What does "KNOCK" mean

[munfordtim]

I am new to this stuff and reading throughout so of the post I seem to see the word "KNOCK" allot, so what does that mean ?

[Mike_Levy]

Knock is preignition. Its when the mixture in the cylinder ignites too soon. Its extremely bad for the motor. It burns pistons, breaks and blows stuff up.

[neidan]

The terms are often used interchangeably..... Knock (that we are talking about) is the result of improper burn, BUT the ignition is 'started' by the spark plug. However because it does not burn smoothly or correctly, the point of highest pressure takes place during the wrong part of the stroke. It can do the same types of damage as pre-ignition, and the 'effect' is very similar, but they are 'technically' not the same as pre-ignition happens without the spark plug firing.

munfordtim, the term KNOCK is used to describe the noise the piston makes as it bounces off the cylinder walls, shaking violently, as it gets forced through the pressure wave created by the fuel that just went up into flame incorrectly.. If you can picture that you can imagine how destructive it could be.

In the case of KNOCK the uncontrolled burn is most often caused by the properties of the fuel that is used. The ignition properties of the fuel are affected by a number of things including temperature, pressure. The Octane rating of the fuel, helps describe the way the fuel will burn under these various conditions. The lower the octane, the less controlled the burn. Ot to say that another way, the more unpredictably the fuel will ignite. For example a higher compression motor requires fuel with a higher octane ratting to ensure that it will light off correctly producing maximum force at the desired point on the power stroke.

Someone may want to verify what I said as I did not proof it on the internet... LOL... It is my understating of the situation however...

[duster360]

The terms are often used interchangeably..... Knock (that we are talking about) is the result of improper burn, BUT the ignition is 'started' by the spark plug. However because it does not burn smoothly or correctly, the point of highest pressure takes place during the wrong part of the stroke. It can do the same types of damage as pre-ignition, and the 'effect' is very similar, but they are 'technically' not the same as pre-ignition happens without the spark plug firing....

Thats close enough.

Main points to remember

1) Knock = Detonation; Common. Occures AFTER combustion is started by spark plug.

2)The rapid pressure rise and heat cause by the early stages of combustion are the precurser of Knock

3) Pre-ignition is NOT Knock ; Its uncommon. Uncontrolled combustion occures BEFORE spark. If it happens too much before TDC, it can bend/break pistons/rods.

[DGatzby1]

I am with Duster and Mike, what is your point anyway? This forum is for people working on or trying to tune their motors and not a scientific discussion, or a debate about words. But my $.02... The knock we are talking about is what Mike responded to with respect to the first post asking the question. It was appropriate, concise and correct. The tool we are discussing correctly identifies a very important part of the functioning of a combustion engine and that is knock and that occurs like Mike L said when the mixture is ignited too soon in the combustion cycle of the four cycles.

FYI between the education and experience some people have on this forum and others, we can write a "reference" if necessary.

[neidan]

I'm not tracking DGatzby1....

The question was asked... Mike replied, gizmodo, said his research says not so much. (I did not read it, still have not)... Anyway I replied, (trying to keep it as simple as I could) see above, Mike pointed out to gizmodo some issues with using Wiki. duster360 gave the nod to my post... (which I asked for and welcome, 40 years playing with cars, I'm still a novice IMHO) I don't think there is any hostility going on? I did not take gizmodo's post to say Mike was wrong, just that it did not jive with whatever he found... Mike did not sound insulted... I think it's all good unless I'm missing something... (been known to happen...)

Tim

[gizmodo]

DGatzby1 I'm a little confused. How can you be with both Duster and Mike? What they say contradict each other. At any rate, there is definitely some good info in this thread and that's what a forum is all about.

[DGatzby1]

Ok, ok, I read everything again. I did not mean to be hostle or sound that way, writing is strange sometimes. I agree with your technical explainations. My point was that IMHO the Wiki reference was just too technical for most readers, since the guy was just asking about "knock", that is all. You guys are correct, with all of the info here and some links outside, everyone can tune a darn aircraft now! I guess that is good. We should be able to make these Hemi's fly. Well, if we had wings at about 160 mph, maybe we could fly. We need some humor.

Mike is half right, ok, pre-ignition can and will destroy an engine, Duster is certainly correct on every point. Tim, nice explaination off the top of your head.

So lets add and tell everyone what the fundamentals are, and that is the tool can read the sensors in the engine that listen for this "knock" (ST KNOCK) and will send a signal to the PCM to pull the timing or "retard" it (lower it) so that the plugs do not fire too soon in the cycle for the conditions. That is what the Forum talks about most of the time. The more octane in the gas allows the engine to run at its full potential that DS has programmed in. If good gas is used and everything is otherwise good with the motor, the sensors will not hear the knock and ask the computer to pull down the timing and thus our motors will make optimum power. How's that?

[neidan]

Works for me DGatzby1!!! I'm with you on the "writing is strange sometimes" thing. I have a tough time sometimes trying to decide on how much information is enough.. I feel like I ether don't go deep enough and so generalize to much, or go to deep and put folks to sleep. LOL...

..... and I suspect you are correct 160 is more than fast enough for liftoff if only we had wings....