Any issues running 98 octane in 4.4 X5?

[JCL]

Originally Posted by m5james

That's quite a spread. What is that converted to AKI?

In round figures, 87 to 92, roughly the same as North America. There is no direct conversion, as the Motor Method and Research method are different test protocols (a test engine at high rpm vs a test engine at low speed). The vehicle is designed to run safely on 87 AKI, but better fuels are recommended.

[m5james]

Considering the heat in his area, along w/ the fact the BMW decided to Essam's car w/ extra climate control measure, I'd run nothing but 98 RON just to prevent knocking in that kind of heat as little as possible.

As for John, his climates aren't any different than what I got in Seattle, so I'd stay in the middle of the 91-98 RON if it was me.

I know there are conflicting opinions about using higher vs lower octane, but using the cheap or too low of an octane in BMW's has never served me well. This is my .02, but using the highest recommended octane will net better gas mileage as opposed to using lower octane in an effort to save money because you will get worse gas mileage, making the cheaper stuff a wash anyways.

[Fraser]

Originally Posted by johninoz2

Hi All,

I have usually always used 91 octane petrol in the past, however, I seem to have got a bad fill and put 98 to give it a boost. After a few fills of 98 the engine response now feels much better but was just wondering if there would be any long term issues constantly running on 98 octane. Does it make the engine run hotter? Does the fuel mixture need re-tuning? Any downsides apart from the extra cost at the pump?

Australian spec 4.4s are designed to run on 98RON. You can also use 95RON while the minimum permissible grade is 91RON. Personally I wouldn't use anything below 95RON except in an emergency.

[m5james]

So maybe it breaksdown like this:

91 RON = 87 AKI
95 RON = 89 AKI
98 RON = 91 AKI

Sometimes I can find 93 AKI, which might make into like 101-102 RON, which I prefer whenever I can find it.

[Fraser]

My understanding is that AKI = RON minus five, or thereabouts.

[JCL]

Originally Posted by Fraser

My understanding is that AKI = RON minus five, or thereabouts.

It isn't a linear conversion. Two different fuels can behave differently on the two different test protocols in the test engine, and so may have the same RON but different MON results, resulting in different AKI figures. As mentioned in post #7, 89 AKI = 95 RON for all intents and purposes, about 6 points. 91 RON is close to 87 AKI, about 4 points. 98 RON is about 91 AKI, about 7 points.

I filled up with 94 AKI at Chevron today, mainly because the idle was a little uneven after a lot of short trips in the city, and I wanted a tank with the maximum amount of Tecron in it.

[JCL]

Originally Posted by m5james

I know there are conflicting opinions about using higher vs lower octane, but using the cheap or too low of an octane in BMW's has never served me well. This is my .02, but using the highest recommended octane will net better gas mileage as opposed to using lower octane in an effort to save money because you will get worse gas mileage, making the cheaper stuff a wash anyways.

That is a blanket statement, and not entirely correct. The best fuel for any vehicle is the lowest AKI rating that doesn't cause the engine management system to adjust the timing to prevent knock. That may be the same as the recommended fuel, but it is unlikely. Fuel quality and specifications in the real world follow a bell curve, so the manufacturers specify a fuel that will nearly always be OK to use. That puts the specified fuel above the average required AKI, if you believe in bell curve distributions.

It has nothing to do with saving money, to me, since the savings are only pennies. It has to do with using the best fuel for the engine, and that isn't necessarily the highest AKI, although it sometimes is.

The safest and easiest is still to use what the manufacturer recommends, but using a lower AKI won't reduce power or mileage until you get low enough that the timing is retarded by the engine management system. That is why my vehicles always ran on 89 AKI (until I got the 535i with the turbos), as it was better performing in winter (we don't get high temperatures here). 89 solves some hard start problems over 91 AKI, in cold temperatures. Best mileage I ever got in the X5 (and the Z4) was on 89. However, we have high quality fuel here, and you may need to get to a pump label of 91 to get the same real world AKI, depending on where you are purchasing it in North America.

[Fraser]

Originally Posted by JCL

It isn't a linear conversion.

I know; due to the different methods to obtain RON and MON. I only suggested the -5 conversion from RON to AKI as a quick guide.

[Essam Khafagi]

In Saudi there is only 91 and 95 RON
In Egypt my home country we have 80 , 90, 92 and 95

[m5james]

80?! Is that a special lawnmower mix