2nd Gen R56 Cooper Mid-grade or premium?

This topic is very debatable, but I think on a forced induction engine using a premium better burning fuel is more important than it is on a naturally aspirated engine. You'll be ok with the minimum recommended octane rating because your engine is never really under that much stress/load. The manual is for both S and non S models correct? That 91 minimum may be aimed at an S model. I run 93 in my S.

 

On boosted engines if it required you have to use it. You can damage the engine otherwise.

Some cars, like my wife's ST, you can use regular and you just get a little less power. Her last car, Nissan Murano and not a boosted engine, said it would run on regular just fine but recommended 91 for best performance. We never found a difference either in performance or mileage. Our experience was the same as many on the Murano board.

So you can run either. Do a little testing to see if you actually get different mileage. There may be a performance difference but chances are that without instrumentation you'll never see it. On non boosted cars it a timing adjustment if predetonation (knock) occurs.

 

My wife's 2008 Cooper only gets premium. The cost difference between grades is negligible. In a car like a base Cooper you need all the power you can get. Having the car cut timing because of a lower octane costs you power.

One other point is that some gas brands do not put the same additive package in their regular gas at they do their high test. Things like fuel system cleaner.

 

You will not harm your engine with lower octane, since the engine computer will make necessary adjustments if "knocking" is detected. Those adjustments will effect your performance and fuel economy, to some degree.

From my experience, my MINI R56 S gets better gas milage on premium, although it isn't a big difference. Since I drive relatively less than average miles (about 10K) per year, I go ahead and use premium without giving much thought to the extra cost.

CD

 

for the record, 89 is mid-grade, not regular. The owners manual for my turbo says 89 is acceptable and that damage may occur from regular (ie. 87)

I mostly run 91 , but i'll do 89 if i get stuck at a station really gouging the price of the premium.

 

If I'm in need of gas & premium is not readily available I put 2 gallons of regular in to get me to a station that has high test. In all my miles of MINI driving that has happened to me once in 126,000 miles. My R53 did not like regular. The difference when high test refilled the take was quite noticeable.

Back on topic: Why regular?

 

Premium only for my car.

 

^^^ What he said, always.