Noob here and a thousand pardons if this is one of those ad nauseum topics, but other than a thread from 2008 discussing octane and some separate threads about carbon buildup I haven't been able to find what I'm looking for.

Most of the posts re. octane are from 8-9 years ago and along the lines of "just use premium, it's not worth it to save $2 on a tank of gas". The difference in price from regular to premium is $0.80 per gallon where I live and I drive 100 miles a day (hence why I now own a MINI) so I am looking at $10-$12 difference per tank, about 1.5 tanks a week.

The only negatives I can find regarding using a lower octane besides less power and possibly less mpg due to retarded timing are some references to carbon buildup in the intake but I'm not sure if those apply to the base hatch. Also I can't understand how carbon buildup happens in the INTAKE before the gas burns?

Can anyone speak to the consequences of using less than premium in the base hatch (mine is a 2009 with 118K)? My manual says as low as 87 octane can be used without damage but I would still like to know what the relationship is between low octane and carbon buildup (if there is any in the base model) before I seriously consider it. I'm thinking about stepping down to 89 unless I find a reason not to.

Thanks!

Carbon build up is due to Direct Injection in the S version. You don't have that problem..

I doubt if there is a strong relationship between octane and combustion deposits, provided you are not using junk fuel. I would expect that power level, ring health and oil to have a much more pronounced effect. Running the engine on the highway at high speeds and loads is a favorable set of conditions to minimize combustion chamber deposits.


Intake valve deposits are a result of crankcase fumes venting into the intake system and coating the valves. The valves get sufficiently hot to cook the oil residue into a solid over time. Integrate deposits over 50k miles and valves get a bad case of "club foot".


According to Google, you have the N16 engine, which has port fuel injection, as opposed to the N14 in the "S" which has direct injection. Thus the N16 intake valves get "washed off" by the fuel, whereas the intake valves in the N14 do not.

FWIW, I've use 87, 89, &93 octane fuel in my Justa and never noticed any difference in performance or fuel consumption. Now I just use 87 all the time.

This is spot on.

If there is one benefit of port fuel injection over direct injection, it is the cleaning of the intake system. Gasoline is a phenomenal solvent and does a good job keeping the valves clean and cool in a port injected engine.

All Prince Engines ( 2nd Gen ) are direct injection.
The last of the Port Injection engines were in the 2002-2006 R50/53 and up to 2008 R52.

You can get away with using that fuel in your MINI since you have an updated engine and valve cover.

If you have a 2nd gen up to Oct 2010 ( and for S model equipped with N14 engine until 2013 ?), I would not be stingy on the low octane fuel. Besides, the DME will eventually detect the presence of low octane fuel, and retard the timing to prevent pre-ignition...using it will decrease MPG's. And it's the chronic pre-ignition you cant hear that will eventually damage your engine...

When in doubt, open the gas flap of your MINI and check for printed suggestions affixed to the gas flap....