Copy & paste:
The M60 Engine
The 740i used a 4.0-liter version of BMW's M60 V-8 engine, making 282 horsepower at 5,800 rpm and 295 foot-pounds of torque at 4,500 rpm. The M60 was ahead of its time, with a lightweight engine block, dual overhead cams, a high-flowing plastic intake manifold feeding four-valve heads. The M60's fracture-fit, sintered-iron rods kept a set of 10-to-1 compression ratio pistons swinging on the crankshaft. However, while the BMW's DME engine management system was sophisticated for its time, it didn't have the provisions or octane sensors needed to make 10-to-1 compression safe with cheaper gas.
Octane Requirements
Use less than 92 or 93 octane in this engine, and the only way the computer will know about it is after the cylinder-specific knock sensors sense detonation occurring. That's like throwing water on a barn fire instead of keeping it from happening in the first place. Best to play it safe and use the best gas available. Besides, if you fill your tank twice a month, you're only saving around $50 a year switching to the cheap stuff. That's about 1/30th the cost of a new set of M60 pistons, which is exactly what you're in for if you allow cheap gas to detonate in your high-performance engine.
Conclusion, don't bother with anything lower than our RON95.
