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Old 03-05-2010, 07:27 AM   #1
lvan
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Octane question about M42/M52

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?
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Old 03-05-2010, 09:01 AM   #2
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My '95 vintage M42 has 89 octane recommended in the owner's manual and on the fuel filler door, actually it says "minimum 89".
I usually put 91 in the summer because there is slight knocking on acceleration with 89 when its hot out.
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Old 03-05-2010, 09:01 AM   #3
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I think its because you are using too high of an octane for your M42. I think its 89 octane.
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Old 03-05-2010, 09:11 AM   #4
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Quote:
Originally Posted by lvan View Post
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.

Last edited by MarkD; 03-05-2010 at 09:14 AM.
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Old 03-05-2010, 10:45 AM   #5
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Got it! Thanks Mark.
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Old 03-05-2010, 06:24 PM   #6
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I'm semi-certain that the E30 version of the motor required 91, while the E36 version was quoted as 89.

Could be off. But it sounds right to me.
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