KR – How To Fight?

Posted by Alex Darmos on

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

  • Reduce boost – Boost is a direct reason for increased cylinder temperatures and thus detonation.
  • Reduce timing (if added) – Timing advance is another direct reason for increased cylinder temperatures.
  • 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).
  • Add water injection – While much harder to tune, this is still an option for reducing charge temps.
  • 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.
  • Keeps 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.
  • Free intake and exhaust restrictions – i.e. cold air intakes, cams, headers, cat-back exhausts, larger throttle bodies, etc.
  • Prevent parts from hitting (i.e. header downpipe with front sway bar)
  • Add more fuel (to a point).

There are more necessary explanations and modifications that can help reduce KR. However, most of them fall under one of these categories. None of these address what the PCM can do to help reduce or eliminate the affect KR has on vehicle performance. Before we get into the PCM I will talk about each of the solutions individually.

I will NOT talk about how good or bad particular products are 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.

  • 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 bottom line to this solution is: since we are all enthusiasts and want more power, this will be our LAST solution.
  • 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 bottom lines 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.

  • 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 bottom line to this solution is this is the ideal way to go!
  • 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 effect on the SC rotors. They are Teflon coated in the Series Two engine and some have experienced de-lamination of the Teflon from the rotor and ultimately damaged their SC, so BE CAREFUL. The bottom line 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.
  • 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 buildup 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. Bottom line to this solution is: very good results against KR but NOT cost effective. Save it for the track.
  • 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. Bottom line 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 creep well past the thermostat temperature. Bottom line for this solution is that it is definitely a worthwhile 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, is three heat ranges colder and makes 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.
  • 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 lb-ft of torque!!! Bottom line is these are ALL excellent mods to do and are the kind you should be considering.
  • Prevent Parts From Hitting – This has already been discussed in the “What can cause FALSE KR” section. See that section for more detail.
  • 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.

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:

  • 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.
  • 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.
  • 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).
  • Spark Advance – Look for values from 15-17 with no knock on a stock PCM. Values below 15 will likely have knock associated.
  • 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.
  • 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.
  • 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?
Now that we have covered the introductory pieces that were needed, we can proceed to the original question.

There is 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:

  • AE (acceleration enrichment)
  • PE (power enrichment)
  • KR attack rate
  • KR recovery rate
  • KR starting voltage level (noise floor)
  • Capping maximum KR
  • 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!!! If you are thinking about doing this please 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)
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 no one 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 exceeds 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. Then I reset up 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.