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j6harris 06-06-2012 05:41 PM

octane rating
does anyone know why the manual recommends to use 89 octane instead of 91 or 93 with a 10.5:1 compression ratio on the 5.7 hemi ?

pogster 06-06-2012 05:58 PM

Because it's not needed. If you pulling a load for example it's better to use 89 than 87 to prevent pre-ignition or knock as its sometimes called. Using higher octanes would be a waste of money.

PaulMys 06-06-2012 07:09 PM

I asked my dealer this same question when ordering my express back in March. He said that the new engines have been engineered this way, and as pogster stated, higher octane has no advantage or disadvantage so just a waste of money.

docgatling 06-06-2012 07:37 PM

In the old days if your motor suffered knock you switched to higher octane. Modern cars (and trucks!) have knock sensors to prevent damage to the engine if to low octane in injected. Your eingine will run but suffer in the preformance department. So, I would run a couple tanks of both and see how your engine performs. Monitor your gas mileage with 89 then try 91. If you don't see any difference use the "cheaper" stuff. There is a good article in Road and Track about octane and its effect on engines. It's not just about compression anymore.

snrusnak 06-06-2012 07:53 PM

The two biggest factors that relate to required octane are compression and ignition timing advance. From the factory with stock tune, 89 octane is recommended. It will run on 87 but may retard timing slightly(slightly less performance/mpg).

GTyankee 06-06-2012 07:57 PM

I have run my HEMI on 87 octane, when the prices were too high
I usually run 89, better gas mileage & more responsive

I wouldn't run the octanes that are higher yet, unless i had something like this

PaulMys 06-06-2012 08:00 PM

GT- now THAT'S a HEMI!

captainchubbzz 06-09-2012 01:30 AM

I've used an 88 octane with no ethanol, it truly made a difference in power and my engine seemed much happier. I could be up in the night though?

j6harris 06-13-2012 11:11 AM

well heres what i came up with .
i tried 3 different types of gas, all 3 were from bp .
89 , 93, and non oxygenated 91

89- the gas mileage is ok, the throttle response is best at low to medium speeds/rpm no load just normal putzing around neighborhood kinda driving or freeway on ramp type area ... anywhere you'd keep the pedal at 3/4 or less it shines , 89 is what the factory recommends but it lacks the full throttle go and mileage of 91 or 93.

non oygenated 91- it lasted forever.. almost 2 weeks on what a tank of 93 oxygenated going the same places would last me in 1 week.. the only problem with that was i couldnt wait to get it out of my tank. there was almost no throttle response. seemed like i could only go all or nothing, it was fine idiling around at low speeds/rpm except when it came to normal driving conditions like accelerating from stop lights and freeway entrances it was very sluggish like i was using syrup for gas but when i opened it up it went like a raped ape! .. that part was nice but i do a good amount of normal driving so this one wasnt for me. plus i hear its more corrosive than oxygenated gas.

93- 93 is what i've been using for a few years now on the 5.7 hemi. the down sides are its more expensive and the low end throttle response isnt as good as with 89 but that is just barely noticable. the gas mileage is a noticable amount better, lasted me 2 days longer than 89 did going to the same places. when you put the pedal to the medal with 93 it goes! i've never felt a vehicle accelerate like this one, it seems like the power curve doesnt curve it just rises.. its nice, fun to drive and i like how it pushes you harder into the seat the longer you've been on the gas.

so.. i decided i'm sticking with my 93 because it suits my driving habits better, i like to open it up from time to time and i have bragging rights to uphold at work when all my chevy and ford loving buddies want to eat some dust i'll be there to serve it to them with that extra ooomph i get from 93, but all around i guess the octane rating really depends on how and where you drive, personally if you do a lot of towing or hauling or friendly races to make your buddies cry i would recommend using a higher octane because it really makes a difference in power when you add more throttle but i'm no expert..
i have a buddy at work that also has a 2012 ram thats been using 87, he doesnt mind it but i dont think he cares or pays attention to the little details in things like i do, after i talked to him about octane he did put a tank of 89 in though haha, i'll have to see if its noticeably different or not especially when he's towing his boat
... my only question now is what will the different octanes do to an engine over time, it would be nice to know how much carbon buildup/ deposits changes between octanes and if certain engine wear patterns or damage occurs between octanes....

snrusnak 06-13-2012 11:21 AM

Wow man. Most of the changes that you noticed between those fuels is in your head. Using 93 octane instead of 89 octane on the hemi will not give you any more power or mpg(at any rpm). It will help promote carbon buildup, though. This is assuming it's running the stock tune.

It's been discussed a million times, and there are reasons behind it. The hemi will run on 87 no problem. It will produce slightly more power and mpg on 89. Slightly does mean slightly; switching from 87 to 89 won't give you 50hp so you can beat your buddy's chevy. The difference likely isn't even noticeable to the driver(maybe 5hp?). Anything more than 89 is just throwing money away and promoting carbon buildup, so you'd actually be doing better for the engine and your wallet to put 89 in instead of 93.

Now I will say that I have no idea what oxygenated or non oxygenated fuel I won't comment on that other than I guarantee you didn't doubled your mpg with it...

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