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      04-13-2014, 07:11 AM   #13
MysticBlue
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Drives: '20 M5C, '09 M3 DCT Coupe
Join Date: Apr 2008
Location: Eastern Oregon... They call 'em rigs here

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If the manual recommends 91, then stick with 91. Using less than 91 will not cause any harm because of the incredible engine management that modern cars are capable of. If an engine is designed for 91, due to it's higher compression ratio, then that's where it will get the best efficiency and performance. Less octane would cause knocking, but the engine management software dials back timing and other parameters to keep it from knocking, which is good... It will do no harm. But, if you are using lower octane because it is cheaper, you are probably losing any savings to lower gas mileage because of the engine management software dialing back the performance. Plus you aren't getting the full potential performance you paid a premium for... More than likely one of the reasons you bought a BMW.
On the flip side, putting higher octane in a car than what it was designed for will do absolutely nothing to improve performance or efficiency and is a complete waste of money.
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M5C 20% tint, Xpel full front, partial sides and rear. Spacers, 15mm front, 12mm rear. Turner Motorsport Oil Cooler Guard.
M3: Active Autowerke filter, Dinan underdrive pulley, Dinan full exhaust and software, 3.45 Differential, Springs, Brakes, Dinan/HRE wheels, 275/30-19 w/10mm spacer, 295/30-19 w/3mm spacer, painted front reflectors, 30% tint, Escort 8500i & Lidar Jammer, LUX H8 V5 halos, Euro rear foglights, 3M ClearBra.
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