who designed your Jeep.

uuuuuhhh....did you read the part about how he actually was one of those engineers...?

I could post more links to official studies but I doubt it would make any difference to you.

I'd be interested in those "official" studies.

 

[Zandcwhite]

Once again, using an octane higher than what is recommended in the owner's manual does absolutely nothing to increase performance if it was not tuned for it. Perhaps you think you know more than the engineers who designed your Jeep. The placebo effect is strong with this one, Yoda.

Also, using a higher octane than what is recommended to reduce knock is just putting a Band-aide on a larger problem that should be corrected.

I could post more links to official studies but I doubt it would make any difference to you.

What part of ”for optimal performance” from the manual makes anyone think that Jeep is not recommending or tuning for 91 octane? Will it run on 87? Of course. Will it have better performance on 91 octane? Even according to the manufacturer, YES. Is it enough of a difference to feel in day to day cruising? Probably not. Will it make a difference in high altitudes, hot days, towing, or blasting up a steep loose hill/ through deep sticky mud/ or making first tracks in deep powder? Most likely yes. You can run whatever you want, but Jeep absolutely does recommend 91 octane in black and white. Mazda and Ford actually publish different power figures for their turbo engines. Mazda gains 23hp and 10ftlbs from 87-93octane. Car and driver tested the F150 eco boost extensively and showed a 20hp difference. 0.6 second quicker 0-60 and a half second quicker 1/4 mile time by a simple octane change. These are not NA engines from the 90’s with no built in tuning adjustments to take advantage of octane increases. That’s not opinion, butt dyno, or emotion, it’s measurable fact period. https://fordauthority.com/2019/08/is-premium-fuel-better-in-an-ecoboost-ford-f-150/

 

[jeepoch]

@RoadiJeff,

There is a lot of science and engineering effort that goes into tuning an engine. Performance is generally only trumped by emission and CAFE regulations. The fuel used is the biggest factor that you as a driver can control in the auto manufacture's and EPA's quest to meet those mandates.

The ONLY reason that you are not required to use a particular, specifically regulated fuel type (by law), is that the powertrain engineers have found the means in which to use a range of fuel types which marginally satisfy the minimum specifications.

If you believe that any fuel is satisfactory just because the 'manual' says so, then go right ahead and sleep soundly at night. The thought of all the sub-optimal things that have to be dealt with in order to achieve the same emissions and fuel economy standards is but a fleeting concern. You are certainly in your comfort zone.

Please don't let me or anyone else take you out of it.

All things run and perform better with good nutritious ingredients. High quality, high octane gas is much healthier for your engine than any less refined petrol.

If you don't see this from a conceptual viewpoint then 'it is what it is'. Feed your Jeep grape juice if it makes you happy.

Jay

 

[jjvincent]

I guess this settles it. The intranets have spoken and now we know. No need to listen to people that make a living at doing calibrations, the armchair calibrators know better. Seat of the pants is much better than data. As we all know, manufacturers just design and build a vehicle, then never do real world testing. They know every owner will ask for the best of the best and use it religiously. I bet all of the 2018 2.0T motors from 2018 in a Jeep with 200K on the clock, have grenaded by now. They have never posted here because they are embarrassed that they threw in regular gas as opposed to premium.

We have a customer with a 2018 and it just hit 200K on his 2.0T Wrangler. He uses our non Mopar oil filters, the non Mopar oil and he runs regular in it. I'll let you know when it blows up as he's doing it the wrong way.

 

[jeepoch]

I guess this settles it. The intranets have spoken and now we know. No need to listen to people that make a living at doing calibrations, the armchair calibrators know better. Seat of the pants is much better than data. As we all know, manufacturers just design and build a vehicle, then never do real world testing. They know every owner will ask for the best of the best and use it religiously. I bet all of the 2018 2.0T motors from 2018 in a Jeep with 200K on the clock, have grenaded by now. They have never posted here because they are embarrassed that they threw in regular gas as opposed to premium.

We have a customer with a 2018 and it just hit 200K on his 2.0T Wrangler. He uses our non Mopar oil filters, the non Mopar oil and he runs regular in it. I'll let you know when it blows up as he's doing it the wrong way.

John,

Not to belabor anything further but the thought becomes "could he have even done better?"

The answer always comes down to "how good is good enough?"

Everyone personally has the absolutely correct right to answer that for themselves. But at least (I hope) some education may have transpired where people may now make a more informed decision.

The concept of a better decision is still up to them.

Jay

 

[Initial-Jeep]

Jay,

How dare you not speak for everyone!

 

[jjvincent]

John,

Not to belabor anything further but the thought becomes "could he have even done better?"

The answer always comes down to "how good is good enough?"

Everyone personally has the absolutely correct right to answer that for themselves. But at least (I hope) some education may have transpired where people may now make a more informed decision.

The concept of a better decision is still up to them.

Jay

What is better? Do you need to use the best of the best for everything? When I see older vehicles, the motor is the one thing that outlasts the rest of the vehicle. Yet on the intranets, we obsess about the internally lubricated engine parts and what gas we put in it.

What I see in a daily basis is this. People keep a vehicle for about 10-15 years. When they have a huge quote of things like replacing the cats, control arms, wheel bearings, brakes and tires, they unload it. Yet the motor is in great shape. If it’s a Jeep, it’s because it’s rusted out. The question is, would have spending the extra money on oil, filters and gas would have been worth it?

 

[jeepoch]

John,

You make very good points. One does not need to always buy the best fuel or best oil or even clean their rig on a daily basis to have a great vehicle. All decisions about operation and maintenance are personal choices based on many factors, where cost is most certainly a major incentive but not the only one.

Can you run 87 octane? Yes, certainly! Will it be better for your vehicle in the long haul? No, it will not. Anything that raises the probability of producing knock by any amount, just increases the chance of piston, rings and cylinder wall damage. And running 87 octane certainly helps promote knock possibility.

However, modern engines with remarkably good engine controller technology will really try and reduce the likelihood of this happening. BUT it still can't completely prevent it. If you tend to be really hard on your motor, your odds of generating knock goes way up even with the best spark timing algorithms. And recall again that any knock whatsoever, even if imperceptible, is detrimental to your engine to some degree at each and every knock event.

So running higher octane fuels will lower the probability of knock regardless of the quality and technology of your engine controller. Is this important? It clearly depends on the driver, and their driving style, circumstance or situation.

Obviously in your opinion, the use of running higher octane is not worth the cost. Whereas I see a benefit.

Yours is a very legitimate position. All I'm saying is that yes from a technical perspective your engine will benefit with higher octane and yes your wallet will not.

It's up to each one of us individually to understand our own pain thresholds. My final point is that it's your choice. Make the best informed decision possible.

I'm not stating you must use 91 or avoid 87 completely, I'm only pointing out the reason why you may (or may not) want to. Is 87 good enough? Neither you nor I can answer that for anyone but ourselves. When is good, good enough? Clearly my idea of good is different than yours.

We have stated our opinions, and I feel between the two of us we have given pretty decent arguments for both.

Neither of us are either right or wrong yet we are also both right (and wrong). Let the individual reader choose their own strategy accordingly. We've given them exactly what this thread intended.

Jay

 

[Hennessey17]

I'm a believer that if you don't want to pay for the better gas... don't buy a turbo.

When I got my Tiguan tuned, I chose the 93 octane tune... because it's abundant in northern IL and southeastern WI... then I drove out of those areas and realized in some places I could only find 91 octane... the difference between the 93 tune and the 91 tune was 5hp.

I have yet to get the Jeep tuned. The manual says 87 is fine, and with gas prices the way they are, that's what I've been using... and it runs just fine... plenty of passing power.

So as a believer of buying better gas, I totally admit 87 is fine for most of us.

I'm more worried about oil change intervals than octane.

 

[SpaceCase]

@jeepoch How does this effect the 4xe? The supplemental manual states to run 87 octane (no benefit stated for higher) and I can only assume that the "boost" from the electric motors is the reason. Does this mean that Jeep re-tuned the 2.0L for the 4xe? Or might I still see an advantage with 91 even with the electric motors?

 

[jeepoch]

@jeepoch How does this effect the 4xe? The supplemental manual states to run 87 octane (no benefit stated for higher) and I can only assume that the "boost" from the electric motors is the reason. Does this mean that Jeep re-tuned the 2.0L for the 4xe? Or might I still see an advantage with 91 even with the electric motors?

As you can tell from the overall conversation in this thread that opinions will be all across the entire spectrum regarding using 87 or higher octane fuels. But since you're asking me, I'll be consistent with my answer.

Yes, 91 will be better for your 4xe, especially since it has the 2.0L Turbo. Do you absolutely need to run with 91 octane? Certainly not. But it's use is still beneficial overall.

Note that the higher octane offerings are generally only provided with the highest quality, premium petrol. These fuels typically are refined and formulated for the best operation of your motor. The premium gas includes detergents and other ingredients to ensure better injector performance as well as helping for cleaner and better combustion. So buying the more expensive offering at the pump can give you even more benefit than just the higher octane rating alone.

Yes, I wholeheartedly agree that your electronic powerplant helps with performance but your four banger is still a very important component.

If you can afford it, the premium petrol will always make your engine a little bit happier.

Cheers,
Jay

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