Leaded or unleaded????

I think I've answered this one before, after you posted much the same question in the "M Power" section....

If you can afford to run the M635CSi, then wherever possible tank up with leaded 4-star. It might cost about £1.10 per litre, but it's original spec petrol, and well worth it.

Otherwise, use Optimax or LRP (or a mixture of both).

See THE LEADED PETROL WEBSITE

Edited by Horsetan

Sohlman wrote: ...Basically the lower octane of unleaded may cause some pinking...

....which is precisely what you are trying to avoid in the first place.

In the worst case scenario, pinking / incorrect detonation will eventually leave you with a hole in your piston crown...

Sohlman wrote: But you won't get pinking if you run on the higher octane unleaded...

...yes, but you specifically wrote "lower octane... unleaded"... which in my book is 95RON. There's no way I'm running my Six on that unless it's a dire emergency.

Edited by Horsetan

Basically the M6 engine is the same engine as an e30 M3 just with two more cylinders. (Stictly speaking the E30 engine is a 4 cylinder version) If the car has not been chipped it should have a good octane tolerance, but the car will obviously run better on the higher octane fuel. If you are really worried about things give Barney at Munich Legends a call on 01825 740456 and he will be able to tell you exactly what the cars can and can't do. He will also be able to tell you things about the fuel lines which i can't answer.

My '6' runs fine on 95 RON, and so do all the others I have heard of. People who have had engine problems (eg over heating) have not been able to pin the blame on unleaded fuel that I know of (anyone know different???).

All the materials in your engine are compatible with unleaded fuel, however; -I am not discounting the possibility of pinking, but IMHO this is only likely if your compression ratio is unusually high. Note, it could be like this if your car does not get driven hard or is burning oil; both these things will foul the combustion chambers and up the compression ratio, cause pre-ignition, or w.h.y.  If it is bad enough it won't matter what fuel you are trying to burn, you'll still get pinking.

Check you spark plugs out; if they have been in for 2000 miles or so they will tell you a lot about the state of the combustion chambers. If your plugs are dirty or have deposits on them, after this sort of mileage, get it looked at. If not, try a tankful of 95RON and see how you go. If you are concerned, have a bottle of octane-boosting additive handy; you can stick it in later if you do get pinking- it will soon mix as the fuel is circulated through the fuel rail and back to the tank all the time the engine is running.   

Leaded fuel is strictly necessary in a small number of engines only; in the remainder the upper cylinder lubrication advantage it gives is marginal, and it definitely causes premature spark plug wear and brain damage.

If you are still concerned, stick in a Broquet fuel catalyst. I still don't know if these are just snake oil or not (and I doubt we ever will for sure) but if it gives you peace of mind it is worth every penny.

Hope this helps.

cheers

Had a inline lead filter on my six. Slowed the fuel flow down and made no difference to the longevity of the engine. Needless to say it is no longer on the car.