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It's detonation. BMW have set the tune in the ECU with way to aggressive ignition timing advance. The 1250 engine now has knock sensors which picks up the detonation noise and the ECU backs off the timing to stop the detonation.
The octane of the fuel you use has no bearing on whether detonation occurs as the ECU advances the timing till detonation is detected.
Two problems here. My engine detonates lots and often. Sometimes the ECU does not adjust to stop the detonation and it continually detonates till you close the throttle or open it up more, neither acceptable when wanting to simply riding at a constant speed. It detonates with mild acceleration out of corners.
My mates 2019 1250 GSA does the same. He has a mechanical background and he (like me) is pissed & frustrated about the detonation and the longer term affects this will have on the engine.
Detonation is NOT good for engines, pistons, spark plugs etc etc and will/may one day destroy the engine. And it will likely be out of warranty so then BMW will say it's the owners problem. Thanks.
My 2017 1200GS ran a 91RON tune in the ECU. Never detonated. With a 95RON tune it detonated using 95 & 98RON. My mates 1200 did the same till he had the 91 tune loaded in his bikes ECU.
BMW need to back off on their obsession to minimise emissions & maximise fuel efficiency and modify the ECU tune to that which actually works in the real world.
Its not BMW obsession, its EURO5 Emission regulations by the EU that drive all manufacturers to make their engines the most efficient!
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