Gas Octane for my 2008 3.0si

[Mike Benvo]

Originally Posted by JCL

No, I don't tune ECUs. Have you recurved a distributor? That might date me a bit.

The speed of sound is a joking reference to the acoustic sensors that listen for knock. Yes, I understand that the processor is using electrical signals, once the acoustic sensors hear the onset of knock.

No, the DME can not adjust timing infinitely. But on these engines (not S54s, with much higher specific outputs, but rather these engines in X5s) I have not heard pinging.

The OP also referenced hearing knocking sounds on startup, using 91. Given that these engines can run well on 89, and often 87, pinging or knocking on 91 suggests that the fuel isn't really 91. All that takes is a fault in the blending system inside the gas station pump. And startup noises aren't likely to be pinging anyway.

Yes, I understand the consequences of crap fuel. See post #2.

Before going further, let's agree that fuel varies a lot geographically. In my area, the fuel is very good. Yours may not be. And the label on the pump is just that, simply a label. It may or may not represent what is coming out the nozzle.

My X5 3.0 operated great on 89 AKI. No change in fuel economy from 91 or 94, measured over several tanks and various elevations and temperatures, by calculating (not relying on the OBD). That suggests to me that the timing was not being retarded, and so with my local fuel, in my driving conditions, there was absolutely no reason to buy more than 89 AKI. 87 AKI was OK, but occasionally I noticed driveabilty issues. But no ping, ever. And yes, I know what pinging is, all the way back from SU carbs, the onset of emissions controls in US vehicles in the early 70s, and so on.

I know your followup question was directed at MD, but I would point out that if there is no discernible difference in performance (and efficiency) then yes, one should run 87 all the time. The correct fuel is the lowest AKI (let's stop calling it octane, since motor fuel doesn't necessarily have any actual octane in it) that doesn't cause knock, or with these control strategies, performance degradation. This recommendation is leaving aside the effects of higher concentrations of detergents in higher grades of fuel, differing amounts of ethanol crap in different grades of fuel, etc.

I don't agree that means that one isn't loading the engine up enough. The engine is there to serve the driver; the driver is not there to serve the engine. IMO.

Thanks for the post. It is good to have an engine discussion again; we had gone awhile with a lot of wheel spacer, window tint, and similar threads.

Jeff

I agree with all of your points - they are all accurate. I like your reference about the speed of sound, upon first read I didn't know you were referring to the knock sensors "hearing" knock.

As far as "I don't agree that means that one isn't loading the engine up enough. The engine is there to serve the driver; the driver is not there to serve the engine." - you are correct about the engine serving the driver, I was just illustrating that if you make a quick switch from 91 to 87 and don't hear pinging, you aren't pushing the car enough (e.g, you aren't hitting areas in the timing maps which advance the timing to a point that which the car may ping). This could be for a number of reasons - it has detected you are running 87 during the course of your normal driving and has pulled it to some degree already (pun intended), as there are static and dynamic corrections (in a sense sort of like STFT and LTFT but pertaining to timing and other motor running characteristics other than fuel trims), or the car is not being pushed enough (in particular environments - high humidity, high altitude, high temperatures, etc.). If you drive normally in favorable environments, it may drive just fine on 87. Higher octane only comes into play when the car is pulling timing. Now, BMW has recommended 91 as a minimum because they know that anything less will cause knock in unfavorable conditions, especially when pushing the car harder. Obviously if the car was tuned for a lower octane it would be just fine, but it is a performance car with higher timing targets than 87 octane can handle. At least soccer mom will be ok