3.6 - For Those Running Premium Fuel

[MrMischief]

For you, 87 is more like 89 for us lower folk.

So why do I get knocking on 85?


EDIT: Sorry! sorry.... I took the bait. I didn't mean to or want to. I don't know enough or have enough knowledge to argue this point. Just here looking for information. Please ignore me. But I did notice knocking at lower elevations as well on their regular which was 85.5 (Montana).

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[AnnDee4444]

FCA does state in the manuals of SRT cars that 91 octane should be used. They do it in the 2.0l Wrangler too. So why not for the 3.6l? Because the people who designed and built the engine, and put hundreds of thousands of test miles on it, found that premium fuel is not necessary.

I searched around a little bit, and the highest compression ratio of any naturally aspirated SRT engine I can find is 10.9:1 (2016+ SRT8). The 3.6 somehow has a higher static compression ratio, yet doesn't require high octane.

 

[TheRaven]

So why do I get knocking on 85?


EDIT: Sorry! sorry.... I took the bait. I didn't mean to or want to. I don't know enough or have enough knowledge to argue this point. Just here looking for information. Please ignore me. But I did notice knocking at lower elevations as well on their regular which was 85.5 (Montana).

Its ok. If you are experiencing chronic knocking you'll want to get that looked at. It could be simple atmospheric factors but it could be a sign of trouble. It's normal to experience short lived minor knock infrequently. It's when it happens repeatedly or for longer periods that you need to be concerned.

I searched around a little bit, and the highest compression ratio of any naturally aspirated SRT engine I can find is 10.9:1 (2016+ SRT8). The 3.6 somehow has a higher static compression ratio, yet doesn't require high octane.

Yes, because compression ratio is not the only factor that determines what octane is required.

 

[Reseg]

I searched around a little bit, and the highest compression ratio of any naturally aspirated SRT engine I can find is 10.9:1 (2016+ SRT8). The 3.6 somehow has a higher static compression ratio, yet doesn't require high octane.

It's not just about the basic compression ratio # on paper. The 3.6L in the Wrangler is likely conservatively tuned @ 285HP, if you consider that in the Challenger it puts out over 6% more HP, @ 303HP (yet the manual STILL recommends 87 octane for the 2021 Challenger, saying there's no benefit to higher octane).

 

[BlackGenesis]

I searched around a little bit, and the highest compression ratio of any naturally aspirated SRT engine I can find is 10.9:1 (2016+ SRT8). The 3.6 somehow has a higher static compression ratio, yet doesn't require high octane.

Car manufacturers go to extended lengths to mitigate pre detonation by utilizing DI fuling and unique design DI pistons. Fuel delivery into the chamber is the key.
Jl does not have what it takes to handle low octane, utilizing fueling system that belongs on 10 to 1 engine. They chose to run garbage timing instead.

 

[madm73]

I switched to jet fuel last week. I am getting 136mpg and the dyno says 890hp/2768torque. Bottom line is you all should be running jet fuel in your 3.6. I win.

 

[TheRaven]

Car manufacturers go to extended lengths to mitigate pre detonation by utilizing DI fuling and unique design DI pistons. Fuel delivery into the chamber is the key.
Jl does not have what it takes to handle low octane, utilizing fueling system that belongs on 10 to 1 engine. They chose to run garbage timing instead.

Its because it's not tuned for high Performance. The Challenger has a smaller range of conditions it needs to operate in, so it can be tuned more liberally. The 3.6l in the wrangler is tuned for operation in a wide range of climates, altitudes, and conditions. Often times that means you end up with a tune that sucks for the conditions you see the most.

The above is why i would never advocate that you shouldn't get a tune, and perhaps Livernois is a great option for a quality tune. It's just that telling all the owners of any vehicle, let alone one of the most versatile and capable vehicles made, that they all need to do a specific thing or they will damage their engines, is pretty irresponsible. That's taking on a ton of liability.

 

[Livernois Motorsports]

It's not just about the basic compression ratio # on paper. The 3.6L in the Wrangler is likely conservatively tuned @ 285HP, if you consider that in the Challenger it puts out over 6% more HP, @ 303HP (yet the manual STILL recommends 87 octane for the 2021 Challenger, saying there's no benefit to higher octane).

Challenger/Charger/300/etc still uses the older, lower compression 3.6L, not everything has gotten the Gen2 engine. the older 3.6L was fine on 87, and actually can gain solid numbers on 87, but this is almost an entirely different engine than the Gen2. Gen2 saw a compression increase, added EGR, and variable valve lift (not just timing), all of it plays into the situation at hand.

 

[BlackGenesis]

I bought my wife a new Rubicon 3.6 manual a little over a month ago. We live at altitude and operate the Jeep between 6000-10,000 feet 95% of the time, it will likely never go below a mile high as it is our around home vehicle.

Our mid grade is 87, as mentioned by others and that is what I have been running in it. After reading a fair amount of this discussion, it is pretty clear to me that going with the available 91 which all has 10% ethanol here may be to our advantage in the long run, and even in daily driving.

The Jeep has just under 2000 miles on it now and from my reading has likely adopted a long term knock control strategy. So if we start running premium exclusively what is the best way to get the ECM to stop pulling timing and start taking advantage of the higher octane fuel? Do we just start running it and wait? Does simply disconnecting the battery for a period of time cause it to reset?

I really appreciate the insight you have provided here. I must say that those who think the manufacturer has the consumers best interest in mind here is a bit humorous. Kind of like thinking the government has your back... Thanks for providing the info, it has convinced me that going to the better grade of fuel is advantageous.

Yes disconnecting battery will reset long fuel trims maps. It will not take too long to get your long term fuel maps to set for premium.
The only issue I see here is that OEM tune is very conservative in order to be able to run low octane and does not have much leeway for advancing spark or vvt if you are looking for any sensible power gain.
Only way to get significant increase in performance and slight better MPG is aftermarket tune (Livernois) with 91/93 octane.
The amount of power it gains on 93 plus tune only leads me to believe that OEM tune is pure garbage. A well tunes NA v6 engine (similar displacement) will be lucky to gain 1/4 of power that Livernois got out of Jeeps v6.

 

[LongTimeListener]

That's taking on a ton of liability.

Liability? For what? For perhaps convincing some people to run 91 octane? So they’d be liable for what, exactly? A few extra bucks those folks spend? Nope.

Or do we mean some kind of slander against Stellantis? Again, almost certainly not. For one, it wouldn’t be worth their time. For two, Stellantis wouldn’t want to expose itself to discovery by suing over such statements. For three, truth is a defense.

People really need to stop oooohinh and awwwing over this. Nothing is going to happen. At all.

 

[TheRaven]

Liability? For what? For perhaps convincing some people to run 91 octane? So they’d be liable for what, exactly? A few extra bucks those folks spend? Nope.

Or do we mean some kind of slander against Stellantis? Again, almost certainly not. For one, it wouldn’t be worth their time. For two, Stellantis wouldn’t want to expose itself to discovery by suing over such statements. For three, truth is a defense.

People really need to stop oooohinh and awwwing over this. Nothing is going to happen. At all.

Nothing to do with slander. Everything to do with "following the manufacturers recommended schedule of maintenance". You have to remember that manufacturers recommendations are tested, for several years, by hundreds of vehicles over thousands of miles. How much testing has Livernois done before making their recommendations? So far it sounds like a few minutes per vehicle over 18 vehicles.

People really need to stop oooohinh and awwwing over this. Nothing is going to happen. At all.

But we agree here. This has been my point from the start. This whole octane debate needs to end. If you feel it's best for you to run premium in your own vehicle, great, do it. Otherwise just run plain ole regular. Nothing is going to happen. At all.

 

[ВџУМεΆј℮℮℗]

I'm still under 2000 miles on my Jeep.. I have been using middle grade. Which I think is 89 octane. I did have highway miles up to San Antonio and back and I got 17.5. And basically daily I'm getting 17 or even Louis XVI(low as 16) however this last tank I put in premium. it got bumped up to 19.5 and that is even filling it up a little more than usual. I generally go to the same pump and just use the Auto Stop. However this time I put in maybe 1/8 gallon more. It's still windy and humid as heck in South Texas. So conditions are less than ideal. Maybe it's a fluke or I still need to wait a while longer till the engine gets good and broken in along with the drivetrain too. One difference is too I changed from my half doors back to the full doors but I doubt that really has any difference.

2021 Willys 2dr 6 speed. Soft top currently

 

[unsavory]

I don't care what anyone else says. I know for a fact that my Jeep runs like absolute dog shit on 87, and so I switched to 91. World of difference.

I have tried going back to 87 for a couple tanks, and same result. 87 pings and knocks all over the place. Maybe others don't actually have any hills where they live, and drive like gramma. But any kind of spirited driving, or up hill grades, and 87 comes a knocking.

I also see at least 1 extra MPG on 91, and typically 2 extra MPG.

 

[F-1shakey]

I don't care what anyone else says. I know for a fact that my Jeep runs like absolute dog shit on 87, and so I switched to 91. World of difference.

I have tried going back to 87 for a couple tanks, and same result. 87 pings and knocks all over the place. Maybe others don't actually have any hills where they live, and drive like gramma. But any kind of spirited driving, or up hill grades, and 87 comes a knocking.

I also see at least 1 extra MPG on 91, and typically 2 extra MPG.

I agree and have experienced the same result with 87 vs 91. Definitely less pinging on 91 and smoother acceleration/power overall.

 

[WranglerMan]

Well I’ve been burning 91 octane for a few months and my avg is 19 mpg but today I jumped back to 89 and will burn that and see what happens but I do know 87 does ping alot.

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