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Old 04-25-2003, 11:46 AM
Duke2.6 Duke2.6 is offline
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Join Date: Mar 2003
Location: Southern California
Posts: 2,010
Re: Here's some good information...

In normal combustion the flame propagates across the combustion chamber in a predicable manner. Detonation is the sudden reaction of the last portion of the unburned fuel air mixture before it is consumed by normal flame front propagation that causes a very sudden rise in pressure and temperature. This creates shock waves in the combustion chamber, and these shock waves are the "knocking" noise we here as they vibrate the engine structure. The real problem with detonation is that it dramatically increases the rate of heat transfer to the combustion chamber surfaces, so sustained detonation can damage a piston or valve.

Preigntion is the initiation of combustion at a point or time other than the spark plug when it fires. It's essentially the same as advancing the timing, so preigntion can lead to detonation. Detonation can also lead to preigntion, because it can create a hot spot that acts as a preigntion source that leads to more severe detontion.

Detonation and preigntion are NOT the same thing, but one can lead to the other in a positive feedback loop. Also, normal flame propagation speeds are the same for all automotive gasoline. "Premium burns faster" or however this goes is a MYTH!!!

There are many variables to detonation, but increases in the following will increase the propensity to detonate on a given octane fuel - compression ratio, throttle opening, ignition timing, inlet air temperature. Compression ratio is the primary parameter in determining recommended fuel grade and you should consult your owner's manual to see what is recommended for your specific year/model/engine.

Detonation usually is most prevalent at low revs. Since turbulence increases the rate of flame propagation, the end gas is exposed to high temperature and pressure for more clock time at low revs and is more prone to detonation. Modern engines with knock sensors that are designed for premium fuel can be run successfully on regular because the engine controller has two igntion maps - one for premium and one for regular. If the knock sensor detects repeated detonation, it will switch to the less aggressive timing map. You might notice a loss of low end torque, but less aggressive drivers usually notice no change.

Though my '88 190E 2.6 was designed for premium and has no knock sensor, I can run regular, but have to change my driving habits - basically just a lower rate or throttle tip-in following a shift to the next higher gear (It has a manual transmission), but I can essentially "drive around" the detonation envelope.

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