Knock and Detenation
 

i kinda understand what knock is.. but i was just wondering how can you tell if you are getting knock? and what causes detenation and what exactly happens?

"Knock" got it's name from the noise that is created from the collision of multiple flame fronts and increased cylinder pressure that causes the piston, connecting rod and bearing to resonate.

This can be caused by carbon deposits or bad gas. High octane fuel is a good way to counter this.

Doesnt it happen often when your lean under boost and the gas burns so fast that the flame can go back up toward the injector before the intake valve is closed. So its exploding oustide of the cyclinder. That is the understanding that i have, so please corret me if im wrong.

Its a VERY distinct sound. When my motor blew I heard it. you WILL know if you have it... its a metallic sound. When I got mine though, its was because I 'spun multiple bearings and threw a rod' 02.gif

Predetonation is when the mixture gets ignited before the spark. It can be cause by exessive heat, poor octane, a/f too lean, bad tuning, ....

the knock sensor is there to see if this is happening. If it is, it will advance or retard timing. That is one of the reason you can get better performance with higher octane fuel, since the knock sensor doesn't detect predetonation, it adjust the timing at a level that would not be possible with lower octane. This is theoretically. On our cars, it doesn't make much of a difference but, for example, on an srt-4, it's a noticeable difference.

knocking, or predetonation is basically when the pressures and tempetures inside the cylinder exceed the spontanious combustion threshold of gasoline. Instead of waiting for the sparkplug to spark and ignite the mixture, the fuel/air misture detonates on it's own. The sound comes from not only the "explosion" but because the piston, rod, and crank are all still traveling towards Top dead center and the pre-detonation is trying to push it back down the way it came.

The reason this can kill an engine comes exactly from that "backwards" pressure. Small amounts will not kill your car, your knock sensing ignition can hear the small amounts of knock that you cannot, and will retard the timing until it goes away. It is the large amounts that YOU can hear that are bad for your engine. At this point, the engine is literally spasiming as it's inertia keeps it running one way and the predetonation is trying to push it the other. It is this that breaks pistons, bends rods, and pushes the head off of the block.

This is actually how desiels work. They do not have a sparkplug, but depend upon cylinder pressure and tempeture to make the mixture explode. As desiel fuel is that much more resistant to exploding, it can take upwards of 14 to 1 compression to achieve this.

Does anyone has a picture of the knock sensor in the engine bay????

It mounted near where the driver's side front strut is. It's horizontal and held with a metal bracket.

Actually, that sensor is to make sure that its not an outside source which it is hearing. The actual knock sensor is inbetween cylindars 2 and 3 pretty close to the top of the block. I will add a picture later today (if I remember).

If I remember correctly there are two knock sensors on the Tibs. One in the engine and one on the strut tower. The one in the engine is to detect engine knock. The one on the strut tower is to make sure the knock the engine sensor detected was actually a knock and not just a bump in the road. Otherwise the igination would retard the timing every time you hit a pot hole or big bump.