Premium vs regular


#1

So the only adverse affect in useing regular gas in a 2011 bmw 328 is subpar performance?


#2

It likely will cost you more to use regular.

When regular is used in a vehicle needing premium, the ignition timing is retarded to prevent detonation. That almost always results in worse gas mileage (in addition to the subpar performance you mentioned).

Look at your mileage before and after switching to regular gas. Then look at the cost difference between the two gasoline grades. Then do simple math. More often than not, it will be obvious that regular is more expensive to use (in vehicles requiring premium).


#3

If the mfr specifies that this engine requires premium gas, there are more possibilities than just reduced performance.

For starters, your gas mileage will almost surely drop. As a result, you might not be saving money by buying regular-grade gas.

And, while the engine’s knock detectors will retard the ignition timing when they detect pre-ignition, knock detectors can–and sometimes do–fail after several years of use. That leaves the engine prone to damage to both valves and pistons.

Additionally, some engines will develop carbon deposits if gas of too low an octane is used.


#4

Usually, on older cars without knock sensors and such, if you had to use regular to get to town in a car that required premium, you could avoid detonation by limiting the amount of throttle opening. At low throttle openings, there is vacuum in the cylinders when the compression starts and the piston has to go pretty near halfway up the bore before the fuel/air mixture is even at atmospheric pressure so you effectively have a low compression engine under those conditions.
Since that’s how you are driving anyway when you are trying to get good gas mileage, there may be no mpg penalty for using regular. It’s only when you put your foot in it that detonation causes the knock sensors to retard the spark.

Long ago, my car developed a fuel leak causing it to run out of gas on me. After fixing the leaking hose, the only gas available within walking distance was Coleman lantern fuel at some hardware store so I limped to the nearest gas station on Coleman lantern fuel, which has an octane rating of about 35 or so, maybe less.
I couldn’t go much faster than 40 mph or it would knock but as long as I drove like there was an egg between my foot and the gas pedal, it ran without knocking.

Just remember to use your common sense and lift your foot off the gas if you hear detonation, even if that means going up that hill in a lower gear and at a reduced speed.


#5

BMW usually REQUIRES high octane fuel, but if your owner’s manual only RECOMMENDS it, yes, decreased performance would probably be the only major downside.

If you can’t afford high octane fuel, however, you probably can’t afford to own and maintain a luxury car, and you should trade it in for something you can afford to drive and care for properly.


#6

Lower performance (along with lower fuel economy) is probably the only adverse effect. But using premium will give absolutely no adverse effects so why would you ever consider not using it?


#7

If there is any pinging involved when the engine is under a load then decreased performance is the least of your worries.


#8

When these “Premium Fuel” cars trickle down to their second and third owners, you can be sure they will be fed “Regular” grade fuel…Last time I checked, 93 octane was selling for $.40 more than 87 octane…That’s about $6 a fill-up…This is a profit center for the oil companies…The only time higher octane fuel makes any difference is under hard acceleration…Cruising at high manifold vacuum, the spark is advanced to it’s programmed limit as detonation is not an issue under these conditions…

If you can not detect any detonation with your ear, rest assured you are not damaging your engine by using regular fuel…Another factor to consider…As these “Premium Fuel” cars age, their combustion pressures decline…By the time they have 150K miles on them, compression readings will have dropped to the point where detonation and the need for premium fuel has passed…


#9

“If you can not detect any detonation with your ear, rest assured you are not damaging your engine by using regular fuel”

Essentially, I agree with you.
However, there are people who never seem to notice odd or new sounds in their car.
And then we have the folks who play their audio system so loudly that they would never be able to hear the somewhat subtle sound of pre-ignition.

Also, there are the folks who have lost some of their hearing acuity
Whether or not they are the same ones who always played their audio very loudly is a matter for debate.

So, I can guarantee you that there are people (possibly many) driving around with frequent pinging noises, but not noticing them.


#10

BMW highly recommends premium gasoline (AKI91). The minimum allowable AKI rating is 87.


#11

So the answer is it will run on regular 87 octane without damage…But it MIGHT accelerate a little better on 91 octane…Try a tank of each and see what YOU think…


#12

@VDCdriver

So, I can guarantee you that there are people (possibly many) driving around with frequent pinging noises, but not noticing them.

That’s true. Also at high speeds, there may be enough competing noise to mask high speed detonation. On airplanes, a sure sign of high speed detonation is rising cylinder head temperatures combined with falling exhaust gas temperatures. The rising CHT promotes even more detonation and you have a vicious circle that leads to engine destruction unless the pilot spots the problem and lowers the power or richens the mixture or both.

Constant speed props can mask the loss in power by automatically de-pitching in order to keep the rpm constant.


#13

VDCdriver is absolutely correct about people not noticing noises; or even vibrations and rough idle for that matter.


#14

In my area, 2011 bmw 328i are going for $20k+, and the difference between 87 and 93 octane is 20 cents.
the only time i would ev er recommend 87 over 93 is if you’re low on fuel and 87 is all the station has. and then it’d be just to get you to a station that has 91+ octane.

If buying premium fuel is that big a burden on you, then you should trade the 328 in for a nice new Ford Focus or Chevy Cruze(the price you probably paid for the used beemer would buy a brand new one of either of those)


#15

In addition to what’s been discussed above, it was brought to my attention that there was a feature on cartalk.com some years back about this:

Premium vs. Regular


#16

Good link, Carolyn. A lot of people still ask the same questions.

I’d like to see a comprehensive summary of myths and truths about ethanol, a link that sticks to the science without the politics and biases. We have an occasional thread on the subject, but it always gets dragged into the political arena. And, of course, what ethanol actually does and what its side effects actually are usually get skewered by our personal biases. Which usually are politically oriented. That isn’t a criticism, only an acknowledgement of the reality. We’re all human beings, and as such we all bring our political beliefs to the table.


#17

I still don’t see why a driver with a car that states “Premium Fuel Recommended” would use anything other than premium. No matter how many different ways or times I do the math I can’t see any worthwhile cost savings. 1,000 miles a month and 20 miles per gallon would yield a difference of $15/month between regular and premium. That’s one lunch, or 2 coffees, or a pack and a half of cigarettes. Woo hoo, big savings!


#18

I agree with you but…
where the heck does coffee cost 7.50?
cigs here are about 5.50 for premium brands, I know they are 10.00 in new York city
15.00 for lunch is too rich for my blood too…


#19

I agree, ASE. But human nature is far, far, far harder to understand than physics… even quantum physics… :slight_smile:


#20

i did have a coffee from starbucks for the first time the other day out of curiosity. i was not impressed. i like half coffee and half cocoa. dunkin donuts has better cocoa, its real cocoa and you can tell the difference