Any issues running 98 octane in 4.4 X5?
 

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?

I would assume that in Australia you are using the Research Octane Number (RON) to refer to octane rating, similar to Europe. In North America, we use an average of RON and the Motor Octane Number, two different test protocols, and call it the Anti Knock Index (AKI)

In North America, BMW recommends 91 or 92 AKI, I can't remember which. Vehicles require a minimum of 87 AKI, with a possible loss of mileage and performance, and many run fine on 89 AKI with no loss of power or performance. Other vehicles don't run well on anything less than 92, primarily because fuel varies widely in quality across North America.

There is no precise conversion between RON and AKI, but it is about 5-6 points usually. So, 98 RON is the same as the 92 AKI recommendation, and many vehicles will run fine on 95 RON, which is similar to 89 AKI in North America.

If you are using the RON scale, it is not surprising that your vehicle would run better on 98 than 91 (which is similar to 87 AKI). I would use 95 RON in any case, and see if you can tell a difference. If you can't then carry on. If you can, then use 98, unless BMW Australia has a different recommendation.

If you are using the AKI scale, and it runs fine on 91, there is no advantage to providing fuel with more AKI than the engine is designed for, apart from the fact that the tank of fuel you purchased may be fresher, or may have higher concentrations of cleaning agents, both of which would explain improved engine response.

Higher octane fuel than that which the engine is designed for does not run hotter, does not contain more energy, and does not benefit an engine, but it does lighten a wallet. The exception to the above is if you have aftermarket modifications that increase the compression ratio, such as different pistons or forced air induction (turbocharging or supercharging).

Higher octane gas also smells nice and sweet when burning :) I've run high octane in the M5 and TL, and they both loved the stuff. I agree completely w/ what JCL has said. I always run the minimum of what a certain vehicles requires because anything less usually causes pinging, causing the engine to back off the timing and in turn you get reduced HP and mileage. Running more than what's needed (if you know that you don't have pinging issues) is usually just a waste of money. I run anything from 91-93 (preferebly ethanol-free) in the BMW's and bike, but I run 85-87 in my truck because its definately not a high compression/HP beast. I have no idea how to convert AKI to RON, but as long as your running the minimum of what BMW put on the label of your gas door, you should be fine. Unless you hear audible pinging (some older cars exhibit it really clearly), putting in the more expensive gas is like buying $$Dinan$$ parts...its a placebo effect that you feel like its faster because you've spent 2x what the average is. :)

So you mean to tell me, my dinan sticker doesn't increase torque?!?! :wow:

ROFL I seem to never be able to find anything thats about 91 or 92 everythign iv seen so far is always 93, and been always going to Hess never caused my engine light to go off unlike Mobile and Shell.. i dunno why but w.e Hess is cheaper and closer

I am using 95 since I bought the car ... 3.5 years now with no problems!

Most likely 95 RON, which is what we call midgrade, or 89 AKI, in North America.

I spoke to an owner of a fuel station and he said that while most of the time fuel all comes from the same places that each seller adds their own additives and that higher compression vehicle will actually react differently.

I don't know about X5's, but I've seen some 740's that had a label asking for 89 and others asking for 91. I'd bet that if Essam's X5 asks for 91 AKI that it'd run even better, especially in the kind of summer heat he gets in his area.

That is true on the distribution side, ie from the production facility up to the filling station. On the retail side, the variables are freshness of the fuel, tank sediment, water in the fuel, and whether the retailer is actually selling what the label on the pump says (whether intentionally, which is fraud, or unintentionally, which is incompetence)