[LukeNumber9]

I suspect this has been asked a few times previously so apologies if I'm asking again - I just couldn't find a definitive answer re UK fuel.

95 fuel is recommended on the petrol cap of my M240i but my question is should I put higher octane fuel in and what the pros of this are (if any of course).

I read on one thread that someone recommended to fill up with 99 every now and then but not every time.

[LukeNumber9]

Answered my own question - put half a tank of 95 octane in and I get this god awful slight tapping noise that I can't 'unhear' now. 99 octane from now on.

[Spinnetti]

An engine is designed for a specific Octane. Some ECUs will adjust down if you put lower octane in and fewer will adjust up if you put higher octane in, but generally you just "use what it says on the tin". Higher octane isn't "better" (marketing drivel), it just depends on what the engine is designed for.

[LukeNumber9]

Originally Posted by Spinnetti

An engine is designed for a specific Octane. Some ECUs will adjust down if you put lower octane in and fewer will adjust up if you put higher octane in, but generally you just "use what it says on the tin". Higher octane isn't "better" (marketing drivel), it just depends on what the engine is designed for.

My cap recommends 95 but sounds better running on 99 so make of that as you will. Probably doesn't make a lot of difference to performance based on what you're saying but can 100% tell the difference in making things sound smoother.

[crabu2]

Originally Posted by LukeNumber9

My cap recommends 95 but sounds better running on 99 so make of that as you will. Probably doesn't make a lot of difference to performance based on what you're saying but can 100% tell the difference in making things sound smoother.

If the cap says 95 RON, that's all it needs. If you got tapping with it, I think you got some bad fuel.

I say this because all modern engines have a knock sensor to prevent damage if the octane is too low. Using higher octane gas than what the car was designed for is a waste because like Spinnetti said, they don't tune engines to perform better because someone uses higher than recommended gas. You'd need an aftermarket tune for that.

As for the tapping.. I have a feeling it was your injectors... And in the past, when I've heard tapping (knocking from not enough octane), it was never at idle... It was always when I turned up the timing and stepped on the go peddle.. Mind you, this was on cars from the 80's, where I could adjust the timing myself with a the help of a timing gun.

[aerobod]

If you don't hear a "tapping" from the high pressure direct injection injectors with the bonnet/hood open, then I'd be more worried. They are quite noisy compared with conventional low pressure injectors.

[crabu2]

Originally Posted by aerobod

If you don't hear a "tapping" from the high pressure direct injection injectors with the bonnet/hood open, then I'd be more worried. They are quite noisy compared with conventional low pressure injectors.

I forgot about the DI tap.. All DI gas engines do it.. It's the nature of the beast.