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Old 10-22-2010, 02:13 PM
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Another Hemi Octane Thread

So I know there have been a bunch of threads on using the recommended 89 octane in the hemi or the "ok" 87 octane.

My question is why 91 or premium is "not recommended". Seems to me that if the computer can adjust for 2 points less octane it should be able to adjust in the other direction as well. Anyone know the rationale behind this? THe reason I ask this is the closest station to me that I prefer to use sells only 87 or 91, no midgrade. Not a big deal I just go down to shell a little further out but still would like to know.
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Old 10-22-2010, 02:41 PM
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Re: Another Hemi Octane Thread

It probably just doesn't work in the opposite direction! All in the programming, and that's what aftermarket tuners are for.
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Old 10-22-2010, 04:22 PM
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Re: Another Hemi Octane Thread

octane ratings are a 'minumum' and they have NEVER posted a max.
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Old 10-22-2010, 04:33 PM
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Re: Another Hemi Octane Thread

where do you get this from I never mentioned a "max". But if you look in the manual on a wk2 hemi it says use of 91 is "not recommended". That is the question i am asking about why is it not recommended.
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Old 10-22-2010, 04:41 PM
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Re: Another Hemi Octane Thread

Better wording would be "not necessary". Basically just that the timing curve that provides optimum performance as programmed doesn't need higher octane fuel to prevent pre-ignition.
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Old 10-22-2010, 04:44 PM
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Re: Another Hemi Octane Thread

Quote:
Originally Posted by Mogul View Post
Better wording would be "not necessary". Basically just that the timing curve that provides optimum performance as programmed doesn't need higher octane fuel to prevent pre-ignition.
I agree if that is the case then "not necessary" or "not required" would be better wording. I just have never heard of premium fuel "hurting" and engine or fuel system. Every fuel leaves some deposits I just wonder if in this engine if there is some evidence that higher octane fuels somehow leave greater amounts of deposits or whatnot.
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Old 10-22-2010, 04:54 PM
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Re: Another Hemi Octane Thread

the higher the octane the more pure the fuel. so LESS DEPOSITS the higher the octane.
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Old 10-22-2010, 06:22 PM
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Re: Another Hemi Octane Thread

The hemi, like the 4.7HO before it, is designed to run just fine on 87. It also has the ability to take advantage of 89 if you run it consistently. Either will work. The engines do not have the ability to take advantage of either 91 or 93 as they come from the dealer. In order to take advantage of the higher octane fuels, you would need to install an aftermarket tune. Running 91 or 93 in a hemi with the OEM tuning is simply a waste of money as the engine will use it as if it were 89.
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Old 10-22-2010, 06:30 PM
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Re: Another Hemi Octane Thread

Using a higher octane rating than recommended typically has absolutely no benefit. With today's gasoline, there is virtually no, if any, difference in the level of engine deposits for 87 octane vs. 91 octane. It's not a question of "worse gas" vs. "better gas."

The octane rating is tied to how much the fuel can be compressed before it ignites. 87 octane gas can handle less compression than 91 octane. Typically, "high performance" engines have higher compression ratios, which is why they require higher octane gas.

Premium (91 octane) gas is "not recommended" with the 5.7L V8 because it will not provide a benefit. If anything, it would provide decreased performance. It won't harm the engine, but will provide less performance than the recommended 89. If anything, you could potentially experience slight engine knocking on occasion.
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Old 10-22-2010, 08:13 PM
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Re: Another Hemi Octane Thread

(Agree with the above)
The 87 octane has more BTU (energy content) than 93 (or 91) octane and therefore, if the engine is designed for 87 octane, the 87 octane will yield the greatest horsepower. Period. Also, if the fuel you use has a high ethanol content, you'll be robbed of some horsepower from the lower energy content in that ethanol.

Higher octane fuels are designed for high compression engines (usually high horsepower engines) that utilize the higher octane to reduce detonation during the combustion process, detonation being an uncontrolled explosion rather than a steady burn expansion. The high output engines that require premium (91 or 93 octane) can run on 87 octane, but engine ignition timing is electronically reduced by knock sensors in order to prevent detonation; this results in less horsepower especially noticeable in small-turbo engines that utilize 10:1 or higher static compression ratios (i.e. the VW/Audi 2.0T).

Conversely, using higher octane in an engine design for 87 octane (the Pentastar V6) yields NO power increase and, due to the reduced BTU (energy) content in high octane fuel, actually will reduce the power that this lower-octane-designed engine can produce. In the case of the Hemi, it was designed for 89 octane fuel for optimal horsepower. I don't suspect much of a drop in power running 91, but it the 91 also won't hurt. If you run 87 in the Hemi, the engine's ignition management system will be constantly battling detonation which might be felt as a rough running engine under load when going into and out of the Hemi's econ mode.
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Old 10-22-2010, 09:12 PM
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Re: Another Hemi Octane Thread

This question comes up for every car that uses premium fuel (91 octane). The question is whether one can get by with a lower octane.
From what I have read from several articles, apparently it depends on the wording.
If the manual states an octane is required, then it is so.
If an octane is recommended, then basically that octane is suggested but other octanes may be used safely.
Apparently modern engine management can easily correct for octane.....so they say.
My '02 4.7L HO GC recommended 91 octane but also stated that 87 octane may be used with reduced power output. I never felt a difference or pinging in the 8 years I had it on regular.
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Old 10-22-2010, 10:04 PM
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Re: Another Hemi Octane Thread

Hi All, I am new here and just got my '11 Overland about 3 weeks ago. I went through the first tank that came with the truck came. Heard all kinds of ticking and clicking...so when it was empty as a test filled it with mobil 91.
It might all be in my imagination but I think it runs better...next fill up I will try 89 again and see what happens.
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