The 98/95 Ron Question- dare I ask it???

[RJ C63]

Unless 95 ron is hard to come by, why would people not want to use 97/98 ron and maximise available performance. The cost difference is negligible in context with the overall cost and running of these cars.

 

[Shaunee]

Regardless of whether or not people have "got away" with running 95 octane fuel, isn't really what the OP is after I suspect.

Yes... the ECU will adapt, but it can adapt only so far. The only way you'll know is by logging the ECU. Finding out how much ignition retard is being "learnt" and whether or not the ECU is able to handle this, without any form of continued detonation.

I'm sure the tuners on here will be able to add further context, as to what they have seen on the ECU, and how far it can calibrate itself as stock.

 

[c63chris]

I think you will find this is exactly where the tuners come in. A standard map from the factory has just this, tolerance to deal with a wide range of fuel. As others have said 95 will affect performance but the car will run fine. An aftermarket map takes away some of this tolerance to give more power on higher octane fuel. The manual for my C63 only said to get the car adjusted if you were always going to use lower octane fuel

 

[rhud]

What about the other way round? My petrol A Class (95 on the flap sticker) seems to run better on super unleaded. Am I doing the engine any harm?

 

[flango]

What about the other way round? My petrol A Class (95 on the flap sticker) seems to run better on super unleaded. Am I doing the engine any harm?

No you are doing it good by using better quality fuel and detergents, not sure you will see the fuel benefit of the increased octane rating though