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ik its a really dumb question i keep seeing every post about how adjust there tune to get rid of the know seen on the trinity what is it and how do i check it ?
 

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Q:What is Knock Retard?

Knock Retard (hereafter referred to as KR) is the response from the PCM to cylinder detonation. KR is the measure of the number of degrees of overall ignition timing advance that must be removed from the engine to prevent detonation from continuing, thus protecting the engine from damage.

Q:What is detonation?

KR is a result of detonation. To have ‘real’ (more on ‘real’ vs ‘false’ KR later) KR, you MUST have detonation. Detonation is the uncontrolled combustion of the intake charge. “Uncontrolled” means that the mixture ignites via a means other than the spark from the spark plug. In most cases, the uncontrolled ignition is due to a ‘hot spot’ in the cylinder. Hot spots can be caused by uneven combustion, spark plugs that are rated too ‘hot’, lean fuel conditions, breathing restrictions (exhaust / intake), bad gas and so forth. One problem in particular that came to light for me was the head gaskets. During one of my engine teardowns, Zooomer from ZZP pointed out that, while my cylinder bores are perfectly round, the head gaskets are NOT made perfectly round. Some of the gasket material actually protrudes slightly into the combustion chamber. Since the head gasket bore linings are made of metal, that little bit that protrudes into the cylinder glows red hot, thus creating the potential for a nasty ‘hot spot’. This is a good area to check and perhaps replace with an aftermarket head gasket. In other cases, the ‘hot spot’ is due to unreasonably high cylinder compression. Either way, the ‘pinging’ or ‘rattling’ sound you hear is the result of the actual collision of the flame front produced by the ‘hot spot’ and the normal flame front produced by the spark plug. Typically, these two flame fronts are opposing fronts, meaning that they are expanding, or propagating toward each other, thus the collision. Real KR does NOT occur without detonation occurring FIRST.

Q:How is knock detected?

Since detonation results in noise (the rattling or pinging sound of the two colliding flame fronts), it can easily be detected through the use of microphones attached to the engine in key locations. On both the L36 and L67 3800 engines, there are two microphones. Each one is located immediately beneath a cylinder bank and are mounted in the block of the engine directly into the cylinder water jacket. As the sound of detonation occurs, the noise is ‘heard’ by the microphones and the signal is carried to the PCM where it is analyzed. The PCM determines whether or not the signal provided by the microphones is knock or just normal engine noise. Knock is detected by the frequency of the signal. The severity of the knock is determined by the voltage level of the signal. Another way to say it is the voltage level of the signal will determine the level of KR. The PCM is tuned to responded ONLY to those signal frequencies that it has been programmed to recognize as knock. Anything else is engine noise.

Q:How does the PCM respond to knock ?

Engineers designed into our engines a safety mechanism for protecting our engines from KR. To do so, the PCM must respond electronically somehow to the knock signal. To electronically eliminate KR, and thus detonation, it is necessary to reduce the heat in the cylinders. Heat is a byproduct of power, so to reduce heat … power must be reduced. The PCM can reduce power electronically by retarding the overall ignition timing. The PCM converts the voltage level to a corresponding spark timing degree (KR) by which the engine should be retarded so that the detonation is naturally eliminated. The higher the voltage, the higher the KR. By doing this, the spark ignition of the combustion mixture occurs much later in the cycle of the piston compression stroke, thus reducing the effort the piston undergoes in compressing an explosion that has occurred ~15 degrees prior to TDC (top dead center). The later the ignition occurs, the less combustion that is compressed, and the less work the engine has to do. The effect of this is to cause the engine to lose power; a noticeable amount of power. The other effect of this is reduced cylinder temperatures which immediately dissipates cylinder ‘hot spots’. With temperatures down and ‘hot spots’ gone, detonation has been eliminated. The KR response by the PCM is limited to not exceed 25.5 degrees.

Q:What does the PCM do immediately after the detonation levels begin to fall?

Once the PCM has retarded timing sufficiently to reduce knock below the currently detected peak level, a changeable parameter in the PCM governs how quickly the overall ignition timing can be restored to normal levels (more on this later). The engine could see a peak of 15 degrees of KR from which the originating detonation may immediately disappear. However, the PCM will not instantly restore timing to pre-detonation levels. Instead, the PCM cautiously and conservatively restores ignition timing at a rate of 0.8 degrees per second. In the event of a 15 degree KR event, it would take nearly 19 seconds for the ignition timing to be restored to pre-KR levels. By the time your car sees full power again, the race is already over. This ‘time’ that the PCM takes to restore the ignition timing is called the Recovery Rate (more on this later). The Recovery Rate will continue in this slow fashion until KR reaches zero, KR increases back above the current recovery value, or the throttle is released.

Q:How much horsepower do I actually lose with KR?

Approximately 2 hp per degree. At 15 degrees of KR, you are subject to lose 30 hp. At 25 degrees of KR, you lose approximately 50 hp. Yes, it is VERY substantial and VERY noticeable. Please note that this is not EXACT hp lost … it is approximate.

Q:Why do I NOT want to have KR (why is it bad)?

Due to the retardation of the ignition timing, KR causes the vehicle to lose substantial power. More importantly, though, the flame front collisions are EXTREMELY harmful to the pistons. These highly volatile areas in the cylinder can cause stress cracks in your piston, which will eventually give way causing an entire CHUNK of your piston to lift right off and begin banging around inside the cylinder. This is why when the spark plug is removed after such an event, the plug end is bent all the way over. The broken piston can be VERY expensive to fix if you are not capable of doing the work yourself. DON’T EVER DISABLE YOUR KNOCK SENSORS. It takes less than 3ms to damage your engine due to knock.

Q:How do I know if I have KR?

KR is an electronically determined value based upon signal input from the knock sensors. As such, the best way to determine whether or not you have KR, and if so how much, is to use a scan tool to actually read that parameter ID (PID) from the PCM. There are three tools readily available : Autotap, Scan Master, and a Tech 2 that can show you your KR value.

Q:What is REAL KR and what is FALSE KR?
Real KR is KR that grows with engine RPM and engine load. It depends entirely on detonation, which is dependant upon throttle position, MAF, MAP, engine load, engine temperature, and RPM. As RPM and engine load increase, the chance for KR (or higher KR) increases. As the vehicle shifts to the next gear, KR will usually make a small jump up as well due to the higher engine load.

False knock is characterized by a sharp spike to an immediately high value of KR followed instantly by the KR Recovery Rate. It doesn’t grow with engine RPM or load, it jumps to a high value on throttle input and then recovers to a low value, or zero perhaps, as engine RPM continues to increase. Note that this is exactly opposite to the characterization of REAL KR. Remember, knock is simply specific noise detected by engine microphones. Because it happens to fall with in the frequency of real KR does not necessarily mean that it IS real KR.

Q:What can cause FALSE KR?

Outlined below is a list of things that can cause false knock.

Sway bar hitting exhaust downpipe – This happens typically with the downpipe of headers because that configuration puts the downpipe in very close proximity to the sway bar much closer than the stock downpipe. The banging noise from the two metal objects hitting may resonate through the frequency band that the PCM detects as knock through the knock sensors. The solution to this is to flip the swap bar over. Because of the curvature of the sway bar near the downpipe, flipping it will allow the sway bar to curve AWAY from the downpipe rather than toward it.

Transmission oil stick hitting exhaust crossover pipe – This typically happens with the crossover pipe of headers due to their large size and proximity as opposed to the stock crossover. The banging noise from the two metal objects hitting may resonate through the frequency band that the PCM detects as knock through the knock sensors. The solution to this is to carefully bend the trans oil stick away from the crossover pipe so that the two do not touch.

Anything loose in the engine or outside the engine may cause noises that drift through the frequency range that the PCM detects as KR. Carefully check your engine! This is very vague and is intended to be vague because just about anything loose in or out of your engine that is making noise could cause this. This includes loose or noisy components in your transmission as well.

Loose knock sensors, or knock sensors that are too tight. Double check that your knock sensors are torqued to spec (14 lb-ft).



Sent from AutoGuide.com Free App
 

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Q:How is knock detected?

Since detonation results in noise (the rattling or pinging sound of the two colliding flame fronts), it can easily be detected through the use of microphones attached to the engine in key locations. On both the L36 and L67 3800 engines, there are two microphones. Each one is located immediately beneath a cylinder bank and are mounted in the block of the engine directly into the cylinder water jacket. As the sound of detonation occurs, the noise is ‘heard’ by the microphones and the signal is carried to the PCM where it is analyzed. The PCM determines whether or not the signal provided by the microphones is knock or just normal engine noise. Knock is detected by the frequency of the signal. The severity of the knock is determined by the voltage level of the signal. Another way to say it is the voltage level of the signal will determine the level of KR. The PCM is tuned to responded ONLY to those signal frequencies that it has been programmed to recognize as knock. Anything else is engine noise.
Was reading this old thread with an excellent description of how engines detect knock. However, in doing some research on knock detection, I came across a video that explains a new type of knock detection being used by many manufacturers that is faster and much more effective than the older system of placing a couple sensors in the block. I didn't know this existed.
Here's the video: CLICKY
If you listen to the portion of the video beginning at11:30 and ending at 14:13, it describes ionic sensing knock detection ignition systems. These new systems send a low voltage signal through the spark plugs in between the "spark events" to sense ionization (caused by detonation) within the combustion chamber. This enables the latest generation of engine control modules to tailor spark advance and fuel management to each individual cylinder based on its needs ALONE, thus increasing potential for hp, mileage and emissions by being able to respond quicker and more efficiently than the old systems that utilized a coupe sensors in the block.

I assume that our beloved HEMIS still use one sensor on each cylinder bank for knock detection. I'm wondering if this new system might be a good thing, and if it might be in the works for future knock detection in FCA motors?
 

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Let's try this again:

I'm wondering if this new system might be a good thing, and if it might be in the works for future MOPAR engines?
 

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Let's try this again:

I'm wondering if this new system might be a good thing, and if it might be in the works for future MOPAR engines?
While there is one knock sensor per bank with the signal from the knock sensor and the signal from the crankshaft position sensor the engine controller can know which cylinder is generating the knock and deal with it on a per cylinder basis and even on a per power stroke basis but adjusting the fuel injector pulse width and adjusting the spark timing.

The system you describe could be a good thing -- I seem to recall it being used in a few vehicles -- but the knock sensor system is pretty darn good and well tested. To accommodate the new system the engine controller firmware would need to be revised and this section of code is at the heart of most critical aspect of engine control. A lot of regression testing would be required before this could be put into service.

The general rule is if it ain't broke, don't fix it.
 

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Some really good explanations posted above. Below is what I was taught in regards to “what is knock?”. But first of all what is “controlled” combustion?:

Deflagration:
The desired combustion process in an engine is known as “deflagration”, originally from a Greek word (then Latin) meaning “to burn down”. “Burn” being the operative word here. This is controlled combustion. We want the fuel/air mixture to burn rapidly – not to knock/detonate (or explode). The flame front of deflagration travels across the chamber between 35mph & to around 130mph (50ft/sec to about 190ft/sec). It’s sub-sonic combustion.

Deflagration is referring to a combustion process that “propagates due to the absorption of adjacent heat”. The spark is the initial heat source, and the unburnt fuel surrounding the spark absorbs heat from that spark causing it to burn, and the process continues as the flame travels across the chamber.

As the flame-front travels across the chamber and expands, it compresses the unburnt mixture ahead of it. If too much compression of the as yet unburnt mixture occurs ahead of the flame-front then part of the remaining mixture can detonate if other risk factors are present.


Knock (Combustion Knock/Detonation), ("Detonation", originally from a Greek word [then Latin] meaning “to thunder down”):
As mentioned in a post above, in an engine detonation is uncontrolled combustion. It’s referring to a combustion process that starts due to heat being generated within a portion of the unburnt fuel/air mixture itself, & at some point after the initial start of combustion from the spark plug. Perhaps due to part of the remaining (as yet) unburnt fuel/air being compressed by the expanding burning mixture. The remaining unburnt fuel/air mixtures becomes superheated from the expanding burning mixture, causing it to spontaneously ignite. Detonation is a post-ignition issue -- not to be confused with pre-ignition.

The flame front of detonation, I believe, can travel at 3000mph (4500ft/sec), & therefore detonation is supersonic combustion. This supersonic combustion produces a supersonic shockwave. The shockwave then drives the combustion process, as the head of the shockwave then compresses more of any unburnt fuel, raising its temperature and causing the detonation process to continue until there is no unburnt fuel left.

The shockwave produces the sonic pinging sound (or pinking as we call it in the UK). In part, some of the sound we hear is that shockwave hitting some of the metallic components inside the engine (piston; valves; cylinder head). Causing those components to ring out for the length of time that the shockwave exists.

Risk factors can be the engine running too hot; timing too far advanced; fuel/air ratio incorrect (too week); compression ratio too high; low octane fuel – (particularly if there is no knock sensor to monitor combustion knock); not enough turbulence in the chamber can also be a factor.

Pre-ignition isn't detonation, they are two different phenomena. However, if pre-ignition is present it can (but not always) be one of the risk factors that leads to detonation.
 
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