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Old 08-06-2009, 08:25 PM   #16
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Join Date: Mar 2008
Location: Oklahoma City
Posts: 1,209
Originally Posted by Lil bastard
Uh... anything which causes the A/F charge to ignite prematurely is detonation!

Too high a CR for the fuel used, too high a charge temperature (intake temp or fuel temp), too advanced a spark, too low an octane fuel for the CR, too little engine cooling, the wrong heat range spark plug, carbon deposit hot spots in the cylinder, or the cumulative effect of any combination of the above.

And, it doesn't only affect forced aspirated (turbo, s/c) engines either, today's naturally aspirated engines are just as prone to it.

Modern engine management software will advance the spark to just shy of the detonation threshold to achieve maximum efficiency. As such, it doesn't take too much of an adverse condition to throw it beyond that threshold.

Once the knock sensor senses the increase in vibration, which it attributes to knock, it increases it's base signal to the ECU which then begins to retard the spark either to a pre-programmed level, or it will advance the spark until detonation is achieved and then retard it back slightly from that mark.

Because of this, naturally aspirated engines are more sensitive than ever to such things as the intake temperature.

I agree with what you said except the intake air temp claim. For ambient air temp to affect the engine it would have to be pretty high, 150-200 degrees. In a naturally aspirated engine I think you will agree that the air temp in the manifold is pretty low, in some instances actually causing frost to form because of rapid expansion. I'd like to know where the temp is taken that Jaay is getting on the Scangauge tool.
Sadly on the outside looking in.
"Drive it like the Doctor ordered"

Last edited by Jaxonalden; 08-06-2009 at 08:28 PM.
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