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  #11  
Old 05-08-2012, 08:55 PM
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snrusnak snrusnak is offline
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Octane is the resistance to detonation(pinging). The higher the number the more resistance, and the slower the burn. You need more resistance to detonation with more compression and timing advance(high performance), and these newer engines are "high performance" in this way because they are trying to get more power and mpg out of them. So to get the full 390 hp out of your engine you need the 89 octane. On 87 octane it should produce less hp(slightly). It has nothing to do with "cleaner" fuel or anything like that. It's simply detonation/pinging related. Now don't worry, your engine will not detonate/ping on lower octane fuel because it has knock sensors that retard timing when detonation/pinging happens, and this just slightly lowers output.

To make a long story even longer:

87 OR 89 is fine. 89 just makes slightly more power and mpg on this engine. And it is SLIGHTLY.
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  #12  
Old 05-09-2012, 09:34 AM
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Thanks snrusnak, that's pretty well what i said, but in better terms, lmao, I'm so lousy when it comes to writing stuff for people to understand lol, But At least I try

Nicely put Snrusnak
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  #13  
Old 05-09-2012, 09:44 AM
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Yeah I honestly didn't even read the responses I just gave mine lol. I'm finding myself having much less time in the evenings to "play" on the computer but still trying to participate on the forum...
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Old 05-09-2012, 11:53 AM
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Quote:
Originally Posted by snrusnak View Post
Octane is the resistance to detonation(pinging). The higher the number the more resistance, and the slower the burn. You need more resistance to detonation with more compression and timing advance(high performance), and these newer engines are "high performance" in this way because they are trying to get more power and mpg out of them. So to get the full 390 hp out of your engine you need the 89 octane. On 87 octane it should produce less hp(slightly). It has nothing to do with "cleaner" fuel or anything like that. It's simply detonation/pinging related. Now don't worry, your engine will not detonate/ping on lower octane fuel because it has knock sensors that retard timing when detonation/pinging happens, and this just slightly lowers output.

To make a long story even longer:

87 OR 89 is fine. 89 just makes slightly more power and mpg on this engine. And it is SLIGHTLY.
I'll agree with you mostly but not about the part where 89 is going to give you more power. Thats too simple a statement. There is no more energy in 89 than 87. As you correctly stated the only difference is 89 is just a little more hard to ignite. If the knock sensors detect knock using 87 then it'll retard the ignition timing to correct which will result in less engine power and slightly lower fuel economy. But if you use 87 and never get knock then using 89 will gain nothing power wise. The truck can't tell the difference between 87 and 89 and will operate as designed if the combustion is correct.

I suspect why Chrysler recommends 89 over 87 is with this engine and the way the trucks are typically used (heavy loads, towing, WOT, etc.) 89 MAY be a better day-to-day choice. However if the truck is only used light duty then 87 is fine. Bottom line is use the appropriate fuel to how the truck is being used.

Last edited by pogster; 05-09-2012 at 12:01 PM. Reason: spelling
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  #15  
Old 05-09-2012, 12:15 PM
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^it has nothing to do with 89 having more energy than 87. The engine is designed with a certain amount of compression and a certain amount of ignition timing advance. These are two of the biggest factors in detonation/pinging and octane requirement. They design the engine to be able to get the benefits(more power and mpg) when using 89 octane through compression and timing, but to also be able to run on 87 without issue(with slightly less power and mpg). It probably is such a slight difference though that it may not even be noticeable.

Same concept as tuners. I run a 93 tune with 93 octane to get more power from my engine. There is not more energy in 93 octane than 87 octane, but there is more ignition timing advance with the 93 tune vs the stock tune, so I get A LOT more power, and slightly better mpg. I could still run on 87 octane with the 93 tune and the knock sensors would retard timing and it'd be just like running 87 octane on stock tune. It's not recommended to do this though.

Also you make a good point about the towing and what not. That is probably a factor in why they recommend 89 octane(more load = more likely to ping/detonate).
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  #16  
Old 05-09-2012, 12:18 PM
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Also, keep in mind that the engine will pick up knock(knock sensors that is) way before you will hear it. You may be running 87 octane and not hear knock, thinking you're getting the most from the engine, but I bet under heavy load and heavy throttle the knock sensors are retarded timing a few degrees. This may be why most don't notice a difference in 87 to 89 octane, because you wouldn't have any difference in normal day to day, light duty driving.

I ran the 93 tune on 91 octane for a while on my truck and monitored the knock sensor. It never retarded timing to and from work. Then when I started hot rodding and burning out, etc. I noticed it would pull up to ~5 degrees at times. Mainly at high rpm and heavy throttle. I never heard a singly ping though...
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