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BMW 3-Series (E90 E92) Forum > E90 / E92 / E93 3-series Powertrain and Drivetrain Discussions > N54 Turbo Engine / Drivetrain / Exhaust Modifications - 335i > octane optimization with knock sensors



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      05-06-2007, 10:28 AM   #1
poiney
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octane optimization with knock sensors

My question is a hypothetical curiosity since I have no intention of running anything less than 91 octane in my 328i. Thought I should state this now, lest I get flamed by someone immediately asking why would anyone buy a bimmer and run low-octane


The OM states that for 328i, it is safe to run regular unleaded gas since the ignition will adjust timing to avoid knocking. Now let's consider running the car at high HP with low-octane fuel.

Since the timing needs to be adjusted produding a less efficient stroke, we can assume some penalty in mileage, which offsets savings of using cheaper gas (although I can't quantify this).

Now consider hypothetical "sunday driving" of the same vehicle. Even with low-octane gas, the engine won't ping. Presumably, the ignition timing wouldn't need to be impacted. Since the energy density of gasoline is nearly independent of octane, I would expect no penalty in gas mileage, and so a proportional savings in fuel cost.

However, I don't know how quickly the engine adjusts timing to resist pinging, so maybe even under low HP conditions, the engine is not running near peak efficiency. That is, maybe the engine utilizes a less efficient algorythm while burning low-octane fuel. Dunno.

If anyone has any data on this topic (either directly or from web) I would be curious to see it.
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      05-06-2007, 10:34 AM   #2
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you sounded smart until you misspelled "algorithm".

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      05-06-2007, 11:23 AM   #3
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Quote:
Originally Posted by poiney View Post
The OM states that for 328i, it is safe to run regular unleaded gas since the ignition will adjust timing to avoid knocking.
The 13B-REW Rotary engines I was working with would eventually blow up due to weakened apex seals if there were only a few ping events. So to be on the safe side, we would do things like always run it a little rich, to help avoid the pinging in the first place.

But regardless of the engine, detonation due to pre-ignition will drastically shorten its life. It seems that a properly tuned engine would not even need to rely on the knock sensor - after all, it's only supposed to be a back-up system in the rare events something goes wrong. I am surprised that your owners manual states that.
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      05-06-2007, 02:09 PM   #4
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The 2007 OM gives minimum safe rating as:

328i -- 87
335 -- 89
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