Another question from owner manual about fuel

[cravin]

I was reading through the manual to figure out the iPod thing, and I came across something that surprised me. The manual says that Premium 91 octane is highly recommended, but it also said that you can use 87 as a minimum with no effect on engine life. Really?

I always thought using regular fuel was a void of warranty or something.

[bigx5er]

You are going to open a can of worms!

I'll shoot out the first opinion and then watch the fun begin

BMW engines can run on 87 octane. Modern engines have knock sensors and computers to adjust to the lower octane. You will see minor reductions in performance and some gas mileage reduction.

I used to argue that if you could afford a BMW you should be able to afford premium. I've backed off on that. A local auto columnist and car junkie says he runs 89 in his premium cars and hasn't noticed a difference.

So for a .20 savings per gallon, I may test a tank or two of 89 and see what happens.

[JCL]

Doesn't have any impact on warranty.

The reason BMW and other manufacturers specify a higher AKI (octane rating) than the engine requires is because fuel varies so much from region to region in North America. I try not to call it octane, as motor fuel doesn't have any. By calling for 91, though, just about everyone will get a fuel that is 'good enough'. You may need 91 where you live, depending on your local fuel quality (and to some degree, on the ambient temperature, and the condition of your engine)

Using a lower AKI won't necessarily cause a reduction in power and fuel mileage. It may do so, because of the variability mentioned above. The only way to know is to test it, tank to tank, and measure the fuel consumption. It the consumption doesn't climb with a lower AKI fuel, it is a safe bet that your timing is not being retarded due to knock, and that the fuel has an adequate anti-knock rating (the actual anti-knock properties, not what it says on the pump). In either case, you won't hurt the engine, you just may spend some of your $/gallon savings in increased consumption, as a trade-off.

Another variable is ethanol, which does reduce mileage. Regular fuels sometimes have more ethanol than premium fuels. Other times they have less. Depends what fuel suppliers in your region are doing.

I ran 89 in my X5 for all the time I owned it. Ran great. Best mileage ever was on 89. 87 didn't run as well. No benefit to using 91 for me, given my local fuel. Same with our 325. Same with the Z4. The 535 got 91 or 94, as that engine (with the turbos) took more advantage of the better anti-knock properties. The X3 is now getting 91, but it runs fine on 89. I just like the fact that our 91 here doesn't have any ethanol at many stations, and the 89 has up to 5%.

Cost difference here is about $0.58 per imperial gallon from 87 to 91, half of that for 89. There is no reason to use a fuel with a higher anti-knock rating than the engine needs (not what BMW publishes, but what it actually needs), as it doesn't help your engine in any way. The other things that often go along with premium fuel (more detergent, no ethanol) can help your engine, but aren't directly related to the anti-knock index (AKI).

I think that the oft-heard line that if you can afford a BMW you should put the best fuel in it is absurd. You should put in a fuel that the engine requires in practice. Anything more is wasteful. You should also use a quality fuel, but AKI is not a quality measure, in the way that sediment, water content, and ethanol contamination are.

[ard]

True. It is pretty close to a 'zero sum game'.

The DME will retard timing a bit to prevent the pre-detonation that lower octane gives you...so power and performance will be down, mileage suffers a bit.

Does the 20 cents a gallon (so 5% here in CA) offset the 5% worse gas mileage?

Even if the above is not arithmetically perfect, you get the idea.

Over 100,000 miles or regualr, will engine life suffer? Who knows? Why bother?

I actually run 95 octane in the M5 and 100 in the 996TT.

[cravin]

Thank you all for the informative posts. I really appreciate it. I just found it surprising since I always assumed that you HAD to use 91 or above to stay in warranty. Main reason for my question was if I'm driving out somewhere remote where I get to a gas station that has run out of premium or super, if a tank of regular would kill the engine. Looks like I'd be fine in a pinch.

I really appreciate this awesome forum. Thanks folks!

[Couper93]

Not sure what the other posters are trying to say so if I repeat, my bad...
Octane is the temperature at which fuel will ignite. Higher the octane, higher the temp to ignite, hence more compression, hence more power. Our engines run a higher compression so in order to optimize performance you need a high octane fuel. Yes modern engines have knock sensors so when you do have bad gas or lower octane, you won't damage the engine but with that , lower octane can result in less timing to compensate for less compression which will lead to less performance.

[bigx5er]

Originally Posted by Couper93

Not sure what the other posters are trying to say so if I repeat, my bad...
Octane is the temperature at which fuel will ignite. Higher the octane, higher the temp to ignite, hence more compression, hence more power. Our engines run a higher compression so in order to optimize performance you need a high octane fuel. Yes modern engines have knock sensors so when you do have bad gas or lower octane, you won't damage the engine but with that , lower octane can result in less timing to compensate for less compression which will lead to less performance.

Complete agree, well said. My original response was more around cost savings and MPG. So far I have noticed a performance difference with 89 octane. The X5 is plenty fast, so it's not a big deal for me. I'm still working on measuring gas mileage to see if it is worth it. As someone else said, if MPG drops 5% then it's a push and I should just run premium

[JCL]

Originally Posted by Couper93

Not sure what the other posters are trying to say so if I repeat, my bad...
Octane is the temperature at which fuel will ignite. Higher the octane, higher the temp to ignite, hence more compression, hence more power. Our engines run a higher compression so in order to optimize performance you need a high octane fuel. Yes modern engines have knock sensors so when you do have bad gas or lower octane, you won't damage the engine but with that , lower octane can result in less timing to compensate for less compression which will lead to less performance.

I wouldn't say it is the temperature at which fuel will ignite, but rather the resistance of the fuel to self-ignition, ie before it is intended to ignite due to the ignition system. So if a given fuel is not pre-igniting, higher octane ratings have no impact except to lighten your wallet.

It isn't a straightforward relationship between more compression requiring a higher octane rating, as that ignores both advances in combustion chamber design, as well as the benefits of multi-stage direct injection fuel systems.

[Couper93]

Your direct ignition, multi stage injection and advances in the combustion chambers has resulted in a lower octane requirement but at the end of the day, high performance = high compression(static or compressed) = high octane, no matter.