why does detonation lower the EGTs?

[A80Avant]

i cannot grasp the idea...

[Nuvo]

Maybe this link will help you?

[Error404]

i think it is becasue the combustion chamber absorbs more of the heat


EDIT: Beaten!

[A80Avant]

Is knocking rythmic?

[Nuvo]

EDIT: Some great info here:

To answer your question (it is periodic):

Detonation causes a very high, very sharp pressure spike in the combustion chamber but it is of a very short duration. If you look at a pressure trace of the combustion chamber process, you would see the normal burn as a normal pressure rise, then all of a sudden you would see a very sharp spike when the detonation occurred. That spike always occurs after the spark plug fires. The sharp spike in pressure creates a force in the combustion chamber. It causes the structure of the engine to ring, or resonate, much as if it were hit by a hammer. Resonance, which is characteristic of combustion detonation, occurs at about 6400 Hertz. So the pinging you hear is actually the structure of the engine reacting to the pressure spikes. This noise of detonation is commonly called spark knock. This noise changes only slightly between iron and aluminum. This noise or vibration is what a knock sensor picks up. The knock sensors are tuned to 6400 hertz and they will pick up that spark knock. Incidentally, the knocking or pinging sound is not the result of "two flame fronts meeting" as is often stated. Although this clash does generate a spike the noise you sense comes from the vibration of the engine structure reacting to the pressure spike.

[A80Avant]

so detonation is not a sound that would follow the rpm range in rythm as the revs go up,it would become faster,right?
Im trying to figure out what it is im hearing at 1st and 2nd gear sometimes,when im at operating engine temp(90C) and im at a stop,beginning to move in 1st gear,usually on uphill(more load),as i start to move the car i hear a knockin sound,cant determine if it is the engine or something else,it is rythmic,goes side by side to the revs up to 2500in 1st gear,then dissapears,and if i change to 2nd gear,it may also sound again,this time no more than 2000rpm where it dissipates and the engine sounds normal again.

i have had that eversince i bought the car.i cant say i payed any attention to it,but i would like to clear that this isnt knocking.And if it is,is it something i should be worried about,since it is only in low revs and for a very short period/occasions?

[Error404]

an interesting read there Nuvo, but never really clears up the difference between detonation and pre-ignition..
As far as i understood it pre-ignition would be casued by overheated spark plugs or something similar, some hotspot in the chamber, but detonation would be ignition of the mixture due to compression - i.e. a lean mixture not supporting compression / boost level

[dan_b]

@PapaVas - the sound you describe, is it a low pitched vibration, and do you feel it actually vibrate through your seat? If so, could be transmission vibration

[EspenW]

Im trying to figure out what it is im hearing at 1st and 2nd gear sometimes,when im at operating engine temp(90C) and im at a stop,beginning to move in 1st gear,usually on uphill(more load),as i start to move the car i hear a knockin sound,cant determine if it is the engine or something else,it is rythmic,goes side by side to the revs up to 2500in 1st gear,then dissapears,and if i change to 2nd gear,it may also sound again,this time no more than 2000rpm where it dissipates and the engine sounds normal again.

I doubt this is detonation. An RS2 turbo (or larger) does not produce much boost at such low rpms in the lower gears, so the engine acts as a low compression NA engine below 2500rpm, thus not very likely to knock.

[Error404]

yer won't be det at that revs.
Could it be the clutch release or a transmission bearing i wonder?

[A80Avant]

i see...
thanks for the input.
No vibration though through the sound..could be something else

[newsh]

Papvas, its the WGFV clicking, tictictictictic, only at large throttle openings and low revs in first and second

[newsh]

The ECU can richen the mixture when it detects knock which will then lower EGT.

[A80Avant]

really?
i have never heard that sound on WOT though!could the sound be covered by the engine noise?

[Greenm3]

Detonation Lowers EGT because as some one stated earlier the engine takes the heat. This is beacuse in normal combustion a lfame fron toccurs and there is a thermal barrier the surfaces and the burning of the gases and fuel. In detonation or knock it is a violent explosion destroying the thermal barrier and this is why pistons are holed etc in extream cases. Just been having fun causing these effects on a adjustable compression ration riscrdo engine. Not nice to hear but fun!
I know this does not help your problem but some background for your question.
Michael