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Old 09-22-2010, 02:28 PM
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Question re Octane

Could someone tell me what the difference in performance would be between using 87 octane and 89 octane gasoline in the hemi engine? I recall asking the Jeep people this question about my 2005 WK and I was told 89 is recommended for optimum performance but 87 is acceptable. (Same thing the current owners manual states.) When I asked if 89 would improve my mpg compared to 87, I was told no. Is this correct?

Would higher octane lower the time it takes to go 0-60? Is this what is meant by "optimal performance?"

Is it worth using 89 octane?
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Old 09-22-2010, 02:34 PM
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Re: Question re Octane

you will get as many answers on one side as the other.

I PREFER to use a higher octane as it is typically a cleaner running fuel. it does PERFORM a bit better too, IMO.

if you goal is the best milage, i would think 87 is your answer.
if you want to get every bit of performance out of it, then 89 or higher.
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Old 09-22-2010, 02:42 PM
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Re: Question re Octane

I have only been able to find 87, 91 and 93. After using 87, I decided to try a couple tanks of 91. The only thing I have noticed is a drop in gas mileage, that may also be related to a heavier foot. Next tank will be 87 again.
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Old 09-22-2010, 02:43 PM
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Re: Question re Octane

The "octane" rating of a fuel is a measurement of a fuel's resistance to preignition. Lower octane fuels tend to ignite by themselves under compression- this is why high compression engines (high performance engines run a lot of compression to get more power from a given displacement), especially forced induction (supercharged, turbocharged) engines require higher octane fuels.

Preignition is BAD. While it's really only audible as a light "ping" or a ticking under acceleration, when the fuel ignites on its own it is lighting off in the wrong place in the cylinder. Basically, you get enough of that and you blow holes in your pistons (hello, new engine). Modern engines can sense pinging and dial back the timing in order to prevent this.

The thing is, when an engine computer pulls back on the timing, you lose power. This means less acceleration. It might be a very modest power loss, but it's still a power loss. By running a higher octane fuel, the engine computer can advance the timing, giving you the most power the engine has to give.

Now, not every engine can utilize high octane fuel- most just don't run enough compression to do it and won't advance the timing far enough for it to become an issue. The Pentastar V6 won't. Some engines will. If your engine isn't designed to run a high octane fuel, you won't get any benefit. Generally, you're best off running the highest octane your engine is designed to use when power is your priority. For the Hemi, that's 89 octane fuel, or mid-grade.
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Old 09-22-2010, 02:57 PM
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Re: Question re Octane

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Originally Posted by Technosavant View Post
...Now, not every engine can utilize high octane fuel- most just don't run enough compression to do it and won't advance the timing far enough for it to become an issue. The Pentastar V6 won't. Some engines will.
So what will happen if Pentastar is fed with 93 gas? Can it get damaged?
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Old 09-22-2010, 03:05 PM
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Re: Question re Octane

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So what will happen if Pentastar is fed with 93 gas? Can it get damaged?
Nope. Feeding an engine higher octane gas won't hurt a thing. There's a SLIGHTLY lower energy content the higher the rating, so there can even be a mileage hit (it may not be noticeable in every vehicle though).

Feeding it high octane fuel, however, is a waste of money. The engine won't benefit. I know that fuel merchants love to talk up how clean and wonderful their premium fuels are, but that's just marketing. Premium fuel is more profitable, since there's precious little difference between the premium and regular (mid-grade is nothing more than mixing the two... the gas station only has regular and premium in their tanks, when you select mid-grade the pump takes from both storage tanks and mixes them).

I just feed my vehicles what the engine is designed to have. My Jeep Patriot gets regular. My wife's Acura TL gets premium. When my Pentastar-equipped Grand Cherokee shows up, if ever, it will get regular. If it were to have a Hemi, it would get mid-grade.
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Old 09-22-2010, 07:21 PM
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Re: Question re Octane

For the past 8 years I have used both 87 octane (mostly) and 91 octane in my GC 4.7L HO and can't feel ANY difference.
I plan to use 87 octane in the Hemi.
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Old 09-22-2010, 08:39 PM
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Re: Question re Octane

The Hemi is designed to use 89 octane. The Hemi achieves maximum power and gas mileage using 89 octane.
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Old 09-22-2010, 09:05 PM
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Re: Question re Octane

Do a search for Octane on www.howstuffworks.com It does a good job of explaining the relastionship between octane rating and the compression ratio of the engine. If you have time go ahead and read the whole gas engine section. Then make up your own mind. Personally I use the recommended 89.
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Old 09-22-2010, 10:15 PM
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Re: Question re Octane

Quote:
Originally Posted by Technosavant View Post
The "octane" rating of a fuel is a measurement of a fuel's resistance to preignition. Lower octane fuels tend to ignite by themselves under compression- this is why high compression engines (high performance engines run a lot of compression to get more power from a given displacement), especially forced induction (supercharged, turbocharged) engines require higher octane fuels.

Preignition is BAD. While it's really only audible as a light "ping" or a ticking under acceleration, when the fuel ignites on its own it is lighting off in the wrong place in the cylinder. Basically, you get enough of that and you blow holes in your pistons (hello, new engine). Modern engines can sense pinging and dial back the timing in order to prevent this.

The thing is, when an engine computer pulls back on the timing, you lose power. This means less acceleration. It might be a very modest power loss, but it's still a power loss. By running a higher octane fuel, the engine computer can advance the timing, giving you the most power the engine has to give.

Now, not every engine can utilize high octane fuel- most just don't run enough compression to do it and won't advance the timing far enough for it to become an issue. The Pentastar V6 won't. Some engines will. If your engine isn't designed to run a high octane fuel, you won't get any benefit. Generally, you're best off running the highest octane your engine is designed to use when power is your priority. For the Hemi, that's 89 octane fuel, or mid-grade.
X2. Good explanation Technosavant.
I generally run 89 octane to let the computer advance the ignition timing to the maximum conditions will allow. When I have put in 87 octane, it is generally 1/2 tank or less, so the impact is not as great. Can't say definitively that I noticed a demonstrably significant difference between 89 & 87 octane for the little time that I checked.

Remember that octane rating now is (R+M)/2, which is the average between Research (R) and Motor (M) methods of testing fuels. Motor is generally lower than Research IIRC. Back in the day, Amoco White Gas was unleaded and 100 octane (M), so every high performance engine could use it.

FWIW, my 5.7L HEMI fuel management approach is:
1. 89 octane as a default choice.
2. 89 octane whenever towing (always);
3. 89 octane for highway driving;
4. If I do use 87 octane, try to refill tank after 1/2 - 5/8 is burned.
5. No reason to waste money on 91 (or higher) octane fuel. That $avings lets me put 110 octane racing fuel in my motorcycles (both of which need leaded, highest octane gasoline to run w/o pinging) at $5-$6/gallon.

My $0.02 worth.
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