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Old 02-04-2006, 08:11 PM   #31
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Join Date: Sep 2005
Location: Minneapolis/St. Paul, Minnesota, USA
Posts: 3,308
Originally Posted by JackG
MNBox wrote: "But, the DME does retard the Spark, that is, produces it Later in the Engine Cycle."

You are correct in your revised statement... The DME *retards* the spark timing when it detects knocking from the use of low octane fuel.

The low octane fuel burns quicker, igniting faster, which forces the retarded timing.

That just flies in your earlier statement, which bears correcting.

Happy motoring!


Not at all, well sort of... I gave an incomplete answer. I was trying to give a brief answer without giving a Course on Internal Combustion Engines. I suppose you could start every explanation with: In the beginning, God created the heaven and the earth. And the earth was without form, and void...then in the 1880's, Gottllieb Daimler and Karl Benz...... just kidding...

In addition to retarding the Spark, the DME interupts the Fuel Injector Cycle, shortening the duration and therby injecting less fuel.

By adding less Fuel, you effectively increase the volume of the cylinder (by the amount of volume the Fuel would have taken up). This means that the Piston can travel Higher in it's Compression Stroke before reaching a Detonation Threshold.

Then the retarded Spark is supplied to add an extra measure of insurance against continued Knock for the reasons I have already stated.

Happy Motoring!... Jim'99

Last edited by MNBoxster; 02-04-2006 at 08:16 PM.
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