When I took delivery of my new car two months ago the saleman told me that in a recent bulletin BMW recommends REGULAR GAS when outside temperature drops below 15 degree C (59 deg F). This is to avoid getting a check engine light or something like that. He didn't give too much of an explanation.

Has anyboy heard anything like that??? It doesn't make any sense to me.

 

I wouldnt listen to your dealer... sounds old school to me... Theres a guy at work that says the same thing to me.. in the winter he uses regular and in the summer he'll use premium... hes pretty old... might be true for old cars but I know nothing about old cars. NEW NEW NEW NEW is in my book. His car recommends Premium gas...

 

No way ... that's half our summer time here in Northern Canada We have never heard of such a thing!

 

Sounds about right to me. Octane is an anti-knock component of fuel, so if you're at higher altitudes where the air pressure is a lot lower (lower mean pressure = less chance of knock) or if the weather is colder outside (added cooling effect of cold air = less chance of knocking), you might not necessarily need higher octane fuels, depending on how the engine is mapped, the compression ratio, and state of tune from the factory. On my '99 Nissan I can run regular or midgrade fuels without a problem in the winter. Once the temps come up to about 50-60F or so I need midgrade at least and preferably premium. In the summer, even with US 93 octane (R+M/2), I still get light knocking in a specific load condition that's repeatable if the temperature is above 90F out.

Like anything else in automotive design, even grade of fuel specified is a compromise. No single grade of fuel is going to cover "all" operational modes optimally. Premium fuel is better for medium to high load operation and you can get more performance and power out of it (assuming the engine is designed and tuned for it, and BMWs are), but at light loads when you're just cruising around it doesn't burn as efficiently as a regular grade gas will since the brake mean effective pressure in the engine is very low. Regular grade gas burns most efficiently at light loads, but doesn't have the anti-knock properties of higher grade fuels with more octane to give maximum performance at medium to high loads. Then there are the huge climate, temperature, and altitude variations all over the world, and a lot of other factors... Doesn't matter if it's an old car or not though. The principle of operation of internal combustion engines has not changed much since they were invented 100+ years ago.

Just experiment and see what works better... I can get 1 mpg (US) better overall with regular grade fuel, and as long as it's colder out I don't feel any drop in performance. In the summer I can also get the same fuel mileage advantage assuming mostly highway driving, but if I step on it there's a big time drop in power since the hot temps means the engine needs more anti-knock in the fuel and it doesn't get it with regular. So in the summer I stick with premium fuel only.

YMMV.

 

I understand that with knock sensors and engine management system you can run the lowest octane without causing any engine damage.

But my saleman said in a recent bulletin BMW recommends using regular gas in cool weather, and it has something to do with the check engine warning. I don't think BMW would do that just to save you money or to hurt the big oil companies.

I should give him a call to ask for a copy of that bullitin, if it in fact exists.

 

I have no idea what the hell you are talking about yet it all makes perfect sense

 

I wouldn't know. There is no such thing as winter in Florida.

 

Read this article, BMW says to use pemium 91 AKI which equates to 87 octane in the US (R+M)/M2 method. I've used regular gas in my last several BMWs since '02 without a problem or knocking. Again your most economical option is to use the lowest grade of gas that doesn't produce knocking. Now if you want additional cleaning agents or think you get better performance with more octane go ahead. In my Big Block Corvette with a 4 barrel carberator, I use higher octane gas, if I go racing it doesn't have a computer to adjust the mixture. Save some money and use 87 octane, if you don't like it switch back, but don't use it because you think BWM says you have to, or becaue it's better for you car, it's all in your mind.

http://en.wikipedia.org/wiki/Octane_rating

 

Is there any information on how "clean" gas at different octane burns? Does one grade leave more deposits than another as most marketing would suggest?

 

Hint... that's when the wind isn't blowing so hard and when flocks of people from the north appear out of no where.

I have never had issues with the check engine light in the winter on my BMWs other than once due to an O2 sensor on my E39 (reset the light and it never came back). The only other time was on an E34 in Tucson (90F+) and it was also an issue with an O2 sensor. But, there is no harm in putting 87 ((R+M)/2) Octane fuel in your car. The BMW engines are mapped for 93+ Octane ((R+M)/2) or 97 (RON) so you will have a little less power with the 87 but it won't harm the engine. FWIW, the check engine light could come on for many more reasons than just engine knock.