87 or 91+ octane?

[Ron93YJ]

87 octane 4 Life

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[Trini]

My JLU has been fed 91+ since the day I took delivery. No complaints.

Same here.

 

[jjvincent]

When gas is cheap, you use 91. When it's expensive, then 87.

If you have an RV, it's really fun as you have to reset the pump at least once as most are still at that $100 limit (some are $125).

On a side note. If you run 87, what happens that is bad? I really want to know as I have never had a real explanation.

 

[Initial-Jeep]

On a side note. If you run 87, what happens that is bad? I really want to know as I have never had a real explanation.

If the gas is good enough (as in it's actually 87), the computer is designed to limit what the engine will do with it, so it's fine. This is also assuming that everything is in good working order, which it should be until the vehicle ages--especially with GDI. If (when?) things get gummed up, you effectively have more compression and greater potential for pre-detonation ("knock") which the computer may or may not be able to handle. So, a firm "probably not bad now, maybe not ever" ?


I like performance and I also look at it this way: gas is cheap; engines are expensive.

 

[Trini]

If the gas is good enough (as in it's actually 87), the computer is designed to limit what the engine will do with it, so it's fine. This is also assuming that everything is in good working order, which it should be until the vehicle ages--especially with GDI. If (when?) things get gummed up, you effectively have more compression and greater potential for pre-detonation ("knock") which the computer may or may not be able to handle. So, a firm "probably not bad now, maybe not ever" ?


I like performance and I also look at it this way: gas is cheap; engines are expensive.

I am in the same boat. I want to take as much care of my engine and the few cents extra is more than worth it.

 

[jmr]

The 2.0L Hurricane is designed to take advantage of Premium but will tolerate Regular. I choose to run Premium since I want the most out of the engine. There is an ongoing discussion with the 5.7 Hemi RAM owners to run Regular or Mid Grade.

 

[jeepoch]

On a side note. If you run 87, what happens that is bad? I really want to know as I have never had a real explanation.

John,

In one simple word, 'knock'.

I've already written a lot of detail on this subject:

https://www.jlwranglerforums.com/forum/threads/87-or-91-octane.89931/post-1873770

If that post was tl;dr and you didn't catch it, 87 octane has a higher probability of producing knock. This is a condition which results in unwanted and unintended mechanical vibrations due to improper combustion. Anything that lowers the chance of this happening is MUCH better for the longevity and reliability of your engine.

Any knock at all (whatsoever), independent of how bad or how often its occurrence, is detrimental to the piston head and all of the sliding surfaces within the cylinder walls. It potentially produces pretty violent forces working against the engine's rotational energy (torque). This not only dramatically lowers engine output power but also, most importantly, leads to the possibility of internal damage of the piston's rings and cylinder walls. Thus potentially yielding a loss of compression and overall catastrophic engine failure.

The best thing you can do (at your direct control) is to use higher octane fuels in order to simply lower the likelihood of producing any knock from happening. This of course won't guarantee prevention but will significantly lower the overall probability.

Lastly, modern engine controllers do a pretty decent job of retarding spark in order to reduce knock, but unfortunately only after it happens. That is, only after it is initially detected (after-the-fact). Any knock at all, regardless of amount, is both detrimental to efficiency and especially harmful to internal operation.

It is really primarily a function of the engine controller itself which permits and allows the practical use of lower octane fuels, especially in high load towing and climbing conditions. Without dynamic spark timing, the only thing you could do to prevent knock is to just run with a higher octane offering. Car companies however tout this capability of the control computer to deal with knock as a way of lowering the vehicle's lower cost of ownership.

Regardless, the very best approach is to try and minimize the chance of producing any type of knock from occurring in the first place. Independent of how the powertrain control module (PCM) copes with it.

As with everything in life, there's always a trade-off between efficient operation and overall cost. Choose wisely.

Happy motoring...
Jay

 

[west tex]

I'm 70, so this Wrangler may very well be my last vehicle.

I take care of my automobiles and want them to last way beyond the end of car payments. In the case of this Wrangler, like I mentioned, it has to last as long as I do. Accordingly, I feed it 91 octane and only fill up at Top Tier stations if available.

You young whippersnappers probably have a different take and more than likely intend to trade vehicles every few years or so. Saving a buck here and there on fuel is more of a priority.

 

[RoadiJeff]

Well, we're old whippersnappers in our 60s and ours gets filled with 87. If 87 octane caused any problems to the 2.0 engine the engineers would not have stated in the owner's manual that it was OK to use. We plan on keeping it for at least 10+ years.

 

[Jim1964]

I’ve alternated back and forth and it *appears* to get about 10% better fuel economy on high octane. I say that with some lack of certainty because I’ve been in the break in period and thus not driving at steady speeds or in a manner to promote fuel efficiency.

To my ears it sounds better running on high octane and the butt dyno thinks there’s a slight increase in acceleration.

Once I’ve completed my ongoing modifications and get the Jeep into a steady configuration I’ll do some actual benchmarking to see if it’s real or wishful thinking.

But I’ll never worry about using 87 regardless of any decline in performance or economy, I trust what the engineers state. In the end I’ll decide based on cost/mile if premium is worth it. Or if there’s a measurable performance difference it may just make me happy.

What I won’t do, unless forced by circumstances, is use the 85 sold in high altitude states. That made sense for old school carbureted engines. Not for a turbocharged engine.

 

[Brsox]

When gas is cheap, you use 91. When it's expensive, then 87.

If you have an RV, it's really fun as you have to reset the pump at least once as most are still at that $100 limit (some are $125).

On a side note. If you run 87, what happens that is bad? I really want to know as I have never had a real explanation.

I don't believe anything bad happens. A lot of modern engines can compensate for lower octane. Higher octane gas isn't really better...it is just needed in higher compression motors to prevent pre ignition. Think about how water will boil at a lower temperature at altitude (lower atmospheric pressure). It has a different boiling point depending on surrounding atmospheric pressure. Different octane fuels will ignite at different temps depending on compression ratios. With a higher compression ratio, lower octane fuel can pre ignite (ignite from temp in the cylinder chamber before spark is given) and cause knocking in your engine, which can cause problems over time. A higher octane fuel will not pre ignite and allow your engine to operate at peak efficiency. If your engine senses a lower octane fuel is being used, it will retard the timing of the engine. This will change the timing of when the fuel/air mixture in the cylinder chamber receives the spark in relation to the cylinder's position within that chamber. This will reduce the compression ratio, preventing pre ignition. Of course, this will reduce the power of the engine as well. It may reduce the fuel efficiency of the engine. I'm not sure how much HP you will lose with 87 octane in the 2.0. As an example, however, in the Bronco, the 2.3 eco boost is rated at 300hp on premium fuel and 270 on 87 octane. So you can run 87 without causing damage, you'll just give up 30hp. I'm not sure if this is a perfect explanation. I'm sure there are mechanics and engineers out there that could explain to better.

 

[Initial-Jeep]

Well, we're old whippersnappers in our 60s and ours gets filled with 87. If 87 octane caused any problems to the 2.0 engine the engineers would not have stated in the owner's manual that it was OK to use. We plan on keeping it for at least 10+ years.

This assumes it's exclusively an engineering decision; it isn't.

When engines age/accumulate build-up (effectively increasing the compression ratio), octane becomes more of an issue.

Knowing that "engineered obsolescence" is a thing and that the powertrain warranty is only 5/60K, how confident can one be about a 10+-year-old GDI engine running outside the engineers' recommended fuel?


"Everything is permissible but not everything is profitable," someone famous once wrote.

 

[Initial-Jeep]

I don't believe anything bad happens. A lot of modern engines can compensate for lower octane.

They compensate after the fact. IOW, after a knock, the PCM realizes "Oh, I shouldn't do that" and backs off.

 

[Squibbles]

The owners manual on my 20 states the engine is designed to run on 87 octane., with higher octane recommended for maximum performance. Mine from the dealership has never had anything other than 87, runs great, gets very good mileage (esp for a Jeep) and has all the performance I want or need. I have no intention of running anything else, period. The oil folks make enough per gallon at best. anything above what an engine is designed to run on is basically a waste. Yea, I know folks say the get more power, better mpg’s etc on 93, but I truly wonder if that’s real or placebo effect. IF I was having any issue with knocking or similar I “might” try something different. Certainly if the owners manual requires 91 or 93 that’s what you should run, however with the Jeep it is NOT a factory requirement. Certainly the engineers that designed the thing should know what fuel is required. I will NEVER again buy any vehicle that requires high octane fuel. YMMV

You will get more horsepower at higher rpm’s and potentially get better mpg because the engine can advance the timing. Will you see an increase equivalent to the increased price, absolutely not. Our Mazda is rated for more horsepower with 91 but the torque ratings are the same, I see 0 difference between fuels. My VW requires 91, I could get away with 89 at my elevation but it’s definitely worth the extra fuel cost with the tune. I’ll likely tune the 2.0 in the wrangler and see how I like it, the SRT tune seems to be lacking torque but it makes reasonable horsepower.

 

[jeepoch]

Again, it's all about knock paranoia.

As Bill @Brsox points out pretty succinctly, it generally depends on the compression of the engine. However, there are of course other factors. Bill appropriately points out altitude (atmospheric pressure) but with a turbo, that factor is greatly mitigated.

There is ambient humidity, any H2O (at any amount, even as vapor) is undesirable. Modern intake filters and plenums do a pretty good job at minimizing that. Another minor factor is air quality (smog). Nitrous particulates are inert and displace the volume within the cylinder's intake stroke of possible O2. This is exactly the concept behind using the Exhaust Gas Recirculation (EGR) Valve. At speed, the introduction of exhaust reduces the amount of available O2 which greatly reduces the combustion emission by-products such as Nitrous Oxide. This reducing smog.

But by far the most important factor behind pre-ignition besides compression, is engine load. Or rather, the overall mechanical dynamic resistance to crankshaft rotation.
Severe engine loads work against normal spark timing by delaying the expected piston location. However, during any condition where the fuel/air mix can ignite prematurely leads to engine 'knock'. This is detrimental to the both the piston's rings and all sliding surfaces due to potentially violent and very undesirable vibrations caused by the explosive energy created before the piston achieves top-dead-center (TDC). Clearly, any pre-ignition causes both unwanted power loss and excessive wear. Any knock whatsoever WILL lower your engine's performance, lifetime and overall longevity.

Furthermore, all fuels independent of octane have exactly the same energy content per unit volume. Octane is just a measure of its flashpoint. The higher the octane the more 'consistent' both the temperature and pressure needs to be in order to ignite.

With higher octane fuels, regardless of compression, allows modern engine control computers to calculate a more precise spark timing solution with very small adjustments. With very accurate spark timing, knock (in theory) could be eliminated entirely. But alas, nothing is ever perfect but with today's technology nearly almost so.

Engine power and efficiency though is greatly enhanced with the use of higher octane fuel. Unfortunately this also requires more refining steps in the petroleum production process. Hence more expensive.

Lastly, the automotive manufactures naturally want to promote a 'lower cost of ownership' for their vehicles. One (if not the) easiest way to claim that, is by using the cheapest, certainly not the most expensive petrol. So yes, with the absolute faith that software will do its magic within the engine controller, by potentially retarding spark, they PROMOTE the use of lower octane fuels as a business decision not necessarily as a scientific or engineering one. See Julian's @Initial-Jeep post above.

Unfortunately, the retarding of the spark equation only occurs when knock has already occurred, a.k.a after-the-fact.

So it comes down to a simple case of paranoia: Can you tolerate some amount of knock or are you budget constrained? If yes, 87 is your go-to fuel of choice. If you're OCD or are looking to squeeze every possible ounce of performance while extending your engine's overall lifetime without worry of your wallet, then higher octane is for you.

Unfortunately, we will always be across the entire spectrum of marginally acceptable vs near perfection vs cost. Pick your pain.

Jay

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