my e63 was 11.3 to one compression. You are a new poster, so don't know what your level of expertise, but here is my understanding of these motors and octane. Contrary to all of the advertisements on TV, Octane does not make your fuel more potent, it makes it less potent. Octane is an ignition retardant, preventing fuel from going up in flames before the spark plug wants it to. At 11.3 to one you are going to get pre-ignition on 87 octane, probably 89 as well. I don't care what you do to retard timing, you are just compressing the hell out of the fuel air mixture and you are running the risk of pre-ignition if you are driving the car mildly hard. The car will do the best it can to pull timing and protect your motor; any car in america has to be able to run on 87 octane fuel. But they don't say run well. You are going to at the very least lose power, at the most damage the motor. If you have a bi-turbo 63 - yes, you are running less compression. But now you are running boost, which is worse. So now the motor, sensing preignition, will pull boost as well as timing, reducing power output. IMHO, run at least 91. Or sell the S63, buy a s550, and run whatever you want as long as it ain't diesel.