E39 540i premium vs regular gas mileage?

[chrveloso]

Has anyone experimented with this? What was the difference? Which do you use?

[Dking078]

I put premium in my 540. i save regular for my work truck.

That's the law 'round here.

[JimLev]

In Elizabeth NJ you should have refineries all around you, use 93 octane.
I did do a test years ago. I usually use 93 and for the heck of it I tanked up with 87, my mileage dropped by 2 MPG.

[bluptgm3]

Regular fuel should produce higher mileage as it is less knock resistant and burns quickly and more completely in the combustion chamber, if your engine can tolerate it. What was the factory recommended octane for your car?


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[chrveloso]

Hi everybody, thanks for the quick replies! I just bought the vehicle and I will be picking it up tomorrow, I'll let you know what the gas cap says. It is a 1998 BMW 540i.

[geargrinder]

Regular fuel should produce higher mileage as it is less knock resistant and burns quickly and more completely in the combustion chamber,

This is absolutely categorically incorrect. Well, the part about 'burns quickly' is correct but all the rest is the opposite of fact-truth.

The reason we have higher octane indeed is because it burns slower, but in fact it is exactly THAT which makes for a more complete and efficient combustion. More explosive combustion is less efficient than the flame front progressively burning across the chamber. More octane means the motor can run more timing which then further increases performance and efficiency. Octane is always good, never bad.

Here's some science. Super clear charts. The paper is basically about "what measurement for octane RON vs MON is a better correlator to performance and efficiency and how should the equations for octane be tweaked for best practical use" but it contains very concise charts showing that higher octane = better efficiency both in regards to power and in regards to emissions and fuel consumption.

https://www.sciencedirect.com/science/article/pii/S2352146516302629

The M62 is a moderately high-compression (at least by old-fashioned standards... basically everything is a high compression engine these days compared to BITD...) engine at 10:1 however that's really not high enough to worry too much about running some crap gas if you absolutely had to (not to mention having knock sensors to keep things in check).

I believe the (USA) spec for all M62's is 91 or better?

I would run 93 as much as I could in any M62... as documented all around the internets, the higher octane lets the computer run more timing which will get you a little more efficiency... but neither would I be super worried if I had to use some 89/91 once in a while. With the stuporcharging I definitely always run 93, and will stay off the fun pedal if I ever have to emergency fill up with 89 or 91... but that's a different story (effective combustion ratio etc.)

[Qsilver7]

My experience over the last 15+ years is to at least use the "recommended" octane (which is different than the minimum "requirement ")...and not so much for MPG but for performance.

The minimum requirement is 87 AKI (in the USA) but depending on how you drive, where you drive, and if you are intuitive enough to feel the difference...using the "recommended" octane fuel (89 AKI/91 AKI/93 AKI) will provide the performance and advertised MPG.

When gas prices first started approaching $3 gallon I remember I was in Albany NY attending the funeral of my uncle. We were about to leave the city and gas prices were hovering just below $3 at the time. I delayed filling up until we got out on the turnpike and to my dismay, gas prices were above $3. At the time, I had a psychological disdain for paying that much...so selected the 87 AKI (instead of the 89 AKI that is recommended for my 97 740iL). As soon as I started off to merge back onto the turnpike...I could tell there was decreased performance. The next fill-up I used 89 AKI and sure enough...the performance I was used to was back.

I have experiemented several times since then and every time I use 87 AKI...I can feel immediately the performance difference. But in full disclosure, I don't do half tank fill-ups...I fill-up within a few miles of the low fuel warning light.

I've also done the same with my 99 540iT and 06 X5 4.4 (which both had a 91 AKI recommendation)...and if I filled up with 89 AKI (1 octane rating down from the recommendation for both vehicles)...as soon as I pull out of the station on to the public street...I can tell the difference in performance.

Know and understand the difference between RECOMMENDED and REQUIREMENT:


http://www.bimmerboard.com/members/q/original/Fuel%20Specification.jpg

{my 97 e38 740iL gas cap w/89 AKI octane "recommendation"}
http://www.bimmerboard.com/members/q/original/E38%20Fuel%20Filler%20Cap.JPG

{my 99 e39 540iT gas cap with 91 AKI octane recommended & 87 AKI minimum requirement}
http://www.bimmerboard.com/members/q/original/E39%20Gas%20Fuel%20Cap%20%20Label%20Octane%20Specs .JPG

{my 06 X5 4.4 w/N62 engine gas cap with 91 AKI octane minimum requirement}
http://www.bimmerboard.com/members/q/original/E53%20X5%20Fuel%20Filler%20Cap.jpg

http://www.bimmerboard.com/members/q/original/E53%20Fuel%20Specification%20N62%20B44%20Engine.jp g

[JimLev]

You tell um GG.
All E39's should have a sticker on the gas cap or in the gas door area.
The non tu V8's (98 and earlier) should say min octane is 89 or 91??
The tu V8's engines (99 and up) specify a min of 87.
Altitude also plays a factor in what min octane you should use.
Out here in the higher elevations you can't find 93.
91 is about the highest available around these parts.
We do have some stations where we can get gas without ethanol, it's still 91 octane.

[edjack]

Mine is labeled "89 AKI min."

BTW, while you're in there, check the fuel flap hinge for cracks. If it breaks, you'll lose the flap. This will require a new flap to be painted to match the car.

[rswapp]

In Elizabeth NJ you should have refineries all around you, use 93 octane.
I did do a test years ago. I usually use 93 and for the heck of it I tanked up with 87, my mileage dropped by 2 MPG.
Octane level has nothing to do with fuel economy, your mpg drop was not performed in a control environment.
If you notice pinging or predetonation on acceleration when running regular bump up one octane level and try it again.
Chances are your 540 will run just fine on mid grade.
I noticed pinging on our 540 with regular, bumped up one level and pinging went away, didn't notice any performance difference and definitely zero mileage difference.
If owners want to just stick with regular because of cost just don't accelerate hard, use the throttle like you have an egg under your foot and you'll do just fine.
The knock sensors if they are good should adjust the spark advance enough to keep the pinging or predetonation under control.
My e39 528 & 530 runs just fine on regular, no pinging what so ever.
Also our e34 540 runs for some reason just fine on regular, have never had pinging with it, as well as our e34 525 which is also a regular burner.
Dumping premium in a vehicle when it doesn't need it is just throwing money down the tail pipe.

[R Shaffner]

Octane level has nothing to do with fuel economy, your mpg drop was not performed in a control environment.
If you notice pinging or predetonation on acceleration when running regular bump up one octane level and try it again.
Chances are your 540 will run just fine on mid grade.
I noticed pinging on our 540 with regular, bumped up one level and pinging went away, didn't notice any performance difference and definitely zero mileage difference.
If owners want to just stick with regular because of cost just don't accelerate hard, use the throttle like you have an egg under your foot and you'll do just fine.
The knock sensors if they are good should adjust the spark advance enough to keep the pinging or predetonation under control.
My e39 528 & 530 runs just fine on regular, no pinging what so ever.
Also our e34 540 runs for some reason just fine on regular, have never had pinging with it, as well as our e34 525 which is also a regular burner.

Dumping premium in a vehicle when it doesn't need it is just throwing money down the tail pipe.

I'd agree with most of this if it were written 30+ years or so ago, or for a car that old. The old rule used to be: If your engine doesn't knock on the cheaper gas, then higher octane won't help.

But with most cars (and all BMW's) since at least the mid-1990's the adaptive systems makes this simple rule wrong. Most of our cars adapt to prevent knock, as GG explained. That's the main point here. If you want more power, and higher MPG during normal driving, then higher octane gas will provide both because the engine adjusts to use it.

That doesn't mean using higher octane gas will be most economical. That would depend on the cost paid for gas, which is a LOT on a percentage basis these days. If you're trying to save money, using the lower or middle grade will do that, as will staying off the throttle.

It's an easy test. On an empty tank put in a half tank of whatever you haven't been using and try it. If you don't notice the difference based on the way you drive, use the cheaper one. The car will adjust to it. But if like many of us you use the higher octane gas, see the difference, and think it's worth it, then you found your answer.

[toybreaker]

Honestly MPG gains are adjusting the nut behind the wheel.
Unless you can get ethanol free gas. That you WILL see an increase in MPG all things being equal.

[BimmerBreaker]

Regular fuel should produce higher mileage as it is less knock resistant and burns quickly and more completely in the combustion chamber, if your engine can tolerate it. What was the factory recommended octane for your car?


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This is correct in theory. For a car not being driven hard enough to cause knocking, a lower octane fuel is easier to ignite and will combust faster and more completely, expelling more energy per unit of time, meaning less fuel has to be provided to output the same amount of energy per unit of time. In theory this will lead to an increase in mpg for the same given driving circumstances.

GG, that article you references compared the fuels at WOT. Thats a different discussion. Of course at WOT you should use the recommended octane. But unless you are knocking, there isnt an advantage to a higher octane. Putting race fuel 100+ octane in a corolla isnt going to make it faster. The engine cant combust it efficiently, it will drop in mpg and power. To a lesser extent the same thing happens on the M62 here - while it can advance timing to use 93, it can also be perfectly happy combusting 87 as long as the engine isnt driven hard enough to knock. Its less work to combust and extract the energy from lower octanes which translates into a (very small) increase in mpg

The M62 also cant make use of octanes too high. I believe it will only advance timing with up to about 92-93 octane. S62 will advance up to 96 octane or so (can blend race fuel with 91/93 and see a gain in horsepower).

A lot of this is "theory" though - in the real world, simple acceleration onto a highway may cause knocking which will in turn mean timing is pulled and then more fuel has to be used to offset that and it cascades into being way less fuel efficient while getting onto the highway, even if once its cruising its _very slightly_ more efficient.

On a test jig, a car at a constant speed - lower octane will give a higher mpg.
In most cases in the real world you are better off using the highest octane your car can make use of. At the end of the day - horsepower IS a measurement of efficiency. The argument that lower octane fuels give increased mpg is simply that we arent talking about the "upper bounds" of engine potential.

[ross1]

[QUOTE=rswapp;30470377]Octane level has nothing to do with fuel economy,

Wrong.
IF the engine didn't self tune to some degree you'd be correct. Our engines will take advantage of the octane available and bump timing accordingly.
Don't believe me? Take a long trip on fast highway and compare mileage when filling w reg v premium. I usually observe 1 or 2 MPG difference.
When I do take a long trip I also use that opportunity for a strong dose of Techron. When that is in the tank it goes up even more.
The economics of reg, premium, additives are another topic.

[JimLev]

^_Same here.
When I lived in MA I made many trips back and forth to Rochester, NY using cruise control.
Mass Pike and NY Thruway, about 400+miles each way. I compared MPG using 87 and 93 octane.
I always got 2 more MPG using 93 which is what I always used in the car.
+2 on using Techron, good stuff.

[StephenVA]

++1^ Higher Octane (up to and just over the Min levels recommended) even with the higher costs will provide a cleaner burn, better tip in acceleration (highest vac, highest combustion temps readings), and the LEAST knock sensor interference on fuel/timing reductions. Ross1's comments/postings have been proven over and over again since the fuel economy runs done way back in 60's sponsored by Shell Oil. GOT IT? 50 year old argument being rehashed...WASTE OF 02.
:deadhorse::deadhorse::deadhorse::deadhorse::deadh orse:

As gas prices are in the tank (pardon the pun), why the hell would you "want to save $$" on this approach? Change your oil, filters, and keeping your tires inflated correctly has a greater impact on fuel/operational economy than cheap gas will ever provide. Want to SQUEEZE the last drop in operational costs out of a 20+ year old BMW? Skinny tires, alignment set to zero toe and camber, rip off your outside mirrors, duct tape your grill shut, keep the windows CLOSED w/ A/C off (see how that works during the summer) and duct tape an egg to your right foot. As you the driver stabbing the pedal has more to do with economy than any "octane" levels in your last fill up will ever have....

The last question I have is who is driving anywhere now days that focuses on gas octane costs vs the Red Death Flu?

Going to get my meds now..... LOL

[ross1]

++1^ Higher Octane (up to and just over the Min levels recommended) even with the higher costs will provide a cleaner burn, better tip in acceleration (highest vac, highest combustion temps readings), and the LEAST knock sensor interference on fuel/timing reductions. Ross1's comments/postings have been proven over and over again since the fuel economy runs done way back in 60's sponsored by Shell Oil. GOT IT? 50 year old argument being rehashed...WASTE OF 02.
:deadhorse::deadhorse::deadhorse::deadhorse::deadh orse:

As gas prices are in the tank (pardon the pun), why the hell would you "want to save $$" on this approach? Change your oil, filters, and keeping your tires inflated correctly has a greater impact on fuel/operational economy than cheap gas will ever provide. Want to SQUEEZE the last drop in operational costs out of a 20+ year old BMW? Skinny tires, alignment set to zero toe and camber, rip off your outside mirrors, duct tape your grill shut, keep the windows CLOSED w/ A/C off (see how that works during the summer) and duct tape an egg to your right foot. As you the driver stabbing the pedal has more to do with economy than any "octane" levels in your last fill up will ever have....

The last question I have is who is driving anywhere now days that focuses on gas octane costs vs the Red Death Flu?

Going to get my meds now..... LOL

Thanks for the laugh.
I bought fuel yesterday, the first time since the first week of March. I think my lawn tractor has used more gas than my car.
I've been using 87(E10) in the Briggs twin.Would I get more acreage if I tried 93?
A friend has an LS GTO for a pleasure car that uses perhaps one tank/yr. He buys race fuel because he likes how it smells. I'll ask him about mileage.

[JimLev]

I've been buying ethanol free fuel for the (LS3) hotrod and sometimes put it in the 540. At $2.90/gal vs $1.39 for the regular Chevron I don't buy it all the time.

[ross1]

I've been buying ethanol free fuel for the (LS3) hotrod and sometimes put it in the 540. At $2.90/gal vs $1.39 for the regular Chevron I don't buy it all the time.
$1.39! I haven't seen that since the Carter administration. I paid $2.10 yesterday and wanted to kiss the pump.
Hell, @ $1.39 I'll daily my '72 MkIV!

[JimLev]

It just went up a few days ago to 1.39. It had been 1.29 for the longest time. Before that it was 1.19.
We've been going on 300+ mile drives at least once a week just to explore the state and (for me) to find old cars on the side of the road.

[R Shaffner]

When my '97 540 got totaled, I test drove the '02 that I have now. First M62TU I'd driven and I was disappointed by the low-end torque. I'd open the throttle wide at lower rpm in 3rd and...nothing. Clearly less than what my old car had.

The gas tank was almost empty. I thought maybe the problem was the gas. I put in a few gallons of 93 and after a few miles viola -- problem solved. Not only did it regain the low-end punch I was used to, but the Vanos gave me even more. I smiled all the way back and bought the car.

After I bought it the seller said he was glad I liked it, but he was personally disappointed by how it drove. I didn't bother to tell him he had the wrong gas in it.

[StephenVA]

Best story yet on this topic of cheap gas performance vs "how cheap can I get" theories.... Still do not get the mind set of running cheap gas to save a $5 on the tank with all the down stream issues. If you must put in the mid grade at least otherwise we have to take up a collection plate for your purchase....

[BimmrMeUpSnotty]

Shoot, regular out here is still above $2.15. Yeah, I never understood the crowd that refuses to believe the differences between regular and premium, my brother is one of them. Can't fix stupid.

[JimLev]

Chevron just upped their price, it's now $1.49. A lot of the Speedway stations are at $1.55 to $1.59.
Down in ABQ some of the stations are $1.89-1.99.
Still cheap. I'm sure many of you remember the $4/gal days.

[BimmrMeUpSnotty]

Sure do, premium was hitting over $5 here in Northern VA that summer.

[mattmar1]

yup oh man when Katrina hit...