Originally Posted by lvan
This is where I get confused about these engines.
I always ran my M42 on 91+. Some owners keep saying that M52 328is run on midgrade (89octane)..
How is it that M42 that came in 1991 uses 91octane fuel ? Than you got M52 1996 + that uses 89octane? I used to think that more powerful M52 would run 91+.... How is this the case?
Simple... the M42 does not have knock sensors and has a 10:1 CR, the M52 has knock sensors but has a similar CR (10.2:1)
If the DME runing the M52 detects knock, it can retard the ignition timing. The M42 DME can not do that, so higher octane fuel must be used.
You have to remember that the M52 is more powerful because it has 55% more displacement from it's two extra cylinders, not because it has a much higher CR.