Fuel Quality => MPG?

[freebird_78]

It could certainly be coincidental or even placebo, but it SEEMS like this particular engine (I also have a Charger with the car version of the 6.4L) is "picky" when it comes to fuel quality. I DON'T mean octane ratings, I mean top tier vs not top-tier gas.

I've tried running higher octane (91 is the best we get here) vs mid (88 here) vs base grade (86 here). Those didn't seem to yield any significant milage change, one way or the other. I tend to drive the same "loop", so that variable is relatively controlled. But what I do seem to see is whenever I fill up with lower tier fuel, I get about 1-2 MPG worse. Right now, I'm struggling to get above 12 with crap gas, where I'm usually above 14 and even 16 MPG on highway stretches. This isn't one tank or one point of data, either. Seems to be a pattern.

Problem is, I can't understand WHY. As I understand it, top tier just means a better additive package (more/better detergents), where a lower tier just cheaps out on the qty and quality of the additive packages. This, in my head, shouldn't have any bearing on mileage. At least not anything significant. I've also not noticed this in any other vehicle. It doesn't make any sense to me.

Has anyone else seen anything like this behavior?

 

[Crusty old shellback]

I use several different brands of gas.
The one thing I have noticed, is that my MPG is directly related to my speed to a certain point. Doesn't matter where I get my gas from. Even when I use the cheap gas in Baja, Mexico on my trips.

Basically anything over 70 MPH and it doesn't really matter, even up to 90 MPH. About 13 MPG.
Anything under and the MPG goes up as the MPH goes down. I've seen 16 MPG as my best, but was doing between 40 and 60 MPH on that trip due to traffic, mostly in he high 40's low 50's.

 

[MEGA HO]

If we aren't talking about octane, the only "additive" that could make a difference on vehicle performance is ethanol content as ethanol has lower BTU comparing to benzine. Less BTU means you need to burn more of it to get the same result.

 

[freebird_78]

Yeah, speed is by far the biggest driver in mileage. It's just physics.

For my observations, I've tried to keep that as constant as possible. Same "loop", same speeds, same driving habits. Apples to apples as much as possible.

 

[freebird_78]

If we aren't talking about octane, the only "additive" that could make a difference on vehicle performance is ethanol content as ethanol has lower BTU comparing to benzine. Less BTU means you need to burn more of it to get the same result.

So ethanol vs fuel is ~79% lower BTU gallon. I think the max you can get, by law, is E10 (10%), so that would be a maximum change of 7.9% total or about 1 MPG, in the ranges we're talking, if you were to compare ethanol free to a max E10. I'm about 99% certain that nowhere that I fill up is anywhere near E0, as there's only a handful of ethanol-free pumps, and they all charge quite the premium for it. So I would expect less than a percentage or two, station to station in difference in ethanol content. Assuming a 2% difference (which I would guess would be the high end). we'd be talking 1.6% difference in energy content, station to station, or less than a quarter of a MPG.

In addition (and I realize I'm opening up a tangential can of worms here...) I periodically pump about 4 gallons of E85 to a tank. Ethanol is a great solvent and "cheap" fuel injector cleaner, the way I see it. At 4 gallons to a tank, this yields about E20, which is still safe from a mixture ratio/code perspective. Been doing that for years. Not a huge impact in mileage, at all.

 

[freebird_78]

My first assumption was the ECU was pulling timing on crap gas. But if that were the case, going to a higher octane would prevent the knock and get me back timing and consequently power.

Perhaps I need to pay closer attention to this and try dumping a half a tank of 91 in there as an experiment.

Perhaps I'll dust off Torque and look for spark retard to see if there's any merit to it.

 

[LateToTheParty]

I can't really say as I have never paid attention to "Top Tier" vs others. But the one impact I notice is the change in seasons. I know around here they state that you should expect something like a 15-20% drop when temps go from 70s to 20s, and that really seems consistent with my experience, and has been consistent across nearly every make/model I've ever driven.

 

[freebird_78]

Took a quick trip, fired up AlfaOBD and logged timing and ST knock retard. Sure enough, it's pulling about 4 deg of timing during cruise.

 

[freebird_78]

Further update...

After seeing 4 degrees of timing getting pulled WAY more often than I'd like, I stopped and dumped 4 gallons of E85.

Again, the math works out so that 4 gallons of E85 mixed with 27 Gallons of 86 yields about E20 with an octane of 88.5.

Graphed timing, and almost immediately, my ST knock retard stayed at 0, with only 2 spikes up to 1 on a much longer drive. Again, as apples to apples as possible (same path, same speed, same driving habits).

That's about as definitive as it gets, for me. Cheap 86 just doesn't quite cut the mustard. Apparently, I hadn't paid enough attention for the couple of tanks I've run in the past, or I was just getting lucky.

In this particular example, "cheap 86" is from Sam's Club. The good news is that Premium/91 at Sam's costs exactly what Shell/Chevron 86 costs elsewhere.

On yet another tangent, I've been using Sam's Premium in the SRT 6.4L for years now. Like I mentioned above, I know that it's cheap because it doesn't have the detergents/additives that a Shell/Chevron do. So, I've been augmenting that with my own "additive pack" on every fill. Sam's Premium/91 + my own "additive pack" (a little PEA aka Techron and a little top end lube) is still cheaper than Shell/Chevron, locally.

I realize this topic is no different than any oil thread. Everyone has a different opinion. This is just mine.

 

[MEGA HO]

FYI ethanol burns slower so dumping gallons of E85 into your tank will bump the octane number up (hence timing retard stayed at 0) but at the same time energy density went down due to ethanol... I'm too drunk to try to figure this all out, just observations...

 

[tru]

seasonal blends, traffic and wind direction will all have an impact. By traffic, i mean drafting...my vehicle loses a few or gains a few MPGs depending on open road vs a congested road. Congested roads as long as they are flowing yields better mileage.

 

[dalet]

Late to the game, but in all of my vehicles I see about a 10% drop in mileage with ethanol blends around 10%. As Mega Ho said, e85 has more octane, which reduces need to pull timing out, but the ECU will add additional fuel so the air/fuel ratio is correct. If you run 100% pure ethanol, you will need to run twice as much ethanol through the system as gas. Learned this on my '87 Grand National. Had to put larger injectors in so I could run e85 and more boost. The thing screams and I scream about the gas mileage from a little V-6.

 

[MEGA HO]

Keep in mind that the stoichiometric ratio for ethanol is quite different than gas

 

[dalet]

Mega Ho,
Thanks man, you just took me back to Chem 114.

 

[unclelala]

For me I don’t care or pay much attention to or about fuel economy although at today’s prices it’s not hard to cry a little.
I figure if I was that concerned about economy I would have found another hobby other than dragging a 5th wheel around all summer.
I drive like a granny, use good quality fuel, do the required maintenance so besides these things I have very little control over fuel economy and I don’t lose sleep over it.
I do have sympathy for people making a living and depend on reasonably priced fuel.

 

[chas0218]

I use mostly Kwik Fill gas the 87 octane, the best I have ever got on the flat open road is 18. That was on the dash meter and cruise set to 55mph and no traffic.
2022 6.4 Hemi with 4.10

 

[whitexc]

I use mostly Kwik Fill gas the 87 octane, the best I have ever got on the flat open road is 18. That was on the dash meter and cruise set to 55mph and no traffic.
2022 6.4 Hemi with 4.10

Was going to say…Kwik Fill is a NY thing. They sell good fuel that they claim is ‘Merican.


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[chas0218]

Was going to say…Kwik Fill is a NY thing. They sell good fuel that they claim is ‘Merican.


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Out of all the fuel I've used Kwik fill, Shell, Sunoco seem to net me the best mileage. Mobil, and FastTrac, are usually the worse but are said to be "top tier". When I bump up the Octane to 93 or 91 I always gain a couple MPG but doesn't warrant the added cost. I might use it if going on a long road trip just out of convenience of not having to stop as often. Don't get me started on the small fuel tank size.