Good stuff Jeff. More things to consider beside knocking... these built-in thresholds can only accomodate so much. Typically with cheaper grade fuel, pretonation is only the tip of the iceberg. You still need to consider the ill effects/after effects of the carbon buildup from carbon deposits on your valvetrain, etc. Over time considering this and the fact the valvetronic motors with the high compression ratios, that $100-200 you save will end up costing you a much prettier penny.
Also the degradation will get more severe causing other things such as further retarding of the timing. You can only go such much BTDC before throwing CEL's, etc.
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Originally Posted by JCL
I fully respect your opinion. Mine is just different. The knock sensors are measuring vibrations representative of pre-detonation, on every firing cycle, per cylinder. It is an active feedback. Any active control system is going to require a feedback loop. It all depends on the threshold setting, how severe the abnormal combusion is (or if it ever even reaches the knock point). It would be reasonable to detect pre-detonation prior to any severe knock condition). BMW does state in their literature that lower octane fuel will not damage the engine. They also rely on the sensors for other than low AKI fuel: they are there to protect the engine from fuel problems, exhaust problems, carbon build up, and other things that cause abnormal combustion. They have to have designed in a threshold level.
As to the $$ that you save, I agree. In my area, it is $0.12 per litre, 80 litres per week, about $10 per weekly fill. $500 per year, and that is for regular. I used 89, for half that saving. (Note: I don't use 89 on the twin turbo 3.0, it is illogical) For the E53, it was completely irrelevant in the economic big picture. For many, it is easier just to pay it and not think about it. For others, there is no option because they require 91 or similar from their local pumps or experience power and mileage loss, due to their local fuel quality.
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