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Discussion Starter #1 (Edited)
I have a 2016 SXT Plus and I regularly track the MPG I'm getting by resetting Trip A every time I fill the tank.

I thought just for the heck of it I'd try premium and see if there was any difference. I have read and watched the articles that it is a waste to use premium in a car that only requires regular and I have always believed that and have evidence from previous cars that it make no difference in performance.

The problem I'm having is with my 2016 SXT Plus I have confirmed that I'm getting 3 MPG more when I use premium. The numbers are repeatable and consistent. I have even driven the same long distance route using regular and then premium (400 miles round trip).

Has any other SXT Plus owners found this or is this something unique with my car? The throttle response is also better when I use premium. At first I thought it was just my perception but over time I proved that it is what is happening.


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The Bacon Hauler (‘12 Cop Charger)
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Your stock tune is configured to run best on the mid-grade (89?) gasoline, but it will run just fine on the regular octane because the PCM will pull the spark timing when the knock sensors tell it the octane isn't high enough (which would lead to knock/pinging under acceleration). That's why you're seeing better MPG with the premium - the higher octane of the premium is basically what the engine is tuned to run on, even though it's a couple of octane points higher than what it needs.

So what you're seeing is the engine running at its peak performance, whereas with the regular octane, you're seeing it run at a little less than peak perf, even though there's no noticeable degradation from your butt dyno's point of view.

Depending on what octane the mid-grade gasoline is in your area, you should be able to run it instead of the premium and see the same +3 MPG over regular gasoline's MPG's numbers. If so, then running the premium truly isn't necessary because you won't get anything above and beyond what the mid-grade is going to give you, performance-wise.

Clear as mud? :nerd:
 

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The Bacon Hauler (‘12 Cop Charger)
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My last reply came out way more convoluted than I wanted, so I'm going to try again with this reply, but this time I'm coming with visual aids!! :eusa_dance:

:11:(I'm going to go into more detail than is necessary here; please do not construe this as me talking down to you. I'm just trying to provide all the backing info necessary to lend credibility to any claims made herein, as I know there may be others that might be reading this thread and need extra details.) :icon_smile:


So going back in time to when you were running regular gasoline (~87 octane) in your car, it felt like it ran just fine and had no problems with the gas, right?

Well, for the most part it did run just fine, except for any time you suddenly accelerated, like for instance executing a passing maneuver on the highway.

In that scenario, when you mashed on the gas pedal and went WOT (Wide Open Throttle), the engine's stock-configured tune was timing the spark plugs' ignition of the gasoline based upon the assumption you were running mid-grade gasoline (~89 octane). But since you were running the regular gasoline with lower octane, the air/fuel mixture had a tendency to pre-detonate, i.e. the gasoline in the mixture ignited and burned the air/fuel mixture before it was supposed to (before the spark plug had a chance to ignite it according to the preset timing configured in the stock tune) from compression alone (kinda like a diesel).

THIS_IS_BAD!!

It's called knock, and if it were left unchecked, it could lead to piston damage and other unsavory things you definitely do not want. When it's bad enough, it can sometimes be audibly heard from inside the car, and it will sound like BBs in a coffee-can - "ping, ping, ping..."

And that is a sound you do NOT ever want to hear because that means things are NOT going well inside your engine!

Luckily, you most likely will not ever hear your engine's knock/ping episodes due to the magic of knock sensors and your car's PCM. The knock sensors will "hear" that detonation taking place, alert the PCM, and the PCM will automatically pull some spark timing so that the gasoline stops igniting on its own (from compression instead of from the spark plug's spark) and starts igniting from the spark plugs like it's supposed to. This is usually just a temporary retardation of spark timing, just enough to arrest the knock, but it can be basically made semi-permanent if the episodes of knock are consistent and bad enough.

So back to your passing maneuver on the highway while running the regular grade gasoline...

During the time your engine is operating at WOT and likely experiencing a few knocks/pings, here's what it looks like from a graphical point of view:

NOTE: the white line is your accelerator position (the flat-lined part is WOT); the yellow line is your RPMs ; the green line is your MPH; the red line indicates Knock was detected.

Passing Lane 87.jpg

From that it is easy to see where your PCM detected some knock while in WOT and as a result it would have pulled spark timing a few degrees temporarily to mitigate it. And that's no good. Any time it pulls timing from its pre-configured marks, you are operating at less than optimal efficiency and thus there exists room for improvement (whether that be in terms of more MPG on a road-trip or a better ET at the track depends upon how you're operating the vehicle at the time in question).

So now let's do the same trip again, but this time you're running premium gasoline (~91 octane) like you did recently, and let's execute the same passing maneuver while driving on the highway:

Passing Lane 89.jpg

Now it is easy to see that the gasoline was not prone to pre-ignition/detonation, and it was able to wait until the spark plugs ignited per the pre-configured timing from the PCM's stock tune, and thus no KR (Knock Retard) was necessary or applied. That's good! Actually, given the PCM's current tune, that's the best you can hope for, i.e. the engine is operating at its peak efficiency, and thus will likely provide better MPG (or ET if at the track) when measured such as you did recently.

Further, the octane required to achieve 0 KR like that is likely a little less than the octane provided by the premium gasoline you were running. Most likely, the mid-grade gasoline's octane (~89 octane) would have been sufficient and yielded the same MPG increase as the premium gasoline did, but that's something you would need to determine yourself by running some mid-grade for a few trips and checking the numbers.


(In reality, with the advent of ethanol as the source of gasoline's oxygenation (instead of MTBP or something like that), the octane numbers you get at each fill-up may fluctuate slightly, and so mid-grade versus premium may not ever be truly settled for your car. It's possible that it may work out to where your car runs best on premium in the summer and can do just fine on mid-grade in the non-summer months, that's just something you'll need to figure out for yourself. And this is still when targeting peak efficiency; in reality it will operate "just fine" on 87 octane as you've already observed).


So again, you can run any octane you wish, and none of them will harm your car's engine. And unless you really pay attention and fine-tune your butt-dyno to recognize the slightest of differences in operation, you will not "feel" any difference between any of the octanes either. A difference will be quantifiable however, as you've seen, by measuring something like MPG, and you will know which gasoline grade is "best" for your car by which one produces the best numbers in your measurements, e.g. higher MPG equals better grade gasoline.

And as you've already experienced, the regular grade gasoline will provide the least amount of performance, which is due to its propensity for KR under acceleration, or something like this:

STKR Spikes.jpg

And as you have also already experienced, the higher octane gasolines will provide the best performance, which is due to its resilience to KR under acceleration, which ideally looks like this:

Proper Octane.jpg

Because any observed KR (by PCM) is going to equal lost HP/TQ to your engine, and that is bad no matter what you're trying to do. :deal:


Hopefully that served more to clear things up than add to any confusion...but please let me know if it had the opposite effect :huh:
 

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The Bacon Hauler (‘12 Cop Charger)
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I get 30 MPG on long trips SXT w/ STP option 20' Wheels
I love it. I only run 87 Gas but this is interesting maybe time to go to 89 Octane
I used to love to complain about the lack of acceleration from a dead stop of the 3.6L + A5 + 2.65 combo my 2011 Challenger had, but that was to be expected honestly. Supposedly the lack of get-up-and-go was offset somewhat by the great MPG attainable on long road-trips (~30-33MPG). Personally, I didn't find the good MPGs by themselves to be a good substitute for the lack of low-end fun.

HOWEVER, what I did find to be a suitable substitute for the lack of low-end fun (or as suitable as I could hope for given the situation) was the car's ability to still return very good MPG numbers on a road-trip (25-28 MPG) in which I drove it like I was an insane asylum escapee. I'm talking WOT every pass attempt, and never letting off until I hit 120+MPH. I'm talking waiting for 2 or 3 cars to line up before I even attempt to pass. I'm talking driving like I had lost my ever-loving mind - pushing the car to its limits in all respects.

...and still getting better than 25 MPG almost every time.

Now that wouldn't have been possible without running the necessary octane gasoline, so that's as given for all this.

I understand the 3.6L + A8 + (whatever rear-end ratio you have) combo runs better than the previous setup, but the same theory still holds - run the recommended octane to get every last bit of fun out of your car. :grin2:
 

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I'll play devil's advocate here. Is the price of higher octane gas going to be offset by a few more miles per gallon with a stock tune?

To the OP, those are Highway MPG's, correct? So that means in everyday driving, you wouldn't see such a big boost in MPG.

I just use the 87 octane that my manual says is fine. Here on the East coast, I can tell you my MPG will dip a couple of ticks when we switch to that winter aerated fuel. :frown:
 

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I started with 87 then moved to 89 and sometimes 91 when I go to Costco because they don't carry mid grade. I've noticed no difference between any of them. I only use mid grade for two reasons. One, I have a Ram R/T and it needs at least 89 so it's just habit. And two, it's so hot here I just figure mid is better for the engine.
 

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The Bacon Hauler (‘12 Cop Charger)
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Yep, in reality, there are so many other variables with this that it will be difficult to really notice a difference between running 87 and 89. The PCM is going to be able to make any difference hard to notice, and that's a good thing. What i went into detail about above is better thought of in terms of being able to extract the absolute maximum amount of performance from the system when operating under favorable conditions, which daily driven commutes don't normally fall under BTW.

If you get a tuner and start running hotter tunes, this sort of thing will be more useful for sure, which is why/how i ever learned it in the first place actually.
 

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The Bacon Hauler (‘12 Cop Charger)
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I started with 87 then moved to 89 and sometimes 91 when I go to Costco because they don't carry mid grade. I've noticed no difference between any of them. I only use mid grade for two reasons. One, I have a Ram R/T and it needs at least 89 so it's just habit. And two, it's so hot here I just figure mid is better for the engine.
Depending on the year model of the Hemi in question, it will absolutely run better (experience less KR) on the highest octane available in a desert climate like that. Hotter ambient temps by themselves will raise the octane requirement of an engine a point or two. Add to that a 2010 or older Hemi setup (bad design for IAT sensor - makes it very prone to heat soak), and you've got yourself a KR machine unfortunately. More octane is one of the easiest ways to combat that, though nothing can completely eliminate it in that type of setup, due to reasons which I won't go into detail on here but which are very real, very hard to eliminate completely, and very preventable with the right IAT design (as is evident by the 2011 IAT redesign Dodge did for all their speed density systems).
 

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Depending on the year model of the Hemi in question, it will absolutely run better (experience less KR) on the highest octane available in a desert climate like that. Hotter ambient temps by themselves will raise the octane requirement of an engine a point or two. Add to that a 2010 or older Hemi setup (bad design for IAT sensor - makes it very prone to heat soak), and you've got yourself a KR machine unfortunately. More octane is one of the easiest ways to combat that, though nothing can completely eliminate it in that type of setup, due to reasons which I won't go into detail on here but which are very real, very hard to eliminate completely, and very preventable with the right IAT design (as is evident by the 2011 IAT redesign Dodge did for all their speed density systems).
I was talking about my SXT. But, the heat issue is in fact one reason why I bought an SXT and not another 5.7. The heat and altitude here just sapped so much power from the Hemi, I couldn't see paying more for an engine that got nowhere close to rated 0-60 and they were 6-speed so I was always running 91 octane, the highest we can get here. I envy the sea level and cool air guys.


BTW, the 3.6 does not seem to suffer from the heat soak issue, so I'm very happy about that. When it was 118 here a couple weeks ago I noticed no difference from when I bought it in September. Good engine.
 

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Your stock tune is configured to run best on the mid-grade (89?) gasoline, but it will run just fine on the regular octane because the PCM will pull the spark timing when the knock sensors tell it the octane isn't high enough (which would lead to knock/pinging under acceleration). That's why you're seeing better MPG with the premium - the higher octane of the premium is basically what the engine is tuned to run on, even though it's a couple of octane points higher than what it needs.

So what you're seeing is the engine running at its peak performance, whereas with the regular octane, you're seeing it run at a little less than peak perf, even though there's no noticeable degradation from your butt dyno's point of view.

Depending on what octane the mid-grade gasoline is in your area, you should be able to run it instead of the premium and see the same +3 MPG over regular gasoline's MPG's numbers. If so, then running the premium truly isn't necessary because you won't get anything above and beyond what the mid-grade is going to give you, performance-wise.

Clear as mud? :nerd:
Really,very clear.
 

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The Bacon Hauler (‘12 Cop Charger)
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Really,very clear.
Impossible!

unless you suffer from the same mental afflictions as I do...:pillepalle:

in which case, I say, "welcome to the party!!" :elephant: :banana:
 

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Impossible!

unless you suffer from the same mental afflictions as I do...:pillepalle:

in which case, I say, "welcome to the party!!" :elephant: :banana:
That was one of the posts I didn't find anything I had to read over. Surprised I haven't run into you yet. I'm the one with the lamp shade on my head.
 

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3.6 + A8 + 3.07 Rear Axle. Gets up and goes fine. I don't hot rod around, but every freeway on ramp is fun time, get to hear the exhaust at WOT. Tunnels too :) I do mostly around town, and only a few miles. I get around 20 MPG. I did get 28 on a freeway trip, but there were still long stretches of stop and go/ slow roll. A Guy
 

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Discussion Starter #16
With my SXT Plus I've never seen better than an average of 27 MPG even with pure freeway driving. I will get periods of 30 or more MPG but it always averages to around 27. When I switched to premium it was the first time my average stayed at 30 MPG. I made the same trip in April and again I took the same trip last weekend. Tuesday I drove non-stop from San Luis Obispo to Los Angeles , approximately 200 miles and my gauge was only down 1/4 tank. My range said I still had a little over 300 miles more on the tank. I was pleasantly surprised. It will be interesting to see what my MPG will be like when they switch to the oxygenated winter blend.

Thanks Nuke. I appreciate your detailed explanation of what is going on.


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The Bacon Hauler (‘12 Cop Charger)
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With my SXT Plus I've never seen better than an average of 27 MPG even with pure freeway driving. I will get periods of 30 or more MPG but it always averages to around 27. When I switched to premium it was the first time my average stayed at 30 MPG. I made the same trip in April and again I took the same trip last weekend. Tuesday I drove non-stop from San Luis Obispo to Los Angeles , approximately 200 miles and my gauge was only down 1/4 tank. My range said I still had a little over 300 miles more on the tank. I was pleasantly surprised. It will be interesting to see what my MPG will be like when they switch to the oxygenated winter blend.

Thanks Nuke. I appreciate your detailed explanation of what is going on.


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With the 3.06 rear-end, 27 is probably right where it is supposed to be, and 30 would be hard to get all the time, but not impossible obviously.

Also, keep in mind that any new tires you put on the back that are different size from the OEs will have an effect on the final drive ratio - basically it's like swapping the gears in your rear-end for a different set. Now, it's going to be a minimal change, but it will change nonetheless.

For example, my Challenger came with a 2.87 rear-end and 225/60/18 tires. I put some 235/60s on the back for a bit, and while the size difference between the OE tires and those was minimal/negligible, it DID make a difference in my final drive ratio; effectively giving me a 2.65 rear-end instead of the 2.87 one.

And even that's not THAT big of a difference. Certainly not a big enough one to really sway my MPG one way or the other once all other variables are considered.

HOWEVER, I could most definitely tell a difference in the acceleration from a dead stop with the higher final drive ratio. Granted, the car was already pretty light on acceleration as it was, which probably made the change more noticeable. Either way, I'm just saying, if you ever replace the OE tires and decide to go wider or taller on the same rims, you'll affect your final drive ratio for sure, even if it's not necessarily noticeable from the driver's seat.

There's a formula you can use to figure out how much affect different sized tires will have on the final drive ratio, but I don't have it handy. It's bookmarked on one of the half-dozen computers I have around here somewhere though, so I can go find it if anyone wants to save it off for future reference.
 

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The Bacon Hauler (‘12 Cop Charger)
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Not All Gasoline is Created Equal

Not only should you always try to run the gasoline grade that will provide your engine with the necessary octane to prevent/avoid KR, but you will need to shop around and find the gasoline brands/locations which will consistently provide you with the expected quality for which you are paying. Here's what can happen if you don't:

Friggin Kroger Gas Sux.jpg

That, my friends, is a friggin' horror show compared to what it should look like, and usually does look like, when I run good quality gasoline. There are several things apparent in that screenshot that are of particular importance:

1. The mere existence of LTKR (Long Term Knock Retard) in that datalog is an unwelcome surprise, as it should not show up ever in my datalogs (because I've modified the tune and run the appropriate gasoline to prevent the possibility of LTKR, usually).

2. The sheer amount of STKR is staggering, especially considering there should be none, or maybe ONE spike of 2.0 degrees AT MOST, on a bad day.

3. The cumulative effects of all that KR can be quantified in this datalog by looking at the Vehicle Speed number at the point at which the graph is frozen - 121.0 MPH. Now I have made this exact same run many times before because it is a fixed distance that I know what my speed should be at the point at which I let off the accelerator. Previously, and consistently, that Vehicle Speed number would be about 131 or 132 at the same point, yet it is clearly much less in this instance.

The fact that a known output (Vehicle Speed) is showing significantly less than expected, is a direct result of the excessive KR (ST & especially LT) we see in the output. I would have to do a lot of fancy math to come up with an amount of HP/TQ all the KR sapped from me on that run, which I ain't gonna do, so I can't say exactly how much performance the crappy gas I was running cost me. But I can say that when operating at maximum output for an extended burst of time, the crappy gas ended up costing me 10+ MPH on a 55-135 run.

UN-ACC-EPT-ABLE!!!

However, it is still my fault, as I know this gas regularly under-performs what I expect of it (93 octane), and I only bought it this time because I was almost out and needed to use some reward card benefits before losing them. Convenience cost me in this case, a lesson which I've already learned, but continue to re-learn for some stupid reason.

-------------------------------------------------

So back to this thread. How does all this apply? Well, contrived as it may be, I think it serves to illustrate just how much effect you can have by running a gasoline with the proper octane versus one with less than needed but still enough for the PCM to deal with.

Q: Will your typical road-trip have drastically different results like this from one tank of 87 to the next tank of 89?

A: Nope. But if you want to maximize the amount of performance and mileage you can get from your car, run the required octane gasoline, and make sure it is of good quality as well.


Q: How do you know quality between brands/stations?

A: You don't...unless you can regularly datalog each different type and get a data set that will tell you.


Q: What if you don't have a tuner to do the datalogs or don't care to generate them from your tuner to attain that data set?

A: Run Top-Tier gasoline brands.


That's no absolute guarantee you'll get top quality, but from the data i've generated over the years with my (2!) Challengers and all the different brands of gasoline in Central, West, and North Texas, I can see that the odds of getting a crappy tank of gas are significantly lower with the Top Tier brands.

And when the occasional bad quality gas is stumbled upon, my data tells me there's a >75% chance it was a non-Top-Tier station that provided it.


Take all that for what you will, but your Challenger sure would appreciate it if you would lend it some credence.

YMMV obviously...
 

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Discussion Starter #19 (Edited)
I've noticed that my Challenger seems to have better performance (like throttle response) with some brands over others. Is the better feeling in throttle response an indication of better fuel for my Challenger? Even among top tier brands I've noticed a difference in throttle response.


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Discussion Starter #20 (Edited)
3.6 + A8 + 3.07 Rear Axle. Gets up and goes fine. I don't hot rod around, but every freeway on ramp is fun time, get to hear the exhaust at WOT. Tunnels too :) I do mostly around town, and only a few miles. I get around 20 MPG. I did get 28 on a freeway trip, but there were still long stretches of stop and go/ slow roll. A Guy


A Guy,

I know you put the Solo Exhaust and the AIRAID high flow engine filter setup on your SXT. After you did the mods did you notice any significant difference in the throttle response or was it mostly an improvement in sound?


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