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I know the owners manual for my 2013 Challenger R/T Classic 5.7 Hemi recommends usage of 91 Octane fuel. Has anyone ran 87 in theirs? If so what have been the issues if any? Just curious. In my 2007 5.7 Hemi truck I purchased new, I have used 87 octane in it most often without any problems thus far. It has a little over 35,000 miles on it.
 

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From the owner's manual:

"The 3.6L and 5.7L engine (with automatic transmission) is designed to meet all emissions regulations and provide satisfactory fuel economy and performance when using high-quality unleaded gasoline having an octane range of 87 to 89. The manufacturer recommends the use of 89 octane for optimum performance."

"The 5.7L engine (with manual transmission) is designed to meet all emissions regulations and provide excellent fuel economy and performance when using high-quality premium unleaded gasoline with an octane rating of 91 or higher."

Realistically, using 87 octane will suffice in most cases, however, I would recommend following the manual's recommendation. If engine conditions (manifold/cylinder pressure and cylinder head temperature) result in detonation the knock sensor will simply signal for a reduction in ignition timing (knock retard) to counter the detonation state. Such reduction is usually most prominent under high throttle/low rpm conditions and exacerbated by high cylinder temperatures. My suspicion for the 91 requirement associated with the 6-spds is for additional detonation protection under the conditions I listed, as gear control is at the discretion of the driver not the PCM/TCM. For automatics, the requirement drops to 89 octane and the transmission will downshift (even in auto-stick mode) if the rpm is too low for the given throttle (manifold pressure) condition.
 

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Always use what the manual states.

With that said, my 5.7 auto calls for 89 but the increased MPG going to 93 offsets the higher price and I have less ST/LT Knock which means I am getting the full rated HP/TQ.

If you go to a lower grade, and lose MPG, your not saving any money and have the potential to do damage. Its just not worth it IMO.
 

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Run at least 91 octane in your M6 R/T.

I ran 92 in my '09 and even then, there was slight spark knock taking off in 1st.

87 will cause timing to pull and you'll get less peak output and lower MPG.

Truck versions have different cam profile and tuning for their purpose. The Challenger M6 / 5.7 has more aggressive spark timing and requires higher octane fuel due to that.
 

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Focus on the larger picture and don't get too wrapped up in the knock retard readings. Some minor timing reduction to counteract light detonation is perfectly acceptable. If you're racing or operating the vehicle in a fashion where every single hp and ft/lb matters then adjust the octane level as necessary. Again, detonation is usually most prominent under high throttle/low rpm conditions where timing is typically actively reduced anyhow. If the advance is too aggressive for the condition the knock sensor will simply detect the detonation state and further refine (retard) the timing. As manifold/cylinder pressure is reduced, and rpm increases, timing is increased. For some engines, it's not uncommon to have upward of 50° of total timing at low throttle cruise...I ran 57° total (19° initial/22° mechanical/16° vacuum) in my 1978 Trans Am with the 6.6L 400 and an aggressive/custom solid lifter cam, however, this was only "all in" under moderate/high rpm and very light throttle. Continued operation under heavy detonation will result in connecting rod bearing, piston, and spark plug damage.

For those with the 5.7L (auto) and 3.6L, this is what Dodge has to say, as directly quoted from the owner's manual....

"The use of premium gasoline is not recommended, as it
will not provide any benefit over regular gasoline in
these engines.

Light spark knock at low engine speeds is not harmful to
your engine. However, continued heavy spark knock at
high speeds can cause damage and immediate service is
required."

Of course, the above applies to a stock engine. Run what the manual states and you'll be fine. After all, Dodge used the respective octane stated in the manual when they rated your engine's hp/torque value and backed it with a warranty.
 

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Focus on the larger picture and don't get too wrapped up in the knock retard readings. Some minor timing reduction to counteract light detonation is perfectly acceptable. If you're racing or operating the vehicle in a fashion where every single hp and ft/lb matters then adjust the octane level as necessary. Again, detonation is usually most prominent under high throttle/low rpm conditions where timing is typically actively reduced anyhow. If the advance is too aggressive for the condition the knock sensor will simply detect the detonation state and further refine (retard) the timing. As manifold/cylinder pressure is reduced, and rpm increases, timing is increased. For some engines, it's not uncommon to have upward of 50° of total timing at low throttle cruise...I ran 57° total (19° initial/22° mechanical/16° vacuum) in my 1978 Trans Am with the 6.6L 400 and an aggressive/custom solid lifter cam, however, this was only "all in" under moderate/high rpm and very light throttle. Continued operation under heavy detonation will result in connecting rod bearing, piston, and spark plug damage.

For those with the 5.7L (auto) and 3.6L, this is what Dodge has to say, as directly quoted from the owner's manual....

"The use of premium gasoline is not recommended, as it
will not provide any benefit over regular gasoline in
these engines.

Light spark knock at low engine speeds is not harmful to
your engine. However, continued heavy spark knock at
high speeds can cause damage and immediate service is
required."

Of course, the above applies to a stock engine. Run what the manual states and you'll be fine. After all, Dodge used the respective octane stated in the manual when they rated your engine's hp/torque value and backed it with a warranty.

Yes, if you are getting LIGHT KR (1-2 ST) then going to a higher octane is not necessary.

However, not one 5.7 that I have seen gets LIGHT KR with the recommended octane.

I have owned three '09+ 5.7 hemi's two were trucks and my current Challenger. Each one was recording 9-12 ST and I believe up to 3 LT KR on the recommended octane. Each degree of KR is an estimated 5hp so you can see how this does not qualify as light knock.

In each vehicle I witnessed a reduction of knock and increase to MPG when going to 93 octane and I am just one of many who have reported the same.

Even still I just recommend using what is listed in the owners manual like I stated in my first comment. But when you want to get into tuning or you are generally interested in performance I recommend logging your KR first, before getting into intakes/exhausts etc.
 

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Always use what the manual states.

With that said, my 5.7 auto calls for 89 but the increased MPG going to 93 offsets the higher price and I have less ST/LT Knock which means I am getting the full rated HP/TQ.

If you go to a lower grade, and lose MPG, your not saving any money and have the potential to do damage. Its just not worth it IMO.

Do you actually see an improved MPG by going to 93 instead of 91 ? I have seen the opposite. In fact, I did a 2 month study with my 93 Stealth R/T Twin Turbo a few years ago. For one month I used 91 octane and the second month I used 93. The MPG and the actual distance travelled with 93 was less than 91. The extra money and the reduced efficiency did not make sense to continue with 93.


Now, granted this is a different car than a 5.7 Challenger. I have done some tank to tank analysis with my Shaker 6sp. The trip odometer is reset at each fill up and the distance on the odometer is always higher with the 91.
 

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Do you actually see an improved MPG by going to 93 instead of 91 ? I have seen the opposite. In fact, I did a 2 month study with my 93 Stealth R/T Twin Turbo a few years ago. For one month I used 91 octane and the second month I used 93. The MPG and the actual distance travelled with 93 was less than 91. The extra money and the reduced efficiency did not make sense to continue with 93.


Now, granted this is a different car than a 5.7 Challenger. I have done some tank to tank analysis with my Shaker 6sp. The trip odometer is reset at each fill up and the distance on the odometer is always higher with the 91.
Havent tried 91. When I started logging on the Challenger it was getting up to 9 ST KR so I went strait to 93 and I was still getting 5 ST IIRC. Then I ran some octane booster to make sure it was not false knock and got a consistent 0 so I stuck with 93 and did a few things to reduce knock further. Once I made the manual changes I got a custom 93 octane tune.

If you are getting 1-2 ST and 0 LT with 91 then stick with it as those are acceptable numbers.

The MPG tracker on my 13 RT is always within 1MPG from the actual value and I believe the newer ones are even more accurate. If you consistently see less MPG with 93 then you may be getting light to no knock on 91, but it would be the first time I have seen/read of it happening.
 

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Yes, if you are getting LIGHT KR (1-2 ST) then going to a higher octane is not necessary.

However, not one 5.7 that I have seen gets LIGHT KR with the recommended octane.

I have owned three '09+ 5.7 hemi's two were trucks and my current Challenger. Each one was recording 9-12 ST and I believe up to 3 LT KR on the recommended octane. Each degree of KR is an estimated 5hp so you can see how this does not qualify as light knock.

In each vehicle I witnessed a reduction of knock and increase to MPG when going to 93 octane and I am just one of many who have reported the same.

Even still I just recommend using what is listed in the owners manual like I stated in my first comment. But when you want to get into tuning or you are generally interested in performance I recommend logging your KR first, before getting into intakes/exhausts etc.
Just because the knock sensor is detecting and retarding the timing does not necessarily result in hp/tq loss, however, continued operation in the detonation state will net power loss. The retard is simply a way of scaling back the timing to achieve the optimum advance for the given condition and terminate the detonation state induced by the over-advanced condition. Greater timing advance does not always equal more hp/tq. The optimum advance is determined by the manifold pressure/dynamic compression ratio, air/fuel ratio, cylinder temperature, etc. The PCMs constant ignition and fuel/air ratio adjustments are simply to obtain maximum cylinder pressure without spontaneously detonating the air/fuel mixture (detonation). One of the possible reasons for your KR value is simply because you live in Texas, where the hot climate lowers the detonation threshold because of the higher intake and cylinder temps. The only definitive way to determine if the timing retard is hampering maximum hp/tq engine is with a dynamometer. In your case, running a higher octane "may" push the detonation threshold back up under your given conditions, but anything without tangible results is pure conjecture/speculation. If you're experiencing abnormal operation or a noticeable power reduction during the KR process there may be other areas you need to address.

Similar to RBOrrell, I did not experience any "noticeable" gains when using a higher octane than recommended in my 5.7 A5 (used 91 when 89 was not available) and mpg dropped slightly (.1-.2) when driving the exact same route with the higher octane--this is at sea level at a maximum outside air temperature of approximately 70° (Alaska) on a 10,000-mile engine.


For those interested in the details of the knock sensor's operation, below is a direct cut and paste from the 2014 5.7L Dodge Challenger Mopar Tech Authority manual:

Theory of Operation

Knock is the spontaneous auto-ignition of the remaining fuel/air mixture in the engine combustion chamber that occurs after normal combustion has started. It can occur under extreme vehicle operating conditions such as;

•excessive spark advance for the given engine operating conditions
•high engine temperature
•high MAP
•low humidity and heavy loads to the engine

Severe, continuous knock may be caused by any of the following, but not limited to;

•carbon deposits
•bad gasoline and/or low octane fuel

Engine power is maximized when the spark advance is calibrated to the maximum torque point or the knock limited point. In order to calibrate spark advance as close as possible to its optimum point, a knock feedback system is used to protect the engine from damage.

The analog output voltage of the Knock Sensor represents the strength of the engine knock. If the engine is running, Knock Sensor voltage will not be at zero, even when knock is not present. This is due to the background noise of the engine. Background noise varies from engine to engine depending upon;

•age
•temperature
•speed
•load
•and other engine operating conditions

An adaptive background noise curve is established by learning the knock voltage versus engine speed under non-knock conditions. When the engine is operated under high load conditions where knock is possible, the knock voltage is tested to decide if it exceeds the knock voltage threshold. Knock has occurred when the knock voltage is at or above this calibrated voltage threshold.

When knock is detected, short term spark advance is reduced by a calculated amount. The amount of spark advance reduction is based off a calibrated severity of the knock event. This reduction in spark advance is used in the next ignition event to prevent further knock events. If the knock continues, an additional reduction in short term spark advance occurs. When the knock stops, the short term reduction in spark advance is ceased. Once ceased, the long term knock compensation is reduced by a calibrated amount to recover some of the reduction in spark advance. This decreases the amount of short term spark advance reduction to optimize engine performance.
 

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hummmmm....age 55, many cars, had 3 challengers so far since 2010......a 2010 se, a 2015 sxt, & now a 2016 rt....2016 rt brakes suck!....the other ones great!...gas, sorry i always got the cheap shet...rt i use da middle shet.......going back too da cheap shet!!!......will say da rt is a great ape!...little shet gets!...sometimes too much!....maybe going back to a V6 with all wheel drive!....2018 look out!....maybe i'm getting old!.....lmao!
 

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hummmmm....age 55, many cars, had 3 challengers so far since 2010......a 2010 se, a 2015 sxt, & now a 2016 rt....2016 rt brakes suck!....the other ones great!...gas, sorry i always got the cheap shet...rt i use da middle shet.......going back too da cheap shet!!!......will say da rt is a great ape!...little shet gets!...sometimes too much!....maybe going back to a V6 with all wheel drive!....2018 look out!....maybe i'm getting old!.....lmao!
...forgot to add I'm getting a flux capacitor drive!:cheers:
 

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Depends on what you are doing. In AZ we have crap gas 85, 87, 91. For years I ran 85 and no issue. You wont have an issue running that low octane unless you are looking for peak performance. Can go full throttle sometimes but don't run the thing hard every day. When I did my cam I swapped to 91 and it is damn expensive over here.
 

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"Just because the knock sensor is detecting and retarding the timing does not necessarily result in hp/tq loss, continued operation in the detonation state will net power loss"

"Engine power is maximized when the spark advance is calibrated to the maximum torque point or the knock limited point. In order to calibrate spark advance as close as possible to its optimum point, a knock feedback system is used to protect the engine from damage."

Exactly... If you are consistently getting ST/LT KR due to detonation, you are losing power. That's why all of the pro tuners will tell you that 1-2 ST KR is acceptable, above that and you want to work to reduce it.

"Similar to RBOrrell, I did not experience any "noticeable" gains when using a higher octane than recommended in my 5.7 A5 (used 91 when 89 was not available) and mpg dropped slightly (.1-.2) when driving the exact same route with the higher octane--this is at sea level at a maximum outside air temperature of approximately 70° (Alaska) on a 10,000-mile engine."

If you know that much about knock sensors then you should also know that just because you dont "notice" something doesnt mean its not happening. Its said that you dont notice any power loss/increase until its above 20 or so HP. A MPG drop of .1 or .2 MPG is within a margin of error.

So what you are really saying is that you didnt record any MPG increase when moving up octane. But what were your ST/LT KR numbers on each octane? That would be more useful data.
 

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Depends on what you are doing. In AZ we have crap gas 85, 87, 91. For years I ran 85 and no issue. You wont have an issue running that low octane unless you are looking for peak performance. Can go full throttle sometimes but don't run the thing hard every day. When I did my cam I swapped to 91 and it is damn expensive over here.
I didn't know any refiner was still producing 85 octane!?! Even that crap pinged in the low compression engines in the 70s (GM had some engines between 7.6 - 8:1 CR in that era)

As others mentioned, an A5 / 5.7 only requires 89 going to 91 won't benefit unless your environmental factors (high ambient temps, quality of gasoline in region, hills / grades) indicate that 89 isn't adequate.

The M6 5.7s have more aggressive timing curves, that's why the power ratings for those a slightly higher over the A5 version and have 91 recommended.
 

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"Just because the knock sensor is detecting and retarding the timing does not necessarily result in hp/tq loss, continued operation in the detonation state will net power loss"

If you know that much about knock sensors then you should also know that just because you dont "notice" something doesnt mean its not happening. Its said that you dont notice any power loss/increase until its above 20 or so HP. A MPG drop of .1 or .2 MPG is within a margin of error.
Which is why I stated nothing is definitive without tangible results (those from precision engine output measuring equipment). I agree, .1-.2 MPG is within the margin of error, however, both times I ran a higher grade than stated in the owner's manual I experienced the same general decreased value, not an increase. Anything below WOT and the PCM is simply adjusting for maximum fuel economy and uses the knock sensor as a facilitator to achieve such. Comparative to the vacuum advance found on older engines, the PCM is feeding in the appropriate amount of timing to achieve peak efficiency under light load/high rpm and removes the additional advance as manifold pressure increases/vacuum decreases...think of this a fuel economy advance. At WOT, the PCM is advancing the ignition with rpm and the knock sensor simply trimming the timing curve to achieve peak efficiency, however at the interest of maximum power output...think of this as power advance. Momentary spikes in KR values are normal as the PCM adjusts to the instantaneous throttle changes; the PCM can't predict your throttle inputs, and as such, operates in a reactive state when the pre-programmed advance baseline is insufficient for the given conditions. If you're experiencing consistently high KR values at CONSTANT throttle and rpm settings then you may need to address other engine issues or possibly move to a higher fuel grade. Also of note is any single cylinder can trigger a KR input. No two cylinders operate at the same cylinder temperature or air/fuel ratio; you have hot and cold, lean and rich, cylinders based on their location, cooling, and other factors (such as carbon buildup). It's entirely possible one problematic (or lean) cylinder is inducing detonation before the others. Again...the larger picture of how it all comes together. Tuning an engine for maximum hp/tq output based on KR values alone is a wasted effort. Yes, the data can be useful, but track times and dyno numbers are the only true discriminators of engine tuning gains/losses and MPG/emissions output values the only discriminators of fuel efficiency gains/losses.

So what you are really saying is that you didn't record any MPG increase when moving up octane. But what were your ST/LT KR numbers on each octane? That would be more useful data.
IF the timing was knock retarding with the 89 and hampering fuel economy/power then there should have been some mpg or power increase when using the 91, of which there wasn't...it was a wash at best. Sine 99% of the driving was at less than WOT, the MPG values serve as the efficiency measuring tool. This brings me to the final and main point of this conversation (and my last reply): Unless you're chasing every tenth or hundredth of a second for racing purposes, expelling the effort and expense to locate and purchase fuel grades above the manufacturer-listed octane requirement for the [stock] engine is frivolous, and even addressed in the Owner's Manual as I posted earlier. The manufacturer tested the engine in a large array of conditions with very precise equipment to determine the octane value they stated. I'm not trying to induce an argument, merely broaden the picture and provide some technical information for others reading this and wanting to further understand the inner workings of their Challenger.

Bottom line...for a stock engine run the octane level listed by the manufacturer in the Owner's Manual.
 

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"Bottom line...for a stock engine run the octane level listed by the manufacturer in the Owner's Manual."

Agreed, this is what I stated in my original post.


"Momentary spikes in KR values are normal as the PCM adjusts to the instantaneous throttle changes. "
"If you're experiencing consistently high KR values at CONSTANT throttle and rpm settings then you may need to address other engine issues or possibly move to a higher fuel grade. "

So either you agree with me, or your comment is saying that 9-12 ST and 3 LT KR is not a high KR value. Thats what I take from your comments. I feel we are saying the same thing but maybe were getting hung up on something I am not seeing..

I dont believe anything you have posted contradicts what I have posted, just that I am saying 1-2 ST and 0 LT KR is the acceptable range and above that you should look into reducing it.

Momentary spikes of 1-2 KR are normal and are no issue. Constant spikes of 9-12 ST which result in LT KR are high (I believe I have read 14 degrees is the max the system can adjust for), and should be addressed. Everyone I have seen post their results of running on the recommended octane get well above 1-2 KR and in my case, on all 3 of my 5.7 Hemi's I have seen a MPG increase when moving from 89 to 93.
 

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Discussion Starter #17
Thanks for all the interesting feedback. I haven't ran straight 87 in my Challenger R/T 6 speed but have added a quarter tank of it to a half to more tank of 93. I haven't noticed any knock with that.
 

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I have the 5.7 5A and run nothing less than 89. Have tried 91 and also 93 and saw no difference in mpg or performance. I did run a full tank of 93 real gas no ethanol and did see almost a 2 mpg more……but the HIGH price of that stuff wasn't worth it plus it was harder to start, found out later that they added stuff I think in winter and thats why for the harder starting, brother in law hauls the fuel and told me.
 

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I've only run 87 octane for the past 3 years in my 2014 RT Plus/ auto and haven't noticed anything. No knocks, pings or anything. I used to have a 87 Oldsmobile Cutlass Supreme 307 ci V8 and ran only 91-93 Sunoco back in the day when gas was cheaper. My mechanic told me to not bother with the more expensive higher octane and started running only 87 octane and I started to get 50 miles per tank better !
 
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