Question about fuel octane level.

[Curmudgeon]

Eh?

View attachment 570694

What's the question? I think @Hemi395 did the datalogging on different octane gasolines. That's when I sat up and paid attention to how the ECM algorithms respond to knock sensing with ignition timing changes.

The ideal octane rating for the 5.7L Hemi?

The manual says 89, but I have read that 89 is just a blend of 87+91 and 91 is more gooder, for, um, reasons.

I just switched once again from 89 to 91. I can't tell a difference but I don't work the engine. I don't tow, haul, or race. I'm already running Top Tier, whether 89 or 91, and being retired I don't drive enough to worry about the price difference between octanes, so I prefer to run what is best for the engine long term.

 

[HEMIMANN]

The ideal octane rating for the 5.7L Hemi?

The manual says 89, but I have read that 89 is just a blend of 87+91 and 91 is more gooder, for, um, reasons.

I just switched once again from 89 to 91. I can't tell a difference but I don't work the engine. I don't tow, haul, or race. I'm already running Top Tier, whether 89 or 91, and being retired I don't drive enough to worry about the price difference between octanes, so I prefer to run what is best for the engine long term.

Ha ha - ideal doesn't exist, which is what my rants were about.

Ideal octane for THE EXISTING STOCK ECM ALGO is 89 dedicated octane - not a blend of pure 87 octane and pure 91-ish octane (whatever the required ratio to "average" 89 octane).

Since I'm an engineer and can't help living in a fact-based world, I use whatever higher octane (without higher alcohol level) is available. Here in Minnesota it seems to be 91 pure octane (@ 10% ethanol). Other states offer higher octane, 92 or 93, due to different refineries in their area. I always try to buy Top Tier brands, they have extra PEA detergent added to keep intake valve lands and injector nozzles clean. We have few left here, most brands pulled out of our flyover country (more expensive to ship to).

So I carry a measuring bottle of PEA (Red Line Sl-1) with me for when I need gas and there's no Top Tier station around. Ratio is 1 ounce extra per 15 gallons non-Top Tier gasoline.

The automotive world has really gone to hell the past 15 years. We are all trying to band aid our way forward.

 

[Docwagon1776]

I know a couple guys who've dropped the truck 6.4 in their 1500's,and they're getting noticably better milege with the 6.4 compared to their old 5.7's,and both claim alot less downshifting from their 8 speeds when unloaded or when towing.On paper it looks like the truck 6.4 isn't a huge upgrade over the 5.7,but in the real world it turns out to be a hell'va upgrade,as the 6.4's torque curve blows the 5.7's torque curve into the weeds.I think they majorily screwed up,by not dropping the truck 6.4 into the 1500's

It makes one wonder why it was never an option. Maybe the test loop mpg vs real use mpg was different? Emissions differences? Stop/start or etorque would be kind of the antithesis of what these were designed for, so maybe that's it? No idea, just spitballing.

In a way, it reminds of the old IHC pickups that used the same motor and transmission as small dump trucks or busses were using. The rest of the truck would eventually rot away, but you weren't killing that motor or transmission without serious neglect and abuse...if then.

 

[Docwagon1776]

The ideal octane rating for the 5.7L Hemi?

The manual says 89, but I have read that 89 is just a blend of 87+91 and 91 is more gooder, for, um, reasons.

All gasoline is a blend, but yes 89 is blended at the station. Octane is varied by the addition of benzene and some other chemicals to the blend (and/or ethanol), the higher the ratio of those additives the higher the octane is. The "base" gasoline is the same regardless.

Some folks believe the additives package contains more detergents with the 91 octane. I think that's more true for some brands and some areas than others, but it's not something I've looked into in years. I try to stick to the 'top tier' rated fuel places and don't worry about it further. Sunoco, Shell, Costco, etc are all top tier in my area.

 

[rzr6-4]

It makes one wonder why it was never an option. Maybe the test loop mpg vs real use mpg was different? Emissions differences? Stop/start or etorque would be kind of the antithesis of what these were designed for, so maybe that's it? No idea, just spitballing.

To preface, this is purely speculation. Do not quote me on any of this.

The HD trucks (2500/3500) are exempt from some of the emissions regulations. I'm sure some of those regulations apply to the vehicle as a whole, and some to the motor specifically. I wonder if by keeping the 6.4 BGE exclusive to the HD models if that also kept that engine exempt from some of the emissions rules and regs. If it's available in the passenger 1500s, that's a whole extra set of testing that they have to be able to pass.

 

[Docwagon1776]

To preface, this is purely speculation. Do not quote me on any of this.

The HD trucks (2500/3500) are exempt from some of the emissions regulations. I'm sure some of those regulations apply to the vehicle as a whole, and some to the motor specifically. I wonder if by keeping the 6.4 BGE exclusive to the HD models if that also kept that engine exempt from some of the emissions rules and regs. If it's available in the passenger 1500s, that's a whole extra set of testing that they have to be able to pass.

At least in the US, it's vehicle specific. HDs have different, more lenient, rules. They used to not count toward CAFE as well, and I *think* that's still the case and why fuel economy wasn't required to be posted on the window sticker. I could be outdated on this, though, haven't looked into for years.

But it would have to be tested in the half ton for sure if it was offered, and be subject to CAFE, though if @Wild one 's information is correct, that would have actually helped their fleet averages.

Chevy offers the 6.2L as an option over the 5.3L in the half ton, but with cylinder deactivation. Ford has the Godzilla motors but only in the 3/4 ton and up and they don't have cylinder deactivation or any of that, but the 5.0 is the only NA V8 for the half tons. So there's definitely some more leniency for 3/4 ton + trucks.

 

[HEMIMANN]

All gasoline is a blend, but yes 89 is blended at the station. Octane is varied by the addition of benzene and some other chemicals to the blend (and/or ethanol), the higher the ratio of those additives the higher the octane is. The "base" gasoline is the same regardless.

Some folks believe the additives package contains more detergents with the 91 octane. I think that's more true for some brands and some areas than others, but it's not something I've looked into in years. I try to stick to the 'top tier' rated fuel places and don't worry about it further. Sunoco, Shell, Costco, etc are all top tier in my area.

Of course all gasoline is a blend.

What I'm saying is that blending an 89 octane gasoline dedicated will not look like a gasoline blended from dedicated 87 & 91 octane gasolines.

The combustion thermodynamics will differ.

 

[Docwagon1776]

Of course all gasoline is a blend.

What I'm saying is that blending an 89 octane gasoline dedicated will not look like a gasoline blended from dedicated 87 & 91 octane gasolines.

The combustion thermodynamics will differ.

How so?

The base is the same. The chemical energy in the gasoline is the same. The only difference octane provides is how much the mixture resists detonation so that the flame front moves evenly vs pockets self igniting. The only difference to get to the desired octane is the percentage of benzene, etc.


Think of it like this: You have a glass of water and you dump enough sugar in it to make it 5% sugar water. Dedicated 5% sugar water. Now you have a glass of 3% sugar water and a glass of 7% sugar water. You stir them together. Blended 5% sugar water. There is no difference and no scientific testing would be able to tell you which was made how, the end result is the exact same. Octane blending is the same. Same base, same overall ratio of additives, zero difference in how you get there.

 

[HEMIMANN]

How so?

The base is the same. The chemical energy in the gasoline is the same. The only difference octane provides is how much the mixture resists detonation so that the flame front moves evenly vs pockets self igniting. The only difference to get to the desired octane is the percentage of benzene, etc.


Think of it like this: You have a glass of water and you dump enough sugar in it to make it 5% sugar water. Dedicated 5% sugar water. Now you have a glass of 3% sugar water and a glass of 7% sugar water. You stir them together. Blended 5% sugar water. There is no difference and no scientific testing would be able to tell you which was made how, the end result is the exact same. Octane blending is the same. Same base, same overall ratio of additives, zero difference in how you get there.

No, the flame front does not move the same because the advanced ignition timing for the given fixed compression ratio is for 89 octane. The molecules that are 87 octane will pre-ignite, and the molecules that are 91 octane will not pre-ignite.

 

[crash68]

If this thread continues, I'll take a large bucket of buttered popcorn, please.

Possibly gonna need a second bucket and some beers!!

 

[Docwagon1776]

No, the flame front does not move the same because the advanced ignition timing for the given fixed compression ratio is for 89 octane. The molecules that are 87 octane will pre-ignite, and the molecules that are 91 octane will not pre-ignite.

You have a fundamental misunderstanding of the chemistry involved here as well as how compounds ignite and burn. If your notion was true, ethanol blended gasoline would have two flame fronts, the base gasoline at one front and the ethanol in a second front (even if we pretend gasoline is a distinct molecular compound). Obviously that does not happen, and obviously pure ethanol has a different octane rating than gasoline.

There is no "87 octane molecule" or "89 octane molecule". Additives of aromatics like benzene don't alter the base gasoline's molecular structure any more than dumping sugar in water alters the water's molecular structure. It's like saying there's a Kool-Aide molecule because you added sugar. The water is the same at the molecular level.

There is no "gasoline molecule" period, it's a mixture of various hydrocarbon compounds. Octane level is adjusted by changing the ratio of the compounds present, not altering the molecules.

 

[BossHogg]

How so?

The base is the same. The chemical energy in the gasoline is the same. The only difference octane provides is how much the mixture resists detonation so that the flame front moves evenly vs pockets self igniting. The only difference to get to the desired octane is the percentage of benzene, etc.

Adding benzene or ethanol to gasoline to increase the octane rating also reduces the amount of chemical energy. While achieving higher octane, you will also need more fuel to do the same amount of work.

The better method of increasing the octane is during the refining process, cracking and reforming hydrocarbons. As I understand it, and it has been a while, the crude quality determines the distillation methods. Which refiners produce what formulations of gasoline is a wonder.

 

[BossHogg]

Possibly gonna need a second bucket and some beers!!

I'm thinking some good old Michigan craft beers.

 

[DanAR]

I just use regular from SAMS (87 octane). They don’t offer a mid-range and premium around here is usually 60-70 cents higher per gallon. I’m not sure how anybody makes the economics of premium fuel work out in a truck that doesn’t require it. I might try to find some 89 for a tank sometime to see if I notice any mileage change. I used premium in my daily driver Mustang GTs, Shelby GT350, supercharged Lightning pickup, etc which was fine. Hard pressed to do that in a 6,000 lb, 233” long pickup that I tool around in like the old guy I am.

 

[Curmudgeon]

Dad? Mom? You know us kids hate it when you argue.

Seriously though, I'm enjoying following this conversation. I can no more
use "the manual says so" regarding fuel octane than I can regarding oil
weight/viscosity, so what I'm seeing here is invaluable to me, because I
really don't have anything better to go on. My background, education,
and experience is all electronics-based, not chemistry or automotive.

I've found that I can learn a lot here, and one of those things I've learned
is that I can generally trust the RamForum "think tank"!

I'll stick with Top Tier, and occasionally add some SL-1 or PEA. I've never
been able to see or feel a difference in octane but I'd like to think I'm paying
for, and using a better product for the engine.

 

[turkeybird56]

I stay with 89 my 2019 runs, butt perception better. When towing ne thing with my light 2,150 lb trailer with wateva inside, I run 93 due to load.

 

[Docwagon1776]

Adding benzene or ethanol to gasoline to increase the octane rating also reduces the amount of chemical energy. While achieving higher octane, you will also need more fuel to do the same amount of work.

The better method of increasing the octane is during the refining process, cracking and reforming hydrocarbons. As I understand it, and it has been a while, the crude quality determines the distillation methods. Which refiners produce what formulations of gasoline is a wonder.

Definitely less chemical energy, it can be noticeable with ethanol, but that's outside the scope of the discussion as to if a blend or "defined" matters. If it needs more fuel, maybe, depends on several factors. If a vehicle is pulling timing earlier the mechanical disadvantage is likely to overshadow the slightly smaller "pool" of available energy, particularly when you think how much of it is wasted as heat in ICE motors. If the vehicle isn't pulling timing, though....less energy is there with no counterbalance mechanical advantage. Hence why running 93 in a vehicle not tuned for it doesn't result in any gain.

I'm nowhere near knowledgeable enough to know all the different ways they crack and form the hydrocarbons for everything from gasoline to synthetic oil, but like you I understand that's how it's done vs the notion there's an "87 octane molecule" out there. Base definitely matters, with something like tar sand "oil" taking a lot more refining and work (and energy and money) than the sweet oil from the Permian basin. Prices for the crude reflect that, and it's a big reason I was against the Keystone pipeline once I researched the bottlenecks in our energy industry. We don't need harder to refine worse crude products, we need more refining capacity. Meanwhile, I'm still making a very reliable 7% dividend on pipeline infrastructure companies.

 

[Hemi395]

Comparing 87 vs 89 vs 91...

Paging @HEMIMANN - or is it @Hemi395 ? I can't keep 'em straight.

Too much Hemi in this gin joint.

That was me

So long story short, our trucks will pull timing on any octane other than 91+. You just can't always hear it.

All of these examples were driving normally and unloaded in the summer. These are also on the stock tune.

On 87, my truck would pull timing almost every time I touched the gas pedal. You could actually hear it detonate audibly which I'm assuming is why they say it's "acceptable" in the owners manual.

89 is better, I never heard it audibly detonate while running it. But the pcm was still pulling timing which suggests its still detonating, just not as bad.

Which brings me to why I always run 93 in my truck. On 93 it rarely pulls timing. Only time I see it pull timing is when im pulling my camper up a long grade.

Since I dont drive the truck daily anymore, the added cost of 93 is 100% worth it for me. However I totally understand why people would want to run 89 if its daily driven.

However I would recommend staying away from 87 unless its the only option.

 

[Hemi395]

I know a couple guys who've dropped the truck 6.4 in their 1500's,and they're getting noticably better milege with the 6.4 compared to their old 5.7's,and both claim alot less downshifting from their 8 speeds when unloaded or when towing.On paper it looks like the truck 6.4 isn't a huge upgrade over the 5.7,but in the real world it turns out to be a hell'va upgrade,as the 6.4's torque curve blows the 5.7's torque curve into the weeds.I think they majorily screwed up,by not dropping the truck 6.4 into the 1500's

The 6.4 truck Hemi is really a nice motor. Its designed from the ground up to be worked.

Reminds me of swapping out the anemic LG4 305 for a 350 in my 87 Monte Carlo back in the late 90s. I actually gained fuel economy from that swap.

Well, when I could keep my foot out of it that is....

 

[Wild one]

The 6.4 truck Hemi is really a nice motor. Its designed from the ground up to be worked.

Reminds me of swapping out the anemic LG4 305 for a 350 in my 87 Monte Carlo back in the late 90s. I actually gained fuel economy from that swap.

Well, when I could keep my foot out of it that is....

Back in the early 80's i swapped out the crappy 301 in my 79 Gran Prix for a HO 400 out of a 71 Gran Prix,and picked up milege,and one hell'va lot more power to pull the 2.29 gears it had,i hated that 301 with a passion,as it was the most gutless engine i'd had up till then,and wasn't very good on fuel economy. I blamed that alot on the 2.29 gears,as the 301 didn't have enough torque to pull them,so you were always deep in the throttle to maintain a decent highway speed,lol