About Knock Retard (KR)

About Knock Retard (KR)

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, 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 on. One problem in particular is the head gaskets. During one of my engine teardowns, Zoom 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 the bit that protrudes into the cylinder will glow red hot, thus creating the potential for a nasty ‘hot spot’. This is an example of an area that should be checked and perhaps replaced 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, causing 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. One microphone is located immediately beneath a cylinder bank and the other is 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 respond 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, the 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). The engine should be retarded so that the detonation is naturally eliminated. The higher the voltage is, 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, the less work the engine has to do. This causes the engine to lose a noticeable amount of power. This also reduces 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?

You would lose approximately 2 hp per degree with KR. 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, only an approximation.

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, 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 dependent upon throttle position, MAF, MAP, engine load, engine temperature, and RPM. As RPM and engine load increases, 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 the engine’s RPM or load; it jumps to a high value on throttle input and then recovers to a low value, or zero perhaps, as the engine’s 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 within the frequency of real KR it does not necessarily mean that it’s 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 transmissions 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 their specifications (14 lb-ft).


Read more

KR-Graph4: What the PCM Believes to Be Real Knock

KR-Graph4: What the PCM Believes to Be Real Knock

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