BMW Repair: octane rateing, cylinder inline, gas mileage

Question
I know that your good at newer BMWs, however I just would like to know the octane level I should be putting in my 1987 325is. Its a 6 cylinder inline, motronic fuel injected, and 2.5 L engine. Right now I put in 87, though I fear I should be putting at least 89 in. Let me know witch one, thank you.
-Ryan


Answer
Ryan the M20B25 engine in your car should be fine on 87 octane because the compression ratio in this engine is relatively low. The higher the compression (or the more heat the engine produces) the higher the octane required to keep the engine from preignition, detonation, or pinging (engine damaging). Some people think that the car runs better or get better gas mileage with higher octane. You be the judge.
-Evan