Do I need the expensive gas???

I am pretty new to this "Ultimate Diving Machine" experience. I want to treat my car right, but I also don?t want to throw money out the window when I don?t need to. My previous cars have never required premium gasoline, but they have never been BMW?s either. I read in the manul to use 91 octane or above to avoid knocking in the engine. But I have also heard reports that unless you are driving a super high performance vehicle that your car won?t know the difference between

87, 89, or 93 octance gasoline. I have a 2003 330 CI, It is the most high performance vehicle i have ever owned, but i don?t know if it qualifies as needing the higher octane gas...and with the gas prices today, if it doesn?t need it, I could save $5-$10 a week by getting the lower gas. What does everyone think? Has anyone had experience with knocking in their engine from lower octane gas?

The manual says 91 or higher. Use 91 or higher. If the manual is incorrect, you have legal recourse. If you cause damage by using the wrong octane you're on your own.

You should use 91. You are right, you don't know if it qualifies as needing the higher octane gas. However, luckily for you, BMW does know. They say use 91. If you use lower grades, your car's anti-ping sensors will reduce your engine's performance until the pinging stops, and you will enjoy reduced performance from your machine. Why somebody would spend $30k or $40k on a car and then reduce its performance to save $10 a week, I have no idea.

What the article was *actually* telling you that if you use an octane level in excess of that which is specified, there is no benefit. This is true.

93 octane will not get your car anything extra. 91 octane will not get my car anything extra, because 89 is specified.

You can put a performance chip in your car, or many other cars. Such a chip will modify the engine operating parameters to use the increased anti-ping capabilities of higher rated gas. In that case, you must use the rating indicated by the Chip manufacturer. In your case, a chip might require 93 octane, and to use less would impair performance. Which would negate the reaons for buying the chip in the first place.

-Russ.

I don't belive that he will get any better mileage, nor fewer repairs, by using the correct grade of gas rather than an under-spec rating suchh as 89. Do you have evidence to the contrary? The only problem will be reduced performance.

-Russ.

"Somebody" wrote

Wrong. He will also get less mileage, because the engine computer will have to run the engine richer to keep the intake charge cool. Retarded ignition timing will also result in less mileage, in addition to lower performance.

Floyd

The owner's manual says: Fuel specifications The engine uses lead-free gasoline only. Required fuel: Premium Unleaded Gasoline, min. 91 AKI. AKI = Anti Knock Index

So the answer is YES, you need to use premium fuel of at least 91 pump octane in the US. If you use less you will get worse fuel economy and run the risk of causing engine damage due to detonation (pinging).

At most stations the added cost of 10 cents per gallon is only about 5% of the cost of the fuel (at ~ $2.00 per gallon). Hardly worth breaking into a sweat about, is it?

-Fred W

Russ,

Yes, actually, part of that reduced performance that you mention will be reduced gas mileage. By retarding the spark you will no longer be optimizing the combustion cycle to extract the most power per potential BTU of energy in the gasoline. The potential power in both grades are (roughly) the same, it's the reduced tuning that causes the reduced fuel economy (as well as reduced power output).

As far as the potential for engine damage, if the octane is low enough that the ECU can no longer retard the spark enough to compensate, or if it gets to the point of dieseling, then yes detonation can cause engine damage.

-Fred W

Your engine has a compression ratio over 10:1 - that is definitely high performance land for an engine. It needs 91 octane gas. What many people don't realize is that while the spark is retarded with lower octane gas, it is also very likely that in many situations (pulling a hill, hot summer days, for example) this may not be sufficient, so the next step for the DME is to richen up the mixture. Lower octane and retarded timing tend to increase cylinder head temp - which the DME is monitoring - so it starts throwing in an overly rich mixture to obtain some extra cooling from evaporation of the excess fuel. Now you dump this extra fuel into the catalytic converter - guess what? It starts getting hot. Remember, all this can happen without any knocking. The DME will 'try' to prevent knocking at all cost.

It's all dependent upon driving conditions, load, air temp, etc, but I sure wouldn't do it. Read an article recently (don't remember where) where Steve Dinan went through the intricacies of providing adequate cooling while testing cars on a dynamometer. The M5 he was testing ran the fuel mixture up to as rich as 9 to 1, if I remember correctly, (as opposed to the 'correct' 14 to 1 ratio) during the test trying to keep itself cool - even though he had a pretty good fan blowing air at it to simulate the effect of road speed cooling air flow.

From my own experience, our 91 octane here in CA is such junk that I will often get major brand gas that just doesn't seem up to par. My 328 runs poorly until I drain that tank and fill with another brand. Any trip over to Nevada or Arizona where I clean the tank of CA gas usually gives me better mileage and performance - until I get home.

What would be really neat, and add greatly to these discussions, would be for someone in this group with an OBDII scan tool to run a test using the real-time monitoring capability for spark advance and mixture ratio, using 89 vs 91/93 octane gas. My money's on it showing a quite a difference in 'tough' situations - say pulling up I-15 from Barstow to Vegas on a summer day.

Just my $.02

Frank

"malt_hound" wrote: > dugies25 wrote: > > I am pretty new to this "Ultimate Diving Machine" > experience. I want > > to treat my car right, but I also don?t want to throw money > out the > > window when I don?t need to. My previous cars have never > required > > premium gasoline, but they have never been BMW?s either. I > read in the > > manul to use 91 octane or above to avoid knocking in the > engine. But I > > have also heard reports that unless you are driving a super > high > > performance vehicle that your car won?t know the difference > between > > 87, 89, or 93 octance gasoline. I have a 2003 330 CI, It is > the most > > high performance vehicle i have ever owned, but i don?t know > if it > > qualifies as needing the higher octane gas...and with the > gas prices > > today, if it doesn?t need it, I could save $5-$10 a week by > getting > > the lower gas. What does everyone think? Has anyone had > experience > > with knocking in their engine from lower octane gas? > > > > The owner's manual says: > Fuel specifications > The engine uses lead-free gasoline only. > Required fuel: > Premium Unleaded Gasoline, > min. 91 AKI. > AKI = Anti Knock Index > > So the answer is YES, you need to use premium fuel of at least > 91 pump > octane in the US. If you use less you will get worse fuel > economy and > run the risk of causing engine damage due to detonation > (pinging). > > At most stations the added cost of 10 cents per gallon is only > about 5% > of the cost of the fuel (at ~ $2.00 per gallon). Hardly worth > breaking > into a sweat about, is it? > > -Fred W

Hey everyone- Thanks...I am sticking with the higher octane, I just wanted hear what others thought. Everyone is correct, saving a few bucks at the pump is not worth ruinning an awesome machine. But at the same time, if the lower octane gas was not helping or hurting, then there is no reason to throw the money away. But since it reduces performance, milage, and could do some long term damage, then it is worth it. Thanks again

I just don't get it. You have spent a fair amount of dosh on a fairly new car. You have clear instructions from the manufacturer what fuel to use for optimum performance.

For the sake of 500 bucks or maybe a bit more per year you want to consider contravening the instruction?

There is an excellent expression in German: Milchmädchenrechnung (milk maid's calculation).

It just doesn't add up.

DAS

For direct contact replace nospam with schmetterling

"dugies25" wrote

Exactly.

BTW, congrats on your purchase, have fun and visit here more.

Floyd

Not enrichment of fuel mixture but retardation of spark. Same impact, lower mileage. Whether the reduced mileage (at lower cost per gallon) is less efficient in terms of miles per buck would be an interesting experiment.

R / John

Another point worth debating is the fact that the octane rating at the gas pump says "minimum octane 91". This implies that the gas could be higher octane but they are only guaranteeing 91.

I have a related question: Where is the gas mixed to give it a specific octane rating? At the refinery or at the gas station? I ask because if it is at the refinery (my assumption), do they send 3-4 different tanker trunks to the stations for the different grades, one tanker with different grades in different fuel tanks, or do they somehow mix it?

Lastly, how big of a difference (other than price) is there between 89 and 91 octane gas? Is 2 octane significant enough to even worry about?

you do get poorer mileage on lower octane as well, because the higher octane burns cooler and longer so the engine also get more energy out of the same amount of fuel.

you save nothing by going to a lower octane, you are just fooling yourself into believing you are.

snipped-for-privacy@canada.com

"Robert La Ferla" wrote >> I am pretty new to this "Ultimate Diving Machine" experience. I want

In the states the octane rating is a strange combination of two different octane tests. Notice it will state (R + M)/2 or something to that effect. Interesting isn't ti.

At the refinery but each truck has more than one compartment.

"330xi" wrote

Sorry, but that is complete crap.

Floyd

The few dollars saved per week will be partly eaten up in poorer gas mileage me thinks. Not worth it in my opinion. I have a 330 and I wont put 87 in it for any reason. I think the "savings" with 89 would not be significant enough in the long run.

Jeff

Alright, that makes sense. I wonder if the decreased mileage offsets the increased cost of the gas or not? I found I had better mileage with my chip installed runningg 93 than I did without it on 89, but the cost of premium easily outweighed those savings.

I'm aware of that possibility but I don't think that could happen by running a BMW motor that specs 91 on only 89, do you? Maybe if you lug it up a mountain pulling a trailer...

-Russ.

As a youth I worked in a gas station and each tank was filled from a different compartment on the truck, they shipped the three grades seperately from the refinery. A tanker truck has several compartments each with their own fill and drain outlets, look along the top and you'll see the fill caps, the drain outlets may be covered or may not be depending on the trailer design.

The difference is important if your car specifies the requirement. Manufacturers don't want to put their buyers through the increased cost and hassel of premium gas if there isn't a solid reason.

-Russ.