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Join Date: Sep 2003
Location: Littleton, CO
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Can the DME optimize for octane?

After reading the recent gas octane topic (which I'm not wanting to revive) I was wondering if the Motronic DME has the ability to optimize based on what it senses is the current octane rating in the tank, as well as air pressure, temp, and humidity.

In other words, will the DME change the spark advance curve map if it senses that engine output can be optimized based on a higher octane fuel? Conversely, can it change the curve when a lower octane fuel is used? Lots of cars now have a ping sensor that does this. I don't think our cars have one.

Back in the day, if you changed gas octane, you had to manually adjust your distibutor accordingly. Another lost art. Anyway, there are many DME gurus here who may have an answer. If this has been discussed before, my apologies.

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Old 10-16-2003, 10:57 AM
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i believe it does optimize somewhat, based on engine feedback, but don't quote me. for a very thorough treatment of the subject, see this web page: http://frwilk.com/944dme/of.htm
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Old 10-16-2003, 01:08 PM
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Yikes! More than I needed to know. I saw a Fuel Quality Switch, where the user, or mechanic can adjust for octane manually. It seems to only retard from the stock setting. What I was looking for is a real-time adjustment. I also saw maps where the advance was based on RPM and load. Where is load determined?

Anyway back to the manual settings, with a higher octane fuel it takes longer for complete combustion, so you advance (?) the ignition timing. Thus the manual settings (that retard) are only useful when you can not obtain the octane recommended by the manufacturer. I'm looking for a way to optimize the use of a higher octane fuel. Otherwise you might as well stick with the recommended grade.

** I just noticed I'm no longer a "junior member". My wife will be so pleased**
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Old 10-16-2003, 02:18 PM
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i believe, but am not sure, that the DME will advance the timing according to the burn, up to a point. also, my car says to use only 91 or better octane, so i don't know what other people are talking about when they say they use 87 octane...

if you want more advance and so on, you are going to have to chip your car. if i am not mistaken, the '87 924s has the 2.5L engine and the same DME as the 944. also, swapping your chip in that car is a piece of cake. that same web page sells chips that should fit in your car. email the guy, he knows all, fr wilks.

d.
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Old 10-16-2003, 02:58 PM
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oh yeah...

i think the retard function on the DME is for use if you can't get the correct octane fuel: read lower than 91 octane.

d.
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Old 10-16-2003, 02:59 PM
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Mine says 91 RON

RON = octane to us americans? I didn't know if that is a euro thing like hp
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Old 10-16-2003, 03:11 PM
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Re: Can the DME optimize for octane?

Quote:
Originally posted by jts924s
After reading the recent gas octane topic (which I'm not wanting to revive) I was wondering if the Motronic DME has the ability to optimize based on what it senses is the current octane rating in the tank, as well as air pressure, temp, and humidity.

In other words, will the DME change the spark advance curve map if it senses that engine output can be optimized based on a higher octane fuel? Conversely, can it change the curve when a lower octane fuel is used? Lots of cars now have a ping sensor that does this. I don't think our cars have one.

Back in the day, if you changed gas octane, you had to manually adjust your distibutor accordingly. Another lost art. Anyway, there are many DME gurus here who may have an answer. If this has been discussed before, my apologies.
951,944S, 944S2 are a yes. All have knock sensors. 944, no. No knock sensor on a plain 944.

Richard
84 FR chipped
Old 10-16-2003, 06:29 PM
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Quote:
Originally posted by TCMdocs944
Mine says 91 RON

RON = octane to us americans? I didn't know if that is a euro thing like hp
I think is euro....donīt remember my manual but i think us is something like MON,lesser octane.
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Old 10-16-2003, 11:54 PM
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the above sounds right to me.

more advance = chipped DME

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Old 10-17-2003, 07:37 AM
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