Expert: Josh Fields - 11/11/2006

Question
I have a chance to buy a very sweet 1991 525i, but I'm not keen on spending the extra 10% on premium fuel the car requires.  What do you think of running this car on regular? Or running one in three tanks premium?  Or is it if I'm not willing to pay the piper, I don't hear the tune.

Answer
Ed,
 I'm not sure that a 1991 525i should use premium fuel.  My father has an '89 and it calls for mid-grade (89 octane where I live).  If you read this in the owners manual then I would stick with it, but I know for a fact that my 1999 328is calls for mid-grade as well.  This 328is uses the same engine as your 525i, just with a few upgrades.

 You cant really "run one tank in 3" with this system.  I'll try to explain why.  In an engine, the piston moves down and pulls air/fuel into the cylinder (like a syringe pulling medicine in).  The intake valve closes, and the piston moves up.  When the piston moves up, it is squeezing all this air and fuel into a little pocket above the piston.  When air and fuel are compressed like this they are more unstable and produce more energy to move the piston down again.  Ok, now high performance engines have compress this air/fuel into an even tighter space to produce MORE power from the fuel being pulled in.  Here's where octane comes in.  87 octane (low grade) is more "unstable" and wont take being squeezed/compressed very well.  It will ignite under compression too early.  89 octane is more stable, and will handle more compression without preignition.  and finally 93 octane (premium) will handle high compression the best, and will wait till the spark plug fires to ignite.  Alot of people think that premium fuel is "better" or "cleaner" fuel.  This is not true.  You actually get more power from low grade fuel than from premium, if you are looking at it "pound for pound".    
 So you are wondering why does it matter if the fuel ignites a little early.  Well if the fuel ignites early, the piston is till moving upward, and the fuel pushes against this upward movement and actually hurts performance, not to mention it stresses engine components as well.
 
 So in summary, I would look in the owner's manual, or on the gas door.  BMW usually has a black sticker on the inside of the gas door that says what grade fuel to run.