KNOCK Can someone really explain what is is and what it means>?

[GstRacer]

yes i'm just starting to tune and i really need to learn about knock and everything about it so please bear with me and help me thatnks a lot

 

[spyder dst]

google ?

http://zhome.com/ZCMnL/PICS/detonation/detonation.html

 

[kpt4321]

Definately read that page, there is some interesting information there.

After you read it, if you have questions feel free to ask us!

 

[Mangold]

Basically Knock is this...

Combustion is the point at which the air and fuel mixed ignites correct... ok

under extreme pressure volatile substances like gasoline can combust without a spark or an igniting factor, so....

your motor can detonate at the wrong time because the fuel and air mixture is under such pressure as the piston is rising... if it combusts before the spark its called knock... if it combusts before the piston reaches top dead center (the point at which the piston is at the height of its stroke) the knock or detonation can put such stress on ur motor that often you will through a rod and or snap a crank so on and so forth and that is what I like to call BAD.

in short, knock is bad and its caused mostly by cheap gas that gives way to combustion easily under pressure... your ECU will monitor the motor for knock and if it detects knock it can automatically chage timing to prevent it from occuring but that can rob you of some power (timing is critical when tuning) so good 93 or 94 octange gasoline is very important... and tuning your car to have precision timing and preventing knock is good

*if you have ne more questions I would be happy to help if i can you might want to ask kpt4321 i believe he knows more than I do, i was mearly trying to put it into simple terms for you

 

[crankbender]

That is more preignition. Knock(detonation) happens usually when multiple flamefronts collide inside the cylinder. The colliding masses (air has mass) cause extreme pressures and consequently temps in small areas of the cylinder. This is why you tend to melt only part of the piston/cylinder not the entire face. Preignition breaks stuff and takes away power....knock can in addition melt your shat.

 

[kpt4321]

Originally posted by Mangold

your motor can detonate at the wrong time because the fuel and air mixture is under such pressure as the piston is rising... if it combusts before the spark its called knock... if it combusts before the piston reaches top dead center (the point at which the piston is at the height of its stroke) the knock or detonation can put such stress on ur motor that often you will through a rod and or snap a crank so on and so forth and that is what I like to call BAD.

Not exactly. If it combusts before the spark, or if a section of the mixture begins to ignite before the spark, you have what is called "preignition."

Preignition can cause detonation, and usually does. Knock or "ping" is simply the audible result of detonation.

in short, knock is bad and its caused mostly by cheap gas that gives way to combustion easily under pressure...

There is so much more to it than this. Timing advance, charge temps, compression ratio, A/F ratio, chamber design, and many more factors can create knock.

*if you have ne more questions I would be happy to help if i can you might want to ask kpt4321 i believe he knows more than I do

You did a decent job, but I wanted to clear some of it up.

 

[crankbender]

^^^^ beat ya.

 

[2GeNTSi]

Ok so would knock occur during only normal driving, hard driving, or what? Wouldn't it only occur during hard driving though? Say if you are venting your BOV and you don't have a AFC to control the A/F ratio, you would run rich correct? I'm a noob, I read that article posted and most of the information didn't make sense to me. I'm still trying to understand this whole knock/detonation stuff so that I can have an idea of where to start as to why my car is slow.

 

[Defiant]

Crankbender and I don't agree on the specifics of the terms, but so far as the phenomenon is concerned, he's not really wrong.

Where I learned cars, "knock" and "detonation" were the same thing, while "preignition" and "ping" were equal. However, this is being blurred by such terms as "knock sensor".

What is crucial, regardless what it's called, is that detonation will literally blow an engine apart. It makes a sound like someone slamming the block with a hammer, and will shake through the whole car. What I call "ping" is the kind of rattling you hear when you try too steep a grade in too tall a gear. It's still not good, but it isn't like detonation. And, it's what a "knock sensor" is listening for.

http://www.dsmtuners.com/forums/showthread.php?s=&threadid=110924&highlight=knock

 

[Mangold]

Originally posted by 2GeNTSi
Ok so would knock occur during only normal driving, hard driving, or what? Wouldn't it only occur during hard driving though? Say if you are venting your BOV and you don't have a AFC to control the A/F ratio, you would run rich correct? I'm a noob, I read that article posted and most of the information didn't make sense to me. I'm still trying to understand this whole knock/detonation stuff so that I can have an idea of where to start as to why my car is slow.

This whole knock vs detonation thing... it all boils down to this, you do not want your fuel igniting before it is suppose to

detonation is when the fuel/air combo ignites, blows up, goes boom!

knock is the term used to describe the external effects of preidetonation in a motor -- ie in a football stadium when you hit the aluminum bleecher it sends a rigging sound running down it, predetintion will send a knock or vibration through your motor

knock is the same thing, because the gas is exploding before it is suppose to it cause abnormal stress on your engine components

think about it this way... when your riding a bike you press DOWN on the pedal to make the bike move foward correct... the pressure of your foot on the pedal is sent through the chain and into the wheel causing motion... that motion of the wheel acts on you and the bike moving you foward

now suppose you push the pedal back down when its on its way back up... what happens? you lock up the chain and "drive assembly" and the tire locks up and you stop (on a simple one speed bike)

in your motor you want the fuel/air to ignite and the exact point that the piston is on its way back down (called the power stroke in a 4 stroke motor) that way the energy from the explosion or burning from the gas/air is converted into mechanical energy when the piston pushes down onto the crank -- if your fuel/air combust before the piston is on its way back down (on the way up) it will force the piston back down THE WRONG WAY -- or in the oposite direction that the crank is moving

this is why knock is such a big thing... predetonation/knock - it is all derived from fuel/air igniting before it is suppose to... and it can wreke havok on your engine components



NORMALLY this would happen under "spirited" driving conditions when your turbo is increasing the amount of air and thus fuel in the cylender... the more air and fuel you cram into that space the more likely you are to experience detonation before spark

 

[kpt4321]

Originally posted by crankbender
^^^^ beat ya.

You cheated, I type slow.

 

[2GeNTSi]

Ok I was getting worried cause I thought that the "ping" is the sound that is made if you are getting knock. Lately I noticed a kind of ringing noise coming from the drivers side when the car reaches 2k. It's only noticeable at low speeds when I'm driving in my neighborhood. I'm thinking it's just something vibrating in the engine bay but I dunno.

 

[Defiant]

Loose heat shield.

As for the bicycle, detonation would be like dropping a 500-pound weight on a pedal.

 

[crankbender]

Crankbender and I don't agree on the specifics of the terms, but so far as the phenomenon is concerned, he's not really wrong.

Yeah semantics can get confusing.


It really seems like all of you are speaking of preignition and not detonation. With preignition the air fuel mixture ignites early but burns through the cylinder pretty much how it is suppose to. With detonation you actually have ignition at more than 1 spot in the cylinder.

Imagine you have a room filled with some gas and you light it. BOOM the room goes up in flame originating at the ignition point and the expanding gasses hit pretty much all the walls equally. Not imagine you light 2 sides of the room at the exact same time. The 2 flame fronts from both ignition sites will collide in the middle of the room. This seriously magnifies the forces at that point and causes unexpected stresses (pushing on only a little spot of the piston instead of the whole thing). Along with this increases pressure comes increased temps (in that small area). The increased temps are what melt pistons.

Either way you don't want this at all.....both are horrible for performance and can kill your engine.

does that help any?

Of course I could be totally wrong on the common nomeclature here.

 

[crankbender]

in your motor you want the fuel/air to ignite and the exact point that the piston is on its way back down (called the power stroke in a 4 stroke motor) that way the energy from the explosion or burning from the gas/air is converted into mechanical energy when the piston pushes down onto the crank -- if your fuel/air combust before the piston is on its way back down (on the way up) it will force the piston back down THE WRONG WAY -- or in the oposite direction that the crank is moving

Actually in modern cars because we understand the dymanics of the oxidizing procedure your ignition comes way before TDC. This is necessary because the combustion process take a finite ammount of time. The reason to retard timing is to make the combustion process complete further down the stroke so the the pressures reached are lower and therefore temps are lower. Lower temps = lower surface temps and therefore less chance of preignition or detonation. This is why once you start knocking you have to retard timing alot to get it to stop (have to cool the surface off).

If you have full preignition on more than one cylinder your engine can run away as long as it has fuel. This is why some cars continue to run even after they are turned off (not common with current fuel systems). But this is exactly how a diesel works.

detonation is when the fuel/air combo ignites, blows up, goes boom!

This is combustion...your car can't run without this. Preignition is when it happens too soon without the spark helping. Detonation is when it happens in more than one place in the cylinder at the same time.

preidetonation

we can fix all the symantics by combining the words!!! j/k man

NORMALLY this would happen under "spirited" driving conditions when your turbo is increasing the amount of air and thus fuel in the cylender... the more air and fuel you cram into that space the more likely you are to experience detonation before spark

Actually the ignition is caused by a hot spot in the engine. As the mixture is compressed by the piston it heats up. This elevated temperature causes the mixture to become more volitile. Eventually you will reach a point when combustion just happens.... A hot spot in the cylinder (dented piston, sharp edge, wall skuff, iridium/platinum plugs, carbon buildup, etc) can cause a local area in the mixture to run a few degrees higher. This is why it doesn't always happen on all cylinders .

What is crucial, regardless what it's called, is that detonation will literally blow an engine apart. It makes a sound like someone slamming the block with a hammer, and will shake through the whole car. What I call "ping" is the kind of rattling you hear when you try too steep a grade in too tall a gear. It's still not good, but it isn't like detonation. And, it's what a "knock sensor" is listening for.

Sooooooo true man. What happens acutally in a high gear with too little RPM is the the advance the computer has programmed into it causes the mixture to complete combustion before the piston is moving down (reduced rpms gives the process more time). You really shouldn't "throttle" your car at less than 30% or so of redline if you can help it.

I believe a "knock sensor" can hear either. Waht the sensor really is is just a mic that is then filtered to listen to only the frequency of the engines current RPM. You "should" be able to tap your knock sensor and op-amp it to a set of headphones and listen to your engine when you drive. I might do this on the racecar when it is getting set up so that I can audibly hear knock. I just don't know how they keep from hearing the normal combustion sound...maybe it checks the pulse width on the sound also....

 

[Mangold]

Originally posted by Defiant
Loose heat shield.

As for the bicycle, detonation would be like dropping a 500-pound weight on a pedal.

Haha, yeah sumin like that... preignition (sorry I have a bad habbit of calling it Predetination-thats the way i first heard it) its bad stuff

NE way im not an expert on exactly how all this works, I just understand where you are coming from and that too many terms can get confusing

When you read my comments Im just tryin to help you get the BASICS of how and why this whole thing occurs... there is a lot more too it (i dont understand everything myself), thats just how ive had it explained to me and it helped me out...

As for that sounds yeah its prolly sumin loose and rattling, usually happens with an exhaust part ie hest sheild on exhaust manifold (ive had mine rattle several times... so ive just taken mine off- looks better that way to me)

Have fun, there's lots to learn but it'll start to come together for you and thats when things get fun!

 

[Mangold]

Originally posted by crankbender
Actually in modern cars because we understand the dymanics of the oxidizing procedure your ignition comes way before TDC. This is necessary because the combustion process take a finite ammount of time. The reason to retard timing is to make the combustion process complete further down the stroke so the the pressures reached are lower and therefore temps are lower. Lower temps = lower surface temps and therefore less chance of preignition or detonation. This is why once you start knocking you have to retard timing alot to get it to stop (have to cool the surface off).

Ok, thanks (im learning too man )

Makes since, I had known that ignition starts before TDC so that the power is trasfered at the exact moment which it can be most beneficial... BUT i did not realize that retarding the timing to cool the surface temp is how the motor works to prevent preignition, i had always figured that it was to decrease the ammount of time the motor had to fill the combustion chamber with feul/air

makes since though, thanks (you learn sumin new every day!)

*ie this is why I said ask the other guys , they know all the ins and outs of this stuff, i just know the basics (PRE-IGNITION IS BAD!)