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Discussion Starter #1
Hey everybody!

My car is a 2012 R/T plus w/ STP, 6 speed and daily driver. I am located in Western Oregon.
I've been running (and loving) the 93 tune for as long as I've had my tuner. I tried the 91 tune and measured a significant decrease in fuel economy at freeway speeds (around town was marginally different). Summer months are rolling around and I am debating switching to the 91 tune vs sticking with the 93.

I run premium only, always fill at the same station. My last car was far pickier, dyno tuned with 11:1 static CR and very aggressive spark timing, so I trust the fuel quality there.
Assuming that I keep running the 93 tune and notice a loss of power, will the ECU pull sufficient timing to protect the engine until I am able to play with the tune? I'm still fairly new to the Hemi world, so I will appreciate any input that comes my way.

Thank you!
 

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Hey everybody!

My car is a 2012 R/T plus w/ STP, 6 speed and daily driver. I am located in Western Oregon.
I've been running (and loving) the 93 tune for as long as I've had my tuner. I tried the 91 tune and measured a significant decrease in fuel economy at freeway speeds (around town was marginally different). Summer months are rolling around and I am debating switching to the 91 tune vs sticking with the 93.

I run premium only, always fill at the same station. My last car was far pickier, dyno tuned with 11:1 static CR and very aggressive spark timing, so I trust the fuel quality there.
Assuming that I keep running the 93 tune and notice a loss of power, will the ECU pull sufficient timing to protect the engine until I am able to play with the tune? I'm still fairly new to the Hemi world, so I will appreciate any input that comes my way.

Thank you!

Not sure what makes you think the engine will like 91 octane gasoline in the summer when it doesn't like it in the winter.



Not sure why you expect a loss of power with 93 octane in the summer. Any loss of power, and if real it will be slight, will be due to the higher ambient air temperature.


(I've driven my N/A Boxster from approx. 70F ambient temperature to 116F ambient temperature and have not noticed any real fall off in how the engine feels. Far and away the biggest contributor to the engine power falling off is a large change in elevation. From near sea level (on I-40 at the CA/AZ state line) to around 7700 feet above sea level (just west of Flagstaff AZ) and making this change in 140 miles and in around 2 hours' driving time the engine feels a tad down on power at the higher elevation. Nothing gruesome but it is noticeable. I have not had my Boxster much if any higher than that but with other N/A cars the power really starts to fall off at around 9K feet and at 11K feet one has to get out and push the car over the pass...)



If you do drop back to 91 octane, the engine controller can probably adapt -- assuming the tune has not put a limit on how much adaptation can take place -- but the engine's performance will fall off due to the retarded ignition timing. This has a side effect of raising exhaust gas temperature which subjects all components exposed to the exhaust gas to higher operating temperatures. Not what I would want to do to an engine at any time, let alone in summer.


Unless the documentation that accompanied the tune calls for you to drop back to 91 octane in the summer I'd just run 93 octane gasoline year 'round.
 

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The DS 93 octane tune probably runs a more aggressive timing curve and more WOT enrichment over the DS 91 octane tune.

It could probably give you a bit more timing advance at steady state cruise in 93 octane tune.

I used to live in Portland and the majority of the fuels were 92 octane in Portland, SW Washington on through the eastern side of both states - made numerous trips all through there.

I presume the area you're in is consistent with grades of fuel (92 octane availability).

In some other states or regions, 91 octane is the highest available. CA is one of those states, due to the reformulated gasoline requirements they have.
 

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Discussion Starter #4
Rockster,

No offense intended, but I believe you misinterpreted my post. I've been running premium only (including the time spent on the stock tune) from day one. As Hal H explained, Premium across Oregon is 92 octane. I should have included that in my original post. Power is not my chief concern, so much as longevity; I am willing to sacrifice a few lb/ft during the months where temps approach and top 90 degrees. The question is whether or not I should revert to the 91 octane tune, while continuing to utilize the 92 octane fuel.

Hal H,

That's what I was assuming about the tune as well. I have read claims of cars "falling flat on their face" during the summer months while using this tune, but I am loathe to trust anecdotal claims without supporting information such as datalogs or a significant difference in carefully logged fuel economy. The fuel down here is indeed labeled as 92 octane, however I had no trouble running it in one of my previous cars which was (presumably) far pickier than the Hemi, with 11:1 static CR and significant spark advance (I had a full standalone ECU and it was first street tuned by me, then dyno tuned roughly a year later).

As our Jeep was totaled last month, the Challenger is currently the only street legal car that we own. I just want to make sure that it is safe to run this particular tune into the summer, or (if need be) revert to the 91 tune until cooler months roll around.

Thank you both!
 

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I can't see why your mpg dropped with the 91 tune, especially running 92 gas. Using 92 with the 93 tune and then throwing in high temps is a recipe for disaster. I wouldn't run the 93 tune without 93 or higher gas , especially in the summer. Hemi's HATE heat and high temps definitely sap horsepower no matter stock or tuned. As a side note, If you're running a DS tuner the canned tunes kick the fans on earlier....
 

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Can't you just datalog and watch for KNK retard?
I had to mix fuel to about 95 octane to (almost) eliminate ST KNK retard on my 09 SRT Challenger in summer.
 

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Rockster,

No offense intended, but I believe you misinterpreted my post. I've been running premium only (including the time spent on the stock tune) from day one. As Hal H explained, Premium across Oregon is 92 octane. I should have included that in my original post. Power is not my chief concern, so much as longevity; I am willing to sacrifice a few lb/ft during the months where temps approach and top 90 degrees. The question is whether or not I should revert to the 91 octane tune, while continuing to utilize the 92 octane fuel.


<snip>


Thank you both!

No offense taken. You are right. Apparently, yes, I did misinterpret your post. Sorry.


Still not sure I understand. Reads like you elected for a 93 tune when 92 gasoline is all that is available.


Given the factory tune is good -- based on my info -- with 91 octane gasoline I would have just stayed with that and run 92 octane gasoline.

While the engine with the factory 91 tune is not able to take advantage of the 1 point higher octane the 1 point helps protect the engine should the nominal octane rating be a bit lower than 92 or should the engine's octane requirement go up due to for instance combustion chamber deposits.


My info is ambient temperature does not materially affect the engine's octane requirement until the temperature is quite elevated.

See attached.

I guess my recommendation would be to revert to the 91 "tune" and run 92 octane gasoline and not worry about having to deal with worrying about the gasoline from season change to season change.
 

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Even with 92 octane - on a mild day (70-80*F ambient)

I'd notice taking off from stop that you'll hear a momentary ping from 1 or 2 cylinders taking off from a stop (this is M6) when you have windows open.

My 5.7 (M6) would do this 10.3:1 CR and the 392 (10.9:1 CR) - so with the timing curves even with the stock tuning there's some ST KR - I had the iTune running live and would notice the instances I see this:

-accelerating from stop
-loading up engine at lower rpm (giving more throttle 1.5k <-> 2k rpm

The system can compensate for it, but the outcome is timing advance will pull to protect the internals so it will cost some power and potentially some MPG during those conditions.

When ambient temperatures get hotter, the potential for ST KR can be greater. At a miniumum, as per FCA - run 91 octane in the 392. Its recommend on the 5.7 (M6) for peak performance as well.

If you've been to the track on hot day - the old cars...they heat soak and often you could hear spark knock (detonation / pinging) take place.

Saw a number of 100* days in Portland over the years and everything ran slower times at the track
 

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The DS 93 tune does bump up WOT timing (spark dwell is bumped up as well) but it also modifies the knock sensor sensitivity (decrease threshold at higher RPMs) along with other tweaks to power enrichment, fans....ect. You would think they would bump up fueling in the higher RPM band but they actually pull back a little. They zero out the throttle spark rate which the stock table actually pulls timing during throttle up.

I have attached image comparing the 93 tune to my stock 2015 M6 RT tune, also added WOT and PE comparisons which show delta between the 93 and stock tune.
 

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Discussion Starter #10
Awesome, all of you guys!

I'll admit that it took a few minutes of review to get my bearings (It's been a WHILE), but I feel a little better about it now. I am a little surprised at how conservative the DS tune is with the failsafe, but it offers some relief to be certain. I'm going to give the 91 octane tune another go, but I'm going to datalog for a few days with the 93 before then. Today was the first chance that I've had to really open her up in a while, made an unnecessarily fast climb up one of the hills outside of town today when she was at ~190* oil temp, climbed to ~210 before dropping back down on the reverse. While I know that isn't the best indicator one way or the other, it's the only real benchmark that I had apart from the good old seat of pants "dyno".

I'll be curious to see the results of the next few days!
 
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