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      11-03-2012, 10:42 PM   #10
328i M-Sport
Private First Class

Drives: 2011 328i M-Sport Sedan
Join Date: Sep 2011
Location: NYC

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Originally Posted by 335BBS View Post
IF engine was designed to do so. This TV show proved that a car designed to run on 87 got worse gas mileage and polluted more on 91 as it was unable to process the fuel correctly.
The problem with their test was that the car looked pretty new. Of course, a new Chevy Cruize will run the best on regular as designed.

As cars get old, especially if they were driven by grandmas and never taken above 2500 rpms, carbon deposits will get inside combustion chambers. Smaller (effective) combustion chambers will act the same way as the engine with a higher compression. And a car that always ran well on regular can start pinging. Direct injected engines create carbon deposits a lot quicker than regular SFI cars.

There are services like MotorVac that can help with the problem but a higher octane gas will work as well.