What octane are we using?

[DonsRam19]

Here's a install video for you guys that want you install a pulsar and have a visual reference.

 

[Barney556180]

Good article on gasoline. Although in a motorcycle mag, a good technical read.

What Gasoline Is And Isn’t, And Why Motorcyclists Should Care

Why can we make more power with a high-octane fuel? Cycle World Technical Editor Kevin Cameron explains the nature of gasoline.

www.cycleworld.com

 

[Dusty1948]

Huh?

The point is, even though our trucks will run on 87, they are designed to run on 89 and higher. Running a lower octane means that the truck will have to compensate for lower than optimal combustion, due to timing adjustments from detecting knock. Running a higher octane allows the engine to use optimal timing curves, and with how advanced engines are nowadays, I wouldn't be surprised if valve timing is adjusted as well. I'm not saying its a "MORE OCTANE, MORE POWER" hard and fast rule, I'm just saying that I'm tired of hearing how octane has no effect in a modern engine. Octane is an anti-knock rating, and detected knock forces the engine to run less efficiently, how are they not related?

(I'm not trying to say this just to you, it's an open discussion)

For this discussion the problem is purely with semantics and understanding.

What automotive engineers say is, that ...using a higher octane fuel than the engine is originally designed for produces no power or performance benefit.

However, conversely, using a lower octane fuel than the engine is designed for will result in lower power and performance, regardless of potential engine knock. Why is this? In simple terms it is because the engine design engineers have determined the combustion chamber peak pressure at the ignition point at operating temperature which determines the expected power output of each cylinder. (There are many other variables involved, of course, such as flame front progression, piston speed, A/F ratio, etc.).

Engine knock is caused by the fuel burning process completing before the piston reaches it's peak pressure point, usually at top dead center. This, too, causes loss of power and performance, sometimes with serious effects. Fuel octane has a direct bearing on the prevention of knock, and engineers design an engine to be below the threshold of knock when operating at design conditions.

With regard to the 5.7 Hemi, Chrysler states that 89 is the recommended octane fuel, allowing that 87 can be used with reduced power and performance. When 87 is used, the knock sensors (if knock is detected) send a signal to the computer to reduce the amount of ignition timing advance. As most know, running a retarded spark of any degree reduces some level of power.

So, using more octane than the design calls for will not produce more power or performance. It is a fact that most gasoline refiners provide a higher level of combustion chamber detergents that may benefit long term performance by keeping fuel system components clean. However, the additional octane is still being wasted unless the engine is designed for that octane.

Best regards,
Dusty

2019 Ram 1500 Billet Silver Laramie Quad Cab 2WD, 5.7 Hemi, 8HP75, 3.21 axle, 33 gallon fuel tank, factory dual exhaust, 18” wheels. Now at: 010977 miles.

 

[dn325ci]

I did fill up with Costco 93 this time, and I don't hear the knock I thought I was hearing on the Costco 87. 1,650 miles on the truck now. I'll have to switch back to the 87 to confirm, but the 33 gallon tank takes awhile to burn through. Truck does feel a little more alert, especially at part-throttle.

 

[Deriggs007]

Dusty nailed it.

When I was rambling on with my Dad about this, he also stated that you see no performance gain from higher octanes unless the manufacturer states it and the vehicle is tuned for it. He did state theoretically, our engines "could" see a benefit above 89, but 89 could simply be the recommended octane. However, RAM would have some sort of documentation if this is the case or this knowledge of performance would become mainstream by now with the lifespan of the 5.7 hemi's and how long they've been to market. Again, this all comes to that fancy word graph he used to describe how this math is crunched.

He doesn't know our engines, but 89 could simply be the recommended because it is the 'sweet spot' on performance ratio, while there could still be an increase using higher octane, it's not the sweet spot if that makes sense. Again, there is no way of knowing without proper documentation directly about our Hemi's, but it's probably out there somewhere.

I'm running a full 89 tank in mine this time and I have seen an increase of roughly 2 MPG in city driving with the change. Would be higher if my truck wasn't sitting in park turned on as I run and do errands and the family stays inside for 15-20min a few times a week. I filled up with 87, reset my average, did my normal commutes and daily routines and typically had 12.6 average by fill-up time. I'm now at 14.2, but still have half a tank to go.

 

[dn325ci]

Let's be clear that I think everyone agrees there is no lifespan or reliability issue to using 87 in the 5.7L Hemi. The engine management will manage ignition timing to protect the engine, and the engine will live a long life as evidenced by many. I don't think it's worth debating that any further. The two issues of interest are optimizing performance and fuel economy, as Ram says in their literature, a clip of which is attached in post #4. I'm not partial to either octane, but I am interested in testing both to see if I can realize notable real-world benefits.

The fuel economy issue is numerically easy to assess. I'm partial to Costco gas because it's a top-tier gas and it's almost always the cheapest in my area, usually by a significant margin. They only carry two octanes - 87 and 93. The price difference between them has been 15-18% recently. I'm a thorough tracker of mileage, and I've been averaging 15.09 mpg so far. So I need to see 15.09 x .18 = 2.7 mpg improvement to call it a wash on dollars.

But the performance improvement also matters to me. Difficult to assess numerically, but I'm looking for two benefits - a detectable change in performance and/or an improvement in the part-throttle knock I think I'm hearing on the 87. I'll have to go back and forth between octanes a few times to work it out in my mind - probably another 1,000 miles.

 

[VictoriaFarm]

Someone once told me there is no 89 tank in the ground. It runs a mixture of 87 and 91/93 to achieve the 89 rating. I have no idea if it is factual.

We used to own a convenience store with gas pumps. There were three tanks in the ground, one for each grade.

 

[VictoriaFarm]

The benefit that I see for running a higher octane fuel is to allow the engine to run in ECO at higher speeds. My truck will not engage in ECO above 60 mph. I assume that it is due to the increased load on the engine is/may cause knocking. The higher octane will allow the engine to have an increased load and avoid the knock. If this is so, an increase in the mpg could be realized. I tried one tank in the past with no difference in mpg, but it could take a couple of tanks to allow the computer to relearn. Question for RamCare.

 

[wildh24]

I put a half tank of 92 in from Costco mixed with 87 from before. Seems like it makes ECO kick in more. Mileage is higher by about 1-1.5 mpg at least in the short term.

 

[mike0012]

Sorry if this was already covered and I missed it. In Colorado at 5300ft above sea level, we have 85,87,91 all with ethanol. What's recommended in this situation?

 

[slimchance]

usually buy my gas at Sheetz ... big distributor around here in Pa .... i get the 88 octane ... E 15 ... talked to a friend who works for a firm that installs tanks and stuff and he inspects the work ... he says there are just 2 tanks in the ground and all grades are a blend from those 2 tanks .... last tank with over 200 miles on wife's trk is showing 19.6 ... our hand calculated avgs are usually + or - 0.2 mpg ... IF we can keep this trk from pulling the trailers before refill maybe we can get a clean mpg ... btw this tank is local Pa driving, which means speeds below 50 mph, lots of stop signs, red lights and construction everywhere, but no 4 lane driving

 

[SoCADesertRat]

I am an engineer, actually. Octane is strictly a measurement of knock resistance. Engines that use high compression, aggressive timing, and/or forced induction will experience knocking (detonation) with insufficient octane in the fuel. This can severely damage or destroy an engine, which is why modern engines detect detonation and adjust for low octane fuels. Maximum power is achievable only with the recommended octane.

Using a higher octane than recommended has no benefit, you're just paying more at the pump. I'm surprised by the card you found and can only assume they're trying to dispel this persistent myth that higher octane will somehow magically make more power in an engine that was not designed for it. There are no harmful effects to your engine by using higher octane than necessary.

Totally agree with your conclusions/recommendations. You are spot on.

 

[SoCADesertRat]

I run 89, usually Arco... I haven’t tried any other grade yet and probably won’t until I get the Pulsar. Using 87 may harm the engine and using 91 is a waste of money. The only thing I’ll do is periodically fill up with Chevron or something like that to help clean out the engine.

ARCO is now officially listed as a TOP TIER FUEL so no need to switch to Chevron or another to clean it out.

 

[Zeronet]

I normally run 87. But several days before towing, I top off the tank with 91 or 93 in an attempt to get the octane up. Then 89 while actually towing.

I have noticed before towing and after I’m done towing, MDS is on more often while running on the higher octane fuel than when on 87. Seems to be able to hold MDS on even going slightly uphill or when lightly accelerating. Haven’t kept the higher octane in for long enough to notice any MPG increase.

 

[silver billet]

This will never be a settled argument because there are too many variables. People making claims like "I see a 1 mpg difference" (no you probably don't; a 1 mpg difference could be from any reason like temperature outside affecting tire pressure, or a more windy week/month, or from subtle differences in driving style because now you're aware of what you're testing etc) or "I don't feel any difference" etc, just too much "feeling" and not enough "hard data". You have to wire in a laptop and actually track what your engine is doing.

(Probably the one exception to this is hearing a "ping" under heavy acceleration/load, that sound is pretty noticable when you hear it and kind of hard to get that one wrong, objectively; but even there, trying to detect less/more ping from brand to brand is still very hard to control, you need a data logger).

And it can vary from brand to brand, and from station to station. Brand X at location A might have much older/crummier tanks/gas than the same brand at location B.

Then there is the issue like if you don't empty your tank completely (and it never is 100% empty) you're not getting true 87 or 89 if you switch octane for one fillup; plus there is the leftover gas in the gas pump's line itself; was that 87 before you threw in 89? You're getting quite a bit of 87 first then.

Throw in other variables like ethanol etc.

Any differences that actually might be differences (mpg/performance) could just as well be due to ethanol or clean/dirty/stale gas instead of octane.

I haven't hooked up any logging software either, but normally I just put in 87 and when I work it in the summer I put in 89/91.

 

[Narg]

89, as the manual with the Hemi 5.7 suggests.

Every V8 truck I've owned for the past 30 years has knocked/pinged on 87 fuel. They all ran better on 89 or higher. Caught my attention when I saw the manual state 89 for this V8, though I'd have used that anyway.

 

[mikeru82]

Huh?

The point is, even though our trucks will run on 87, they are designed to run on 89 and higher. Running a lower octane means that the truck will have to compensate for lower than optimal combustion, due to timing adjustments from detecting knock. Running a higher octane allows the engine to use optimal timing curves, and with how advanced engines are nowadays, I wouldn't be surprised if valve timing is adjusted as well. I'm not saying its a "MORE OCTANE, MORE POWER" hard and fast rule, I'm just saying that I'm tired of hearing how octane has no effect in a modern engine. Octane is an anti-knock rating, and detected knock forces the engine to run less efficiently, how are they not related?

(I'm not trying to say this just to you, it's an open discussion)

Our engines were designed to run on 87 to 89, not 89 and higher. In addition, 89 octane is recommended, not required. There is no mention of any octane higher than 89.

I have run 87 in my current truck since day 1. I didn't read the owners manual regarding which octane to use because there is no sticker inside the fuel door telling the user which octane to use. So I made the assumption that 87 was good enough. I used 87 octane on my 2017 Rebel for two and a half years, and achieved almost 17 mpg with that truck. My 2020 Laramie is getting about 14.5 mpg after 2600 miles. Now that I know 89 octane is recommended, I'm planning to switch to that and see if my mpg's improve. My take-away from this is it's okay to use 87 octane. But for maximum perfomance and fuel economy you should use 89. Basically summarizing the owners manual. In other words, I haven't done any damage by running 87 octane, but I'll be able to realize my truck's full potential by running 89 octane.

 

[silver billet]

Our engines were designed to run on 87 to 89, not 89 and higher. In addition, 89 octane is recommended, not required. There is no mention of any octane higher than 89.

I have run 87 in my current truck since day 1. I didn't read the owners manual regarding which octane to use because there is no sticker inside the fuel door telling the user which octane to use. So I made the assumption that 87 was good enough. I used 87 octane on my 2017 Rebel for two and a half years, and achieved almost 17 mpg with that truck. My 2020 Laramie is getting about 14.5 mpg after 2600 miles. Now that I know 89 octane is recommended, I'm planning to switch to that and see if my mpg's improve. My take-away from this is it's okay to use 87 octane. But for maximum perfomance and fuel economy you should use 89. Basically summarizing the owners manual. In other words, I haven't done any damage by running 87 octane, but I'll be able to realize my truck's full potential by running 89 octane.

Well if you hear a pinging when using 87 octane under load, is that a "good thing"? I have no idea, but my guess would be "no"; it wouldn't surprise if that was somehow negatively affecting the engine.

 

[mikeru82]

Well if you hear a pinging when using 87 octane under load, is that a "good thing"? I have no idea, but my guess would be "no"; it wouldn't surprise if that was somehow negatively affecting the engine.

No need to get passive-aggressive about it. I never said anything about pinging, nor have I ever heard anything like that on this truck or my previous one. Of course damage could be happening if you actually are experiencing that.

 

[ncgrl1]

Auto magazine recently did dyno test on this very subject. These are actual rear wheel horse power & torque figures. It would translate to a lot more at the crank where our trucks are rated. I think you will find the results interesting!