87 or 91+ octane?

[Zandcwhite]

As many others have stated, the Jeep increases performance by advancing the timing. The problem is, there’s no octane meter built in to the fuel system. The ecu will continually increase the timing to it’s peak parameters unless or until it detects knock. Obviously running lower octane fuels will have it reaching the point of knock far more often. Does it allow enough detonation to cause appreciable wear or damage? Maybe not. Is even the slightest bit of knock hard on an engine? Absolutely. I’ll stick with the fuel that allows for the best performance AND is easier on the engine. It’s a $60k Jeep, I’ll pay an extra $5/tank.

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[Xspurt]

Instead of the see-saw between 87 and 91, what about 89? It sits squarely between the two in octane rating and additional price. Yes, it's a half measure ... a convertible sofa (not a great sofa, not a great bed) but it will have less knock. The adjustment system varies timing continuously, not in preset notches (at least AFAIK).

 

[Initial-Jeep]

A good question. I'd say not in a turbo and go with what's recommended.

 

[Brsox]

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

That's correct. The small amount of knock that occurs, after the computer detects it and makes an adjustment, isn't going to cause any harm to your engine. A prolonged, consistent knock, might. The engine has been designed to adjust and run on 87 octane.

 

[Brsox]

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.

So, if all engines age and accumulate build up, thereby increasing compression, then all engines require premium fuel at some point in their lifespan?

 

[Brsox]

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

Excellent explanation, my friend.

 

[Old Dogger]

I purchased a new Cadillac Escalade several years ago. The owners manual said 87 octane was fine to use, but the recommended for more power was 91 octane. I ran it for approx. 90K on 87 octane, and never had a issue with it. That also included a lot of mountain driving. So it is a choice.

 

[jeepoch]

So, if all engines age and accumulate build up, thereby increasing compression, then all engines require premium fuel at some point in their lifespan?

Compression can typically not go higher. This is limited by displacement and bore stroke. Perhaps just a little based on carbon build up over time. This would nonetheless be miniscule compared to piston ring and cylinder wall surface abrasion. The net result is generally a downward trend towards lower compression.

Once the rings are damaged and you start to burn oil, compression is compromised to the point that every combustion event becomes a crap-shoot. The introduction of lubricating fuel into the combustion chamber will result in a much dirtier burn, resulting in a dramatic drop in power and much higher emissions output.

The only thing you can do then to compensate (other than a complete engine overhaul) is to run the highest octane racing fuel you can get your hands on. Otherwise, the engine controller's spark timing solution would be all over the map.

The takeaway here is to keep those precious piston rings healthy, for as long as possible. With modern fuel additives such as detergents, this does a remarkably good job at keeping the injectors and piston heads clean. But knock, of any amount, harms the piston's health more than just about anything.

Happy motoring...
Jay

 

[Initial-Jeep]

So, if all engines age and accumulate build up, thereby increasing compression, then all engines require premium fuel at some point in their lifespan?

Please take my posts for the manner and context I intended.

 

[Initial-Jeep]

This could be useful:

https://www.pure-gas.org/

 

[Dyolfknip74]

Service advisor at the Fort Collins Jeep dealership told me "Do not use 91 octane. You should be using 85 octane"

Lol, that job title doesn't really carry any merit with me, sorry. Not to mention, what is 85 Octane?

 

[Dyolfknip74]

Just use what ones owner manual says to use.........

Lol, it says both. Did you even read this thread?

 

[Zandcwhite]

So, if all engines age and accumulate build up, thereby increasing compression, then all engines require premium fuel at some point in their lifespan?

All forced induction engines absolutely. Although with the higher starting compression of most modern engines, I'd be hesitant to run 87 past 100k on anything I owned.

 

[jmccorm]

Just use what ones owner manual says to use.........

Lol, it says both. Did you even read this thread?

 

[hiimmike]

Lol, that job title doesn't really carry any merit with me, sorry. Not to mention, what is 85 Octane?

High elevation fuel.

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