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Hi all, as some of you know I have a 2001 300m engine in my 2004 intrepid. Just noticed that the 300m manual says it needs premium unleaded fuel, while the intrepid says regular. Well which one should I go for? FYI I've been using regular all the time since the engine swap, did it do any damage to it?
 

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Hi all, as some of you know I have a 2001 300m engine in my 2004 intrepid. Just noticed that the 300m manual says it needs premium unleaded fuel, while the intrepid says regular. Well which one should I go for? FYI I've been using regular all the time since the engine swap, did it do any damage to it?
My 1998 manual suggests 89 Octane for the 3.2 and 87 for the 2.7.
My 2004 manual suggests 89 for the 3.5 and 87 for the 2.7.

I don't see any reason to pay for anything higher than 89.

No, you haven't done any damage. I'm sure that 99% of these
cars left on the road are being run with 87 or in some states I've even seen 85.
 

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My 1998 manual suggests 89 Octane for the 3.2 and 87 for the 2.7.
My 2004 manual suggests 89 for the 3.5 and 87 for the 2.7.

I don't see any reason to pay for anything higher than 89.

No, you haven't done any damage. I'm sure that 99% of these
cars left on the road are being run with 87 or in some states I've even seen 85.
Also, kind of off topic, but still in the octane rating nonsense. 300m special PCM's were tuned for 91 octane weren't they, compared to the regular 3.5L pcm which was just 89. That's why supposedly the special has a slightly higher output.
 

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Also, kind of off topic, but still in the octane rating nonsense. 300m special PCM's were tuned for 91 octane weren't they, compared to the regular 3.5L pcm which was just 89. That's why supposedly the special has a slightly higher output.
Octane has no bearing on power output. Power output has a bearing on Octane required however.
 

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my parents have had a 1999 300m since it was new and never ever have they put anything besides 87 octane in it...my 00' R/T has only run 87 in it for the last 12 years I've owned it....I tried putting in higher octane gas and it didn't change anything from power, MPG or how the exhaust smelled, nothing....I run the cheapest gas I can find and it runs consistent
 

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I believe the Owner's Manuals for 3.5 equipped cars specify standard 87 octane except for the Special which specifies 89 octane.

I'd have to look at both of my manuals but I'm almost certain that's the case.
 

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Ron is correct. The manual specifies 89 octane for 300M Specials, all other LH models require 87 octane. I believe the reason for this is the different fuel maps on the Special engine computer, but I could be wrong. I put 87 in my 2.7 and 3.2 Intrepids without issue. Most people stick with 87 on 300Ms, even Specials. I put 89 in my Special since it seems to run better. 87 in an Intrepid using a 300M engine is not a problem at all. Carry on!

Typically higher performance engines will require higher octane levels to avoid pre-ignition. It has nothing to do with making the power, just supporting it to avoid damage. Typically in high-performance V8 engines, as well as most turbocharged and supercharged engines. It doesn't apply to any LH car other than the Special, and even that is up for debate.
 

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But the higher octane will allow the PCM to have to retard the timing less under high load to keep the knock sensor quiet.

More advanced timing with higher octane fuel should get you more power because the piston starts pushing earlier and the power is released in a more controlled/usable manner without the earlier and higher peak pressure (of lower octane fuel) setting off the knock sensor.

It's the area under the pressure-time curve which creates the total power during a single power stroke. More area (total power) results from longer, more even pressure duration (from higher octane) than a shorter duration, higher-peak pressure curve (because it's that quicker-rising and higher peak that the knock sensor interprets as knock, which, again, retards the timing in future cycles, delaying the start of that nice, more even pressure curve).

The Special and non-Special engines are the same stroke, bore, and compression ratio, so pre-ignition should not enter into the discussion (of which grade of fuel to use). Pre-ignition is the fuel detonating (purely from pressure and temperature) before it sees the spark - hence the term "pre-iginition". Timing changes due to closing the loop on the knock sensors can do nothing for pre-ignition. A Special and a non-Special engine are no different as far as tendencies to pre-ignite with any particular grade of fuel.
 

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I just double checked my 2002 Trep manual and it clearly states 87 without regard to the engine size.

I know I read 89 in the 1998 manual for the 3.2 (but that's in Indy).

I wonder if I read 89 in one of the Chrysler 300 manuals (also in Indy).
 

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87 would be fine in a trep running a 3.5 HO, as long as you aren't running the special PCM. Then I would use 89
 

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Hmm interesting Wikipedia states the special was designed for premium (91+ octane). Sounds like that needs corrected!
Such a reliable source....

I've done the higher octanes, no performance increase, if anything, loss of MPG. (maybe I was on the gas too much to feel the difference).
 

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So far I've just kinda sat back and watched this thread. I was surprised to see the thread be labeled as regular vs premium and would have rather expected it to be 87 vs 89 octane considering premium is usually 91. Back when we had a local Kum & Go, they offered super unleaded in 89 and regular in 87. Between the two, last time I figured it up the cost of running super unleaded with 10% ethanol, the additional fuel needed (mathematically) didn't quite break even to the cost of running regular unleaded. However, when calculating my avg. miles per gallon I never noticed hardly any difference (my driving is very consistent with 96% being interstate miles 2 hours a day).

With that being said, I really didn't notice any significant difference in fuel economy even with the ethanol vs no ethanol, but I did find that the engine ran just a slight bit smoother when running on the 89 octane super unleaded (felt the vibration with my hand on the fender when judging the difference between the two). Anymore it doesn't really matter to me considering the Kum & Go was bought out by Casey's and their options are super unleaded 87, regular 87, and premium 91.
 

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The octane rating of a fuel indicates its resistance to preignition. Preignition occurs when the compression ignights the fuel before the plug fires. Preignition can cause catastrophic damage to an engine, usually a hole in the piston. The higher the octane rating the greater this resistance. The higher the octane rating the more fuel is used to make an equal amount of power. The knock sensors job is to retard the timing when it sences that preignition exists. The retarding of the timing is what sucks power from the vehicle.

You can use whichever fuel you like as your not knocking, the reccomended fuel Is the fuel that will maximize performance. In some engines you can't use a lower octane because it will cause knocking.

Many drag racers are turning to e85 because it is cheap compared to $9.00 a gallon race fuels and it has a high octane rating. We just went through converting a 9 second he i cuda last week.
 

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You have to distinguish between pre-ignition and detonation because, again, pre-ignition has nothing to do with the knock sensor and the resulting retarding of timing. If you are getting true pre-igntion, you could turn the spark off and the fuel would still ignite (and cause damage if repeated often enough).

On a given compression-power cycle, the fuel does not have a crystal ball whereby it can predict when the spark is going to occur - so it can't decide to ignite itself (without a spark) based on when it knows the spark is going to occur in the future (after it has already started igniting) - because it has no way of knowing that.

The octane rating of a fuel indicates its resistance to preignition. Preignition occurs when the compression ignights the fuel before the plug fires. Preignition can cause catastrophic damage to an engine, usually a hole in the piston. The higher the octane rating the greater this resistance. The higher the octane rating the more fuel is used to make an equal amount of power.
With you so far, except higher octane also slows down expansion when it does ignite from either cause (i.e., 1) pressure and temperature or 2) spark).

The knock sensors job is to retard the timing when it sences that preignition exists....
Pre-iginition no. Detonation - yes.

In normal ignition (i.e., the spark starts the fuel igniting), pressure rapidly builds in the cylinder - and that pressure further causes the fuel to "explode" faster - which is detonation - so the higher octane also helps prevent that by slowing down the rate of the fuel's igniting (preventing pressure from building up too rapidly before the piston is down further and some volume expansion has occurred).

So - again, ignoring pre-ignition (because it is not affected by timing), with lower octane fuel, when the spark sets it off, the pressure increase occurs too rapidly - before the piston travels down the cylinder a bit to allow expansion and reduce pressure peaking, and you get detonation - which the knock sensor picks up. Higher octane slows that rate of pressure build up, so you don't get the knock at the retarded timing setting - therefore the timing is advanced until the knock does occur, and that advance is starting the ignition earlier (than it would with the lower octane fuel).

The retarding of the timing is what sucks power from the vehicle
Yes.
 

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Here regular fuel's octane rating is 85! Premium is 87, and we don't have anything higher than that.
Ee also have "SP95-E10" which should be the same octane rating as unleaded 85. I'm using regular 85 or E10 everyday in my M without a knock. I tried puting 87 in it a few times but I couldn't feel any difference...

Anyway, just a dumb question, I've not been searching TBH but what about bioethanol in our cars? I'm using E85 when I can find a pump and for now the M doesn't mind. MPG decrease a bit but as E85 is half the price of regular 85... It still worths it!
Do you know the octane rating for E85.
 

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I don't think it is a good idea to use E85 in a vehicle that is not designated "Flex Fuel"
E85 can be from 51%to 85% ethanol and vehicles designed for it use special fuel system parts. Seals, orings, hoses and any lines that contact fuel are made of materials that can tolerate the high levels of ethanol.
10% does not appear to have an effect but I would be concerned about higher levels.
 

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Cool I just started another thread about this lol but I'm going to track this one as well. The manual said 89 Recommended so do you guys think that's just marketing for the gas companies or has relevance to the performance. I've got the 3.5 and the car has a tank of 87 so I'm gonna burn all of off and see if there is any difference. Marking Kms
 
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