Knock Retard


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. 


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 propogating toward each other, thus the collision.  Real KR does NOT occur without detonation occurring FIRST.


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.


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.



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.


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.


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.


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.


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.


What can cause FALSE KR?


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


1.        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.

2.        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.

3.        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.

4.        Loose knock sensors, or knock sensors that are too tight.  Double check that your knock sensors are torqued to spec (14 lbft).



How do I FIGHT KR?


The simple and most basic answer is in one of the following (in no particular order):


1.        Reduce boost - Boost is a direct reason for increased cylinder temperatures and thus detonation.

2.        Reduce timing (if added) - Timing advance is another direct reason for increased cylinder temperatures.

3.        Install an intercooler - This is the best solution of all … no doubt.  This will reduce the intake charge temperature by approximately 100 degrees F (results vary among intercooler manufacturers).

4.        Add water injection - While much harder to tune, this is still an option for reducing charge temps.

5.        Run race gas (or at least the highest available octane gas) - Always a solution, high octane gas slows the burn rate of the combustion, thus acting inherently as a cooling agent.

6.        Keep your engine running cool - A cool engine helps to reduce the chance for 'hot spots'.  Things like lower temperature thermostats, larger radiators, etc will help.

7.        Free intake and exhaust restrictions - i.e cold air intakes, cams, headers, cat-back exhausts, larger throttle bodies, etc.

8.        Prevent parts from hitting (i.e. header downpipe with front swaybar)

9.        Add more fuel (to a point).


But there is much more to many of these and MANY necessary explanations and modifications that can help reduce KR … but most of them fall under one of these categories.  What none of these address is what the PCM can do to help reduce or eliminate knock altogether or help to decrease the affect KR has on vehicle performance.  Before we get into the PCM, let's talk about each of the solutions in the list individually.


I will NOT talk about how good or bad a particular product is or compare them to other similar products across manufacturers.  That is not the purpose of this document.  The purpose is to talk about KR and how to reduce or eliminate it.


1.        Reduce Boost - As you increase boost, cylinder pressures will increase because more air is being forced into the engine.  As pressures increase, the temperatures will naturally increase as well and will lead to detonation.  By lowering boost, you lower the cylinder pressure and temperature and thus deter the likelihood of detonation.  The bottomline to this solution is … since we are all enthusiasts and want more power, this will be our LAST solution.


2.        Reduce Timing - If you are experiencing KR and you want to get rid of some (perhaps all) of it, AND you have the ability to add/retard timing to the engine (via a MAF Translator Plus, a DHP PCM, a ZZP Mini-AFC, or the ZZP ICCU), then start by reducing to 0 (or until KR is gone) the added ignition timing.  If you still have some KR, you can then start RETARDING the timing some until KR has been eliminated.   Unfortunately, if you are on the edge of having KR then adding timing is the FASTEST way to get KR … and a LOT of it.


It is common knowledge that KR is not exactly a one-to-one ratio to the amount of corresponding ignition timing that is pulled.  In my experience, for 5 degrees of KR … 6-7 degrees of ignition timing is pulled.  The bottomline to this solution is … since we are all enthusiasts and want more power, this will be our second to last solution.


Maximum timing advance allowed by the stock cals is 17 degrees.  Typical values seen at WOT is 15 degrees.  Adding timing on top of this will only improve power, assuming you have no KR.  This is one good reason why it is important to have something like an intercooler to control KR.  With KR under control, a substantial amount of hp can be added by spark advance.  The amount possible is under some scrutiny, but everyone agrees that it is not less than 1hp per degree, and not more than 3 hp per degree of added timing.  This means that by adding a maximum of 10 degrees of timing, 10-30 hp will be seen by nearly everyone who is successful at implementing it WITHOUT KR.


3.        Install Intercooler - This is the most reliable and recommended solution to KR in a force fed application.  The available intercoolers for the L67 (Thrasher, ZZP) will drop about 100 degrees off the lower intake manifold air temperature.  There is some consequence though.  You will drop some boost across the intercooler (either intercooler) as it IS a restriction.  The gains you will see are instantaneous if you had KR previously.  If you did not have KR previously, you may not see any initial gains.  The REAL gains to the intercooler are what it ALLOWS you to do as later mods.  For instance, with an intercooler KR is MUCH less an issue, therefore running smaller pullies is MUCH easier.  Additionally, timing can now be added to the engine, again without as much worry for KR.  Intercoolers are self-contained so you don't have to worry about depleting or adding anything to it later, unlike water injection.  The bottomline to this solution is … this is the ideal way to go!


4.        Install Water Injection - Not as many Grand Prix owners have done this.  While some have been successful, others have struggled to tune it such that the car runs well.  The idea is to inject a small, very fine mist of water into the air as it passes through the intake, or into the lower intake manifold after it has already passed through the supercharger.  The water will absorb the heat in the air, thus cooling it, and then become vaporized in the cylinder and pass harmlessly out the exhaust as steam.  The amount of water we are talking about here is very small.  A negative effect of running any liquid in the air stream prior to the supercharger is its affect on the SC rotors.  They are teflon coated in the Series Two engine and some have experienced delamination of the teflon from the rotor and ultimately damaged their SC … so BE CAREFUL.  The bottomline to this solution is … it is viable and can work, but can take time to tune and should only be used in the air stream AFTER the SC.


5.        Run Race Gas - This is a GREAT solution that has EXCELLENT results in fighting KR.  High octane gas slows the burn rate of the combustion mixture, thus reducing the rate of heat build up which helps to cause 'hot spots'.  The downfall to this is cost.  At an average price of $4.00 per gallon, it is not a realistic choice for everyday use.  Bottomline to this solution is … very good results against KR but NOT cost effective.  Save it for the track.


6.        Keep Your Engine Running Cool - Since detonation is typically caused by 'hot spots' in one or more of the engine's cylinders, running the engine cooler can help reduce the chance for a 'hot spot' to occur.  Simple ways to do this is to run a lower temperature thermostat (more on this is a second), a larger radiator, an intercooler, water injection, and to lesser extents headers, cold air intakes, less restrictive exhausts and so forth.


Lowering the thermostat temperature to a 160 or 180 can help some.  As I remember, a local club member tested this mod and found an average gain of 2.5 hp by lowering the temperature.  This is roughly equivalent to a little over one degree of KR.  Of course, even with a lower temp thermostat, it is likely that your engine temp will STILL go well above that temperature simply because the radiator does not have the capacity and heat dissipation ability to keep the coolant THAT cool.  You may not see that problem during the winter months, but during the summer, and in a lot of stop and go traffic, that temperature is going to climb regardless.  So don't expect it to stay at 160 degrees just because you install a 160 thermostat.  Bottomline on this mod is it is so inexpensive and easy to do, it is well worth it to save a degree of KR.


Another way to run your engine cooler is to install a larger radiator.  With additional capacity for cooling, this can go a long way toward controlling your engine temperatures.  This combined with the thermostat mod is worth the effort.  The radiator will help to keep engine temperatures down near the thermostat temperature during those times when it wants to creap well past the thermostat temperature.  Bottomline for this solution is that it is definitely a worth while effort, but the radiator does take a fair amount of time to install.


Additionally, running a cooler spark plug will help.  Autolite 103's, for example, are three heat ranges colder and make an excellent choice for highly modified 3800 engines.  These colder spark plugs have less area exposed to the combustion chamber and do not heat up nearly as quickly as hotter plugs.


7.        Free Intake and Exhaust Restrictions - Allowing your engine to breathe more easily will help.  Installing headers, removing the U-bend, removing the resonator, installing a cat-back exhaust system, cam shafts, cold air intake, and ported & polished throttle bodies or larger throttle bodies from other vehicles like the Corvette (75 mm LS1 TB) can help.  All have the same effect, but to varying degrees.  The engine has to work LESS to breath in MORE air and pump out MORE exhaust.  Less work equals less heat over the same period.  A local club member ran dyno tests regarding the U-bend (installed and then removed) and found that removed, 5 hp was gained across most of the RPM band  along with 2.5 lbft of torque!!!  Bottomline is these are ALL excellent mods to do and are the kind you should be considering. 


8.        Prevent Parts From Hitting - This has already been discussed in the "What can cause FALSE KR" section.  See that section for more detail.


9.        Add More Fuel - The best way to tune your vehicle when adjusting your air/fuel ratio is on a dyno.  Most dynos have a wideband O2 sensor that can reliably measure your engine's actual air fuel ratio across your entire dynoed RPM band and displays it on the computer for your analysis.


Fuel Background

The stoichiometric ratio for any internal combustion four stroke gas engine is 14.7:1.  That means 14.7 units of air to one unit of gas.  This is the perfect combination of air and gas AT IDLE.  The PCM will command this combination.  Due to inherent inefficiencies in the engine, the PCM can't simply command 14.7:1 and leave it at that.  The engine will naturally drift a little in one direction (more rich or more lean) based on the last commanded a/f value.  To control this drift, the PCM actually needs to MONITOR the oxygen content of the exhaust gas so that it knows when the engine drifts off of 14.7:1 and by how much.  This information is used by the PCM to counter those drifts by commanding more or less gas depending on the direction of drift.  This whole procedure is indicated by the PCM parameter ID (PID) called LTFT or Long Term Fuel Trim (and STFT or Short Term Fuel Trim).  This parameter indicates how much the PCM is adding or deleting fuel to/from the engine over the long term (and short term for STFT).  A value of 0% indicates that the PCM does not have to make any adjustments.  A positive value indicates that the PCM is adding fuel because it is running lean, and a negative value means the PCM is removing fuel because of a rich condition.  The PCM IS limited, however and can only adjust up to 16% additional fuel or 23% less fuel.


At WOT (wide open throttle), the story is completely different.  The PCM relies on static fuel tables to determine what a/f ratio to command.  The PCM never uses the oxygen sensor under WOT conditions.  As a result also, the LTFT is never used under WOT.  This is when it is necessary to use a wideband O2 to determine your true a/f ratio and tune accordingly.  The O2 sensors used by the engine are in their nonlinear region at those O2 voltage levels which is why they are not used.  However, as a RELATIVE value for YOUR car, YOU CAN use them to get an idea of where you are at RELATIVE to previously known GOOD values that you may have correlated to a dyno.


As such, adding fuel under the right circumstances can have a positive impact on KR.  There is no clear cut formula for the do-it-yourselfer because of the unique conditions that everyone's vehicle is running under.  Fuel can be added through several methods such as the MAF Translator, a DHP PCM, the ZZP Mini-AFC, the ZZP ICCU, or simply increasing the fuel pressure at the rail through an adjustable fuel pressure regulator.


What should I monitor with a scan tool?

The following values, at a minimum, should be monitored with an Autotap, Tech 2, or Scan Master (some parameters may not be available with the Scan Master) when tuning your car for spark or fuel:


  1. Engine RPM - Useful for monitoring your ICCU various fuel and spark segmentations if you have a ZZP ICCU, or if KR is occurring at only certain RPMS and so forth.
  2. B1S1 O2 Sensor - For WOT adjustment.  Typical accepted values range between 0.88 - 0.93 volts.  Higher values indicate a richer mixture, while lower values indicate a leaner mixture.
  3. Injector Pulse Width - Must be less than 23ms at 5200 rpm, 21.4 ms at 5600 rpm, and 20ms at 6000rpm.  Anything equaling or greater than these values at these RPMs indicate that your stock injectors have gone static (i.e. always on at WOT).
  4. Spark Advance - Look for values from 15-17 with no knock on a stock PCM.  Values below 15 will likely have knock associated.
  5. Knock Retard - Best is obviously 0.  Most authorities agree that approximately 2 hp per degree of knock retard is lost.  An intercooler is the best choice to take care of this.
  6. Long Term Fuel Trim - Used to determine if your engine is running within the adjustable limits of the PCM.  LTFT should never read as low as -23%, nor as high as +16%.  Anything between means that the PCM is able to correctly adjust for engine input/output variations.
  7. Throttle Position - Used to see when you have gone to WOT, at idle, or at cruise.  Range should run from 0 to 100%.



What can the PCM do for me in the fight against KR?


Ok … now that we have covered the introductory pieces that were needed, we can proceed to the question that you have read through six pages to get to. 


There are a LOT of calibrations in the PCM.  I might say thousands.  It is a whole sea in itself of parameters, many with meaningless descriptions, and some with very meaningFUL descriptions, and some you wonder why they are even there.  The following outlines some of the important parameters that DHP adjusts (in a more descriptive format) that can affect KR:


  1. AE (acceleration enrichment)
  2. PE (power enrichment)
  3. KR attack rate
  4. KR recovery rate
  5. KR starting voltage level (noise floor)
  6. Capping maximum KR
  7. Disabling knock sensors.


There is even the ability to ignore KR at specific RPM values.  For instance, some individuals have had very strange KR occurrences.  One example is the unexplainable instantaneous 15 degrees of  KR at exactly 6000 rpm.  It would happen pretty consistently and only instantaneously.  Once the engine pushed through 100 or so rpm starting at 6000 where the 15 degrees occurred, it would recover to 0 and the engine continued on normally with no KR.  After much investigation, the decision was made to simply ignore any KR that occurred at exactly 6000 rpm.  It is amazing what you can do with the PCM.


Ok, let's take an individual look at each of the seven items from the list above.




Acceleration Enrichment is that little splash of gas that is provided during throttle movement.  The idea here is that you add a splash of gas right at the moment you press the throttle so that any detonation that MIGHT have developed is less likely due to the cooling effect of the AE splash.  One of the parameters in the PCM allows this amount of splash to be changed.  This has been a very effective countermeasure in the battle against KR and is used widely in DHP PCMs.




These tables provide the fuel for the engine at WOT.  This is where some magic can be worked for additional horsepower gains, a/f ratio tuning and so forth.  PE = Power Enrichment.


KR Attack Rate

Like the title sounds, this is how aggressively KR is instituted.


KR Recovery Rate


Like the title sounds, this is the rate at which KR recovers from its peak knock level down to 0.  For the stock PCM, this rate is 0.8 degrees per second.  This means that with 15 degrees of KR, it would take nearly 19 seconds to recover to 0.  This recovery rate can be changed to any value.  Some PCMs have this value set to 2.5, while others have it set to 5.0.  At a recovery rate of 5.0 degrees per second, it would take only 3 seconds to recover from 15 degrees of KR to 0.  This is a VERY nice change to have in the PCM!!!


Capping Maximum KR


Simply put, this parameter limits the amount of KR that can be invoked by the PCM.  For a stock PCM, KR is limited to 25.5 degrees.  Many DHP PCMs have this value set to 15 degrees of KR.



Disabling Knock Sensors


DON'T DO THIS!!!  Not that I would know anything about this, but I strongly urge any idiot that is even thinking about doing this to reconsider!  The knock sensors are there for a reason.  If you have knock, KR is there to PROTECT your engine.   This is its ONLY function, period!  It takes only 3ms for your engine to be damaged by detonation if the knock sensors are not enabled to protect it.  Typical damage is indicated by a chunk of your piston (usually cylinder 1 or 3) breaking off and 'banging' around inside your cylinder.  You will be lucky if your cylinder does not become scored so that you only have to change a piston.  Otherwise, you are looking at a whole new short block.


KR Starting Voltage Level (Noise Floor)


OK … this is the one that I wanted to really make mention of.  For those of you that were following the thread on ClubGP about the LS1 throttle body kit from ZZP know that I mentioned a particular modification that was done in the PCM to help keep my most recent KR issues at bay.  I mentioned that I needed to get some scope shots to help better illustrate what I wanted to talk about.  I also mentioned that I wanted it in a different thread so that the information would not be lost or buried in the LS1 thread.  So here we are.


The noise floor for KR is the voltage at the PCM will start to recognize knock, at least this is the theory.  I say 'theory' because noone is 100% sure what EXACTLY this parameter does, but given the description and the results, we have ideas.  The stock setting for this parameter is 0.5 volts.  This means that anything below this value will be ignored by the PCM.  The normal idle knock sensor signal level on MY car was closer to 1.0 volts.  Since my idle signal levels were 0.5 volts HIGHER than the stock detection settings, DHP raised that value from 0.5 volts to 1.0 volts.  The results were ASTONISHING to say the LEAST.  I will get into these results in a moment.


Before we get into the graphs, let me explain what I did.  I connected a wire from the front cylinder bank knock sensor and routed it into the cabin and terminated it with a female BNC connector so that it could be connected to one channel of a dual channel oscilloscope.  I connected a wire from the rear cylinder bank knock sensor, and in the same way as the front, I routed it into the cabin and terminated it also with a BNC for use with channel two on the scope.  I took readings at idle, at cruise and at WOT with and without KR, and repeated them MANY times.  The particular scope I used has a built in floppy disk drive and selections for converting the scope shot into an Excel CSV file.  For each scope shot, I saved it to the floppy as a CSV file AND as the proprietary scope file so that I could also call it back up on the scope later.  I set the scope up to trigger on the leading edge of any positive going AC voltage level that exceeded 14.1 volts.  Because of this, I had to generate KR that would exceed this value.  If this value was not exceed, the scope would show nothing … it would still just sit there and wait for the trigger voltage.  In this way, I could correlate KR readings from the Tech 2 with a TRIGGERED scope shot.  I also wanted to trigger high KR at LOW RPM so that the actual knock signal would not be hard to find buried in high engine noise.  Since I know my car very well, I knew what I needed to do to generate KR under these circumstances.  During these tests, whenever my KR exceed 14.1 volts, the scope was triggered and the signal snap shot was captured instantly on the pulse that exceeded 14.1 volts.  Each time I successfully triggered a scope shot that I wanted to save, I exited the highway, pulled into a gas station and saved the signal to the floppy disk.  The I resetup the scope for the next trigger and started again.  The idle shots were taken while idling in the gas station.  The weather was cold (28 degrees) under mostly cloudy night-time skies.  The time was between 2:00AM and 4:00AM when there would be the least amount of traffic on the highway.


For reference purposes, my car has the following engine and trans mods:


Intercooler             LS1 throttle body kit            Ported and Polished SC      Custom PCM

Custom CAI          Mild CAM                             Headers                                 Offroad Pipe

No Ubend              No Cat                                    2.6" Pulley                             3.29 Gears                             

LSD                        Raybestos Clutches            Mild Shift Kit                        10 Deg Added Timing

38lb Injectors        Mini-AFC


Note - Since I have control over the timing, and I was NOT running race gas, I did not have the extra degrees of timing dialed in.  I was running the stock 17 degrees of ignition timing.


The Knock Graphs


The graph below (Graph 1) shows the knock sensor voltage level of my car at idle.  Essentially, the signal is simple engine noise, but note the scaling on the Y axis.  There are some peaks up near two volts, but the bulk of all of them are down near a volt or below.  There is no actual knock in this particular signal.  The X axis is scaled in units of time.



Graph 1



The next graph (Graph 2) below shows what the knock sensor signal looks like at WOT with no KR.  Notice again the scaling on the Y axis.  This time it is VERY high.  But of course I was careful to ensure that there would be no knock during this run.  This is simply engine noise at high RPM.



Graph 2


The third graph below (Graph 3) shows what the PCM believes to be real knock (not just engine noise).  This knock is indicated by the spikes in the signal along the graph.  The largest spike by far is the one on the far left.  Note the scales on BOTH axis.  This is significant signal strength at VERY high frequencies.  We are talking in the nanosecond range.  Time separation between distinct knock pulses is about 23 microseconds.


By expanding the time base of Graph 3, we can zoom in on the large spike to the left as shown in Graph 4.  This is what the actual knock pulse looks like.  It is a high frequency, very short burst with one occurring every 23 ms as shown in Graph 3.  Within the burst, the waveform cycle varies anywhere between 120 ns (yes NANOseconds) and 300 ns, but is consistently within those time frames in the testing that I've done.  The actual frequency range of this corresponds from 8.3 MHz (120ns) to 3.3 MHz (300ns).  The actual pulse frequency does not matter so much, except to know that it is a VERY high frequency.  We aren't talking microseconds here … we are talking NANOseconds and megahertz.  Look at the voltage on that initial spike.  It is  nearly 16 volts!


So What's The Bottomline … why do I want to change the KR noise floor?


The point to all of this really is to show where the knock voltage is at idle, which in my car is at about 1V Peak.  Prior to DHP changing my noise floor, I was getting HEAVY KR by just accelerating away from a stop normally.  It was terrible.  Every time I accelerated … even lightly, I got substantial KR (10-15 degrees).  I never had it that bad until right after my trans was rebuilt.  Since then it has been very bad.  DHP asked me what my noise floor was on the knock sensors, and from my scope shots, I was able to tell him approximately 1 volt.  Dave then raised the stock setting of 0.5 volts up to 1.0 volts.  Since then, I have to REALLY work at getting KR.  I have learned since then exactly what I have to do to get this kind of KR to occur.  Thankfully, this fix in the PCM has prevented me from seeing KR under normal driving, spirited driving and racing.  However, if I perform the right circumstances, I can repeatedly get KR.  Fortunately, it is something that I have to really work at to have happen.

Graph 3


Graph 4

Now, is this noise floor parameter REALLY "fixing" KR?  Is it really ELIMINATING KR?  No.  Absolutely not.  What it is doing is simply masking KR up to a 1 volt.  Notice that these graphs indicate by definition that this is FALSE KR.  It is KR that jumps immediately to a BIG value and then recovers to 0 as engine RPM increases.  It appears that this parameter does MORE than just mask KR up to 1 volt because it is SO much harder for KR to be generated in the car.  Unfortunately, information about this particular parameter is limited, and it is currently unknown what it really does.  I can say, that the system still detects knock at all levels above a volt (I can generate KR from 1 degree up to 15 degrees).  When I add timing without race gas, I start to see KR at around 6 added degrees pretty easily (23 degrees overall ignition timing), so the system is working and working well.  I have had this change to the PCM since October of 2002 with no ill effects in about 5,000 miles of driving  (I put on a lot of miles).  The fact that my heavy KR started immediately after my trans rebuild also lends itself to false knock (extra noise generated by the trans).


So, the bottomline to this mod is this …. it should NOT be your first mod, but if you have tried everything else and have similar KR issues that I have described (large KR spikes in small pedal travel that recovers as engine RPM increases), then you may want to look into this one.  If I didn't have THIS one done, I would not be able to run at the track very well at all.  With the help of this PCM change by DHP, my vehicle is running between 12.62 and 12.84 at the track.


Good luck to everyone, and I sincerely hope something out of this crazy document helps somebody out there.




Bill Hooper

319 hp @ wheels

~395 hp at the crank

12.62 @ 110

MIGPC Member #3