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Unread 08-27-2014, 09:41 AM   #1
ivorycruncher
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Fuel economy boost with premium?

I have a 2011 Liberty, which I bought new,and fitted with a K&N CAI and Dynomax catback exhaust very shortly after taking delivery. I also had a very similar setup on a 2008 Liberty. This combination of mods has typically netted me 1-2 mpg of fuel economy increase on the highway, but not much effect for city driving.

As it stands right now, I can typically get 20-21 mpg with pure freeway driving (~70 mph), and up around 23 mpg for regular highway driving (~60 mph). In-town is lousy, generally 12 mpg or less, usually less. FYI, these numbers are according to EVIC info.

I recently decided to switch to premium fuel, filling up after the low fuel light came on on the dash, just to see what would happen. It's either 91 or 92 octane, but I don't know whether it's ethanol-free or not (most likely not). I live in Minnesota, where almost everything has ethanol. Only select gas stations carry premium without ethanol, and generally advertise it as not for use for anything other than small engines and collector cars, and there was no such warning where I filled up.

With this change in fuel, I was a bit surprised to see that I managed to squeeze out 23 mpg on the freeway, and even more surprisingly, so far am seeing 13-13.5 mpg in-town. There's a lot of nay-sayers out there that claim there's absolutely zero benefit to running premium fuel in an engine rated for regular 87 octane gas, and I have somehow just proved otherwise, at least for my specific situation. Not only that, but the engine seems to run smoother, and even a bit quieter at cruising speeds, as if it isn't laboring as hard.

I'm just wondering, though, does the fact that I'm running premium in conjunction with the CAI and catback exhaust make any difference? Would I not see the increase in mpg without those mods? One way or another, the numbers don't lie, so I think I'm going to continue running premium for now. I've done the math, and with a 2 mpg increase, I will actually end up spending less on fuel, as the fuel savings outweighs the increase in cost per gallon. I'm just curious just exactly why I'm seeing this increase.

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Unread 08-27-2014, 10:02 AM   #2
Gageraid
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I'm not gonna bash ya for this, but it DOES say in our manual to only use regular 87 octane.
If you run a Hypertech programmer (which you can't cuz yours is a 2011) you can tune it to run on Premium safely.
I am curious of those mpg numbers since I'm always on the hunt to figure out our Libbys a little more.
Try calculating at the pump next time. Gallons used to fill divided by miles driven.
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Unread 08-27-2014, 11:47 AM   #3
ivorycruncher
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Hmm, good to know, but I would need to see some actual scientific evidence to back up the claim that it's unsafe to run on premium fuel just because it isn't specifically rated for it. Aside from the numerous claims that it offers zero benefit, the only claims I see regarding octane levels and harm they can cause to an engine is if you use a lower octane fuel than what the engine requires, as that can lead to premature detonation (i.e. pinging/knocking). Running an octane level higher than the recommended minimum should never harm an engine.

And I honestly had doubts too about any benefits, but a 2 mpg increase seems to be a pretty clear benefit, especially in city driving. Believe me, I drove the 2008 Liberty for a bit over 3 years, and the 2011 for another 3 years, and EVIC accuracy aside, I have never seen 13.5 on the EVIC for in-town driving, on either one. In the best circumstances, maybe in the 12's. Of course so far it's only been 3 days of in-town driving, so obviously a longer test period is needed to prove the trend, but so far I'm optimistic. I still suspect the improved air flow with the CAI and catback are part of the equation though.

I'll try to remember to calculate manually too, but will have to start after next fill, as I don't know what the starting mileage was on the current take. FWIW though, when calculating manually, I've found the EVIC's reported average mpg to typically be around 0.7-1.0 mpg on the generous side, so I expect that trend to continue, regardless of the increase I'm seeing.
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Unread 08-27-2014, 12:12 PM   #4
josworth
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Well Ivory, I'm sorry but I just have to post into this.

#1... 2 to 3 mpg difference over a short period of time is not really valid. Your MPG can vary with just a formulation change at the pump. You would need to run it like that with multiple fuel sources and in a much more controlled environment.

#2... The manual is correct...but not for the reasons you think. If you feel you are getting value from running 91 or 92 octane, go ahead and run it to your hearts content. You will not hurt anything... back a decade or so, you could cause issues with carbon build up, but those days are long gone. That said, the PCM does not have the ability to compensate enough for the higher octane to see any benefit.

#3... The only way to get the benefit of higher octane in an engine like the 3.7 is to reprogram it. Now some variable valve timing engines can "tune" themselves to various octanes, but those normally are in vehicles specifically tuned to run premium... so the converse is true there. You can run 87 but you will impact performance as the engine de-tunes itself for the lower octane. (my iVtec Honda actually spelled that out in the basic owners manual).

Again, run premium if you wish, but with all I have learned in the past decade about engine tuning, I personally will not.
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Unread 08-27-2014, 12:23 PM   #5
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Quote:
Originally Posted by ivorycruncher View Post
...if you use a lower octane fuel than what the engine requires, as that can lead to premature detonation (i.e. pinging/knocking).
Mostly corrected in the past 5 to 10 years........

But you do run the risk of cylinder residues due to the PCM overcompensating and pulling back the timing..

So, MOST will stop knocking, but also do not combust the fuel properly..

So, this is where it gets a little strange. In my post above I noted that running premium in a regular octane vehicle used to be bad.....

Well, it was sort of the same thing... only you would get carbon build up in the EGR system due to poor combustion and "soot" being expelled because the PCM couldn't compensate enough for the high octane. (It almost voided a warranty back in 1991 on my GF's car)

Since then advances have helped to fix the 92 octane in an 87 octane engine but most 92 octane engines will still have adverse issues when running 87 octane.
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Unread 08-27-2014, 12:33 PM   #6
ivorycruncher
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I appreciate the feedback, and yes, I do realize that more testing is necessary for scientific proof. I just wanted to point out that to my credit, I have filled up at numerous gas stations across several states, but never premium, and after more than 6 years of that, I have never seen 13.5 average in town, and certainly not 23 on the freeway at 70 mph. Ultimately I will run premium for a little while and see whether I can debunk this or not. My original question was more in regards to whether or not, assuming the results are accurate, the intake and exhaust mods had anything to do with it.
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Unread 08-27-2014, 04:15 PM   #7
tjkj2002
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Originally Posted by josworth View Post
#2... The manual is correct...but not for the reasons you think. If you feel you are getting value from running 91 or 92 octane, go ahead and run it to your hearts content. You will not hurt anything... back a decade or so, you could cause issues with carbon build up, but those days are long gone. That said, the PCM does not have the ability to compensate enough for the higher octane to see any benefit.

#3... The only way to get the benefit of higher octane in an engine like the 3.7 is to reprogram it. Now some variable valve timing engines can "tune" themselves to various octanes, but those normally are in vehicles specifically tuned to run premium... so the converse is true there. You can run 87 but you will impact performance as the engine de-tunes itself for the lower octane. (my iVtec Honda actually spelled that out in the basic owners manual).

Again, run premium if you wish, but with all I have learned in the past decade about engine tuning, I personally will not.
Little of what you typed is true and some not.

Any vehicle model year '96 or newer has the ability to run any gas(from 85 octane to 93 octane) with little to no issue regardless of what is recommended.Can it help or hurt? Yep,both ways sometimes.The higher the octane the higher resistance to flash burning(pre-ignition) and if you go to high you will not burn all the fuel.Well OBDII can adjust the ratio so your not wasting fuel so yes it can lead to better mpg's but the downside you will loose some power.Just like a dirty air filter can and will increase mpg's(will never degrease mpg's till past 85% clogged) but reduce power for the same reason.

Then there are other factors like altitude,weight,and how you drive.They state you need less octane at higher elevations,not my Jeep with the 3.7,needs more octane since it's over 2000lbs heavier then stock,plus it states in the manual to use higher octane gas when towing and I'm basically towing 2000lbs+ all the time.Then you get my '03 Town Car,states to use 87 octane regardless of altitude due to engine damage,yep stated right in the owners manual(also states entire warranty is void on engine if you use a oiled air filter).
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Unread 08-27-2014, 05:15 PM   #8
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Quote:
Originally Posted by tjkj2002 View Post
Little of what you typed is true and some not.

Any vehicle model year '96 or newer has the ability to run any gas(from 85 octane to 93 octane) with little to no issue regardless of what is recommended.Can it help or hurt? Yep,both ways sometimes.The higher the octane the higher resistance to flash burning(pre-ignition) and if you go to high you will not burn all the fuel.Well OBDII can adjust the ratio so your not wasting fuel so yes it can lead to better mpg's but the downside you will loose some power.Just like a dirty air filter can and will increase mpg's(will never degrease mpg's till past 85% clogged) but reduce power for the same reason.
So basically what you're saying is that the computer will adjust the fuel mixture so that it uses less fuel, in order to make sure that all the fuel that is provided gets burned properly, thus increasing fuel economy, but in doing so, horsepower drops because there's less fuel actually being burned (smaller bang = less power). That sorta makes sense.

However, I have not, as of yet, noticed any significant difference in power, regardless of whether it's more or less. The vast majority of my time is spent poking around town though, and I pretty much never tow anything, so losing a couple horsepower is not too big a deal. Will see if I still feel the same way after having more time to drive in more scenarios, and see if the numbers hold up.
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Unread 08-27-2014, 06:08 PM   #9
josworth
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Quote:
Originally Posted by ivorycruncher View Post
My original question was more in regards to whether or not, assuming the results are accurate, the intake and exhaust mods had anything to do with it.
Well, along the same path... that debate rages on.... lets just say based on my experience and research the simple answer (without retuning the PCM) is no.

Quote:
Originally Posted by tjkj2002 View Post
Little of what you typed is true and some not.

Any vehicle model year '96 or newer has the ability to run any gas(from 85 octane to 93 octane) with little to no issue regardless of what is recommended.Can it help or hurt? Yep,both ways sometimes.The higher the octane the higher resistance to flash burning(pre-ignition) and if you go to high you will not burn all the fuel.Well OBDII can adjust the ratio so your not wasting fuel so yes it can lead to better mpg's but the downside you will loose some power.Just like a dirty air filter can and will increase mpg's(will never degrease mpg's till past 85% clogged) but reduce power for the same reason.

Then there are other factors like altitude,weight,and how you drive.They state you need less octane at higher elevations,not my Jeep with the 3.7,needs more octane since it's over 2000lbs heavier then stock,plus it states in the manual to use higher octane gas when towing and I'm basically towing 2000lbs+ all the time.Then you get my '03 Town Car,states to use 87 octane regardless of altitude due to engine damage,yep stated right in the owners manual(also states entire warranty is void on engine if you use a oiled air filter).
Based on that where am I wrong???

I didn't get into details but I did say essentially the same thing...

Two low an octane in a car tuned for premium will get you knock and the PCM will compensate. Degrading performance. Older cars did not have the range to compensate and knock could cause damage. (my 85 Dodge Turbo)

Put high octane fuel in a car designed for regular and normally you just won't see a gain that warrants the expense. Now in extreme cases you will get build up from un-burnt fuel. I mentioned that previously as well.

I understand that some engines tuned to run on 87 will benefit from a higher octane when placed under extreme loads. That also demonstrates the limits
of that engine's PCM where loads produce a knock scenario using 87 octane that the engine can't compensate for.

One of the neatest things I've seen in a relatively modern set up is like in Hondas with Vtech where their PCM has enough flexibility that it actually rates HP numbers for both 87 and 92 octane. But that gets into the sophistication of computer controlled variable valve timing.

Also, OBD II and PCM programming are actually two different things... OBD II (On Board Diagnostics Two) is a protocol that dictates what is readable through standard codes and what is transmitted. Basically everything has to send to a reader a set of measurements and report faults in a universal code format. Just about every car company goes beyond the mandate. In an effort to increase both economy and power they have the PCM control more than the US Government standard of OBD.
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Unread 08-27-2014, 07:46 PM   #10
tjkj2002
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Quote:
Originally Posted by josworth View Post



Based on that where am I wrong???

.
Quote:
The only way to get the benefit of higher octane in an engine like the 3.7 is to reprogram it
Not true at all,stated already.

Quote:
the PCM does not have the ability to compensate enough for the higher octane to see any benefit
The limits is not why,the PCM will know when it's under heavy load and if using higher octane gas it can richen the mixture to help keep the combustion temps lower to help emissions(NOX mostly).

Quote:
Now in extreme cases you will get build up from un-burnt fuel.
Not in a engine built and sold in the US in the last 18 years.

Quote:
Also, OBD II and PCM programming are actually two different things
While yes technically one dictates the other through having to meet requirments.The actual programming maybe different protocol's(OBDII is not a protocol,PWM and CAN Bus are protocols) the end result must be the same regardless.The different protocols are to keep the DIY'er out of the system.
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Unread 08-28-2014, 03:13 AM   #11
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Let me clear the air on some things... first of all, tjkj2002, I am not debating this to argue, so please don't take me wrong. This is just one way I use to keep learning. While what I type may come across as arrogant at times, that is not my intention. Those that know me face to face can attest to that.

The other point to remember as to the original post, I did not say that using high octane fuel would cause harm to his engine. In fact I said that it is fine just that my personal experience says that the benefits just aren't there.

So, if I am reading your comments correctly, you are saying that using 92 octane fuel in the 3.7 will give you measurable benefits? When I state "limits" that is a collective statement. Meaning the software is limited by the mechanical components and the fact that it expects certain conditions. For example... the cam profile in the 3.7 is not variable so the computer can't effect valve duration and timing. Also, it expects a specific injector size and spark plug. While down stream and upstream sensors tell the PCM what is happening along with things like knock sensors, it also knows that it can only vary spark timing and air to fuel ratio to a limit. At the edge of those limits you can get minor issues that aren't enough to trigger a code but still cause ill effects in the long term. Ill effects like shortened catalyst life or carbon buildup. Again, these effects can vary greatly from one engine to another depending on it's makeup. A rule that may apply to a Jeep with a 3.7 does not necessarily apply to say a Jeep with the 3.6 or Hemi, or a Honda Accord with a 2.4 iVtech engine. Again, based on what I have learned so far, it is my understanding that there is very little "potential" in the 3.7. Not saying that it is bad... it is just limited by both software and mechanical makeup. Please correct that statement if it is wrong (I would love nothing more than to increase the power available and would spend within reason to get it).

There is also what many describe as the "placebo effect".. I too fall victim to that at times, it is just human nature.. You spend money on an improvement, install it, then swear it made a difference... where if you put it in a lab, there is none. I am at that point now with the Hypertech tuner. I have it, and swore it made a difference when I installed it. That said, me being the usual worry wart with warranties, I kept deprogramming it every time it went back to the dealer for the vibration issue. I didn't want them to hook up their system and come back at me with "you modified the computer". I bring it up because each time I did that, back and forth, I really don't feel the difference anymore. Granted it is their "economy" tune designed for 87 octane, not the "performance" or 92 octane tune. But I keep thinking... with the mechanical limits and 87 octane does it really make a measurable impact?

This is also where consumers fall into traps..... myself included.... you can change one or more of those hard points.. spark plugs, fuel injectors, intake, exhaust... where they will impact a specific "point in the range" with an improvement... but many times that positive impact at that small data point is counter acted by a negative impact at other points in the range. That little rule has been around forever... IE... why an extreme cam in a hot rod makes it idle like crap.....

Also, I will now stop using the words "regular" and "premium" (yea, right) when talking fuel. That is just a marketing ploy used by oil companies. They use the term "premium" for the simple reason that consumers equate that with "better". BS. Engineers that I have had the pleasure to learn from have spelled it out chemically. There is nothing better about "premium" vs "regular". It is just an excuse to charge more for a different octane. If I'm not mistaken, it is primarily a North America thing... hence why many European cars specify "premium" is because they are designed to run on higher octane fuel, and higher octane fuel is the standard there. Oil companies could very well have multiple octanes at the pump and charge the same for each.... but why do that when they can get a "premium" price for "premium".


OK, enough for now... man I really should know better... the basic rules of a vehicle forum... stay away from conversations about fuel and oil....
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Unread 08-28-2014, 08:24 AM   #12
tjkj2002
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So, if I am reading your comments correctly, you are saying that using 92 octane fuel in the 3.7 will give you measurable benefits
It can,the "normal" driver will not really see any besides better idle and if used for some time can increase mpg's a little.Higher octane gas is generally better controlled then the cheap gas,more consistent and better additives which can be a huge difference over the "cheap gas".I have run into at work all the time from people that hunt for the cheapest gas and have issues from it,bad gas is becoming a issue these days since the prices have gone up over the last decade.


Those that are more aggressive drivers and some environmental factors will make higher octane gas a better choice and will give you very noticeable increases.When I moved to CO I used 87 octane(mid grade here) and was okay around town since I was on the flatter side of town but that 1st trail run was a adventure(10,000'+).Pinged and knocked badly which 91 octane gas cured.I don't even want to try the 85 octane junk here.
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Unread 08-28-2014, 08:31 AM   #13
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Okay guys, please play nice, no need to start a war here.

FWIW, I feel like I'm learning a lot by reading all these comments, so thanks for that. I still am not seeing any major red flags to running higher octane fuel, at least in the short term, so for now I will continue the experiment. BTW, as of this morning, my EVIC now shows 13.9 average, still all in-city driving, but perhaps not as much stop-n-go as some times. Of course more time is needed to include all typical driving conditions, but still, I've never come anywhere near 14 in the past no matter what I've tried.
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Unread 08-28-2014, 09:32 AM   #14
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Originally Posted by ivorycruncher View Post
Okay guys, please play nice, no need to start a war here.

FWIW, I feel like I'm learning a lot by reading all these comments, so thanks for that. I still am not seeing any major red flags to running higher octane fuel, at least in the short term, so for now I will continue the experiment. BTW, as of this morning, my EVIC now shows 13.9 average, still all in-city driving, but perhaps not as much stop-n-go as some times. Of course more time is needed to include all typical driving conditions, but still, I've never come anywhere near 14 in the past no matter what I've tried.
I'm reading quietly over here
Your gas mileage is still pretty bad
Eventually I'll try running premium fuel, but with my programmer set to the setting for it.
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Unread 08-28-2014, 09:36 AM   #15
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Originally Posted by ivorycruncher View Post
Okay guys, please play nice, no need to start a war here.
Not even close..... war was the clown that jumped on this forum bashing Jeep....

I am enjoying this... and learning as well......

I won't even go into the whole chemistry thing regarding fuel branded as premium... LOL.... But a comment inserted by tjkj2002 regarding "cheap gas" is very true. While the difference between say Marathon gas and Shell gas is alarmingly small.... the crappy little corner budget place is a great risk to the health of you vehicle.

Most of my comments are generalizations that I have learned... some of the additional commentary actually has me tempted to run a few tanks of 92. I want to see what happens.... knowing I won't be hurting anything.......
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