How does knock affect performance.
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Thread: How does knock affect performance.

  1. #1
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    How does knock affect performance.

    Sorry for the really long post. If people want to really learn what is Knock Retard or how does the ECM monitors it, I read that a while ago on another forum when I had my 480 HP 2001 grand prix turbo.
    It refers to the 3800 buick engine but applies similarly to the Juke.


    You will see how important it is to properly tune the engine and not only RAISE BOOST.








    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. 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.


    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. 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.


    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.




    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.




    How do I FIGHT KR?


    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.
    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.
    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 swaybar)
    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.
    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 , 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-line to this solution is, since we are all enthusiasts and want more power, this will be our second to last solution.




    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:






    AE (acceleration enrichment)
    PE (power enrichment)
    KR attack rate
    KR recovery rate
    KR starting voltage level (noise floor)
    Capping maximum KR
    Disabling knock sensors.






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


    AE


    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.


    PE


    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.

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  3. #2
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    My gp gxp ran 13.85 and dynoed 265 whp 254 fpt had major problems with knock seeing more then -8 degrees on a normal day driving home in traffic...knock really does kill hp

  4. #3
    Senior Member ugean's Avatar
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    Great write up! Thank you!

  5. #4
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    One thing that I've always wondered about engine knock is, would I know if it is happening? I don't think I've ever heard a car knock. The owner's manual says that it's normal for the engine to experience light knock under load, but I've never heard anything that sounded like knock.

  6. #5
    Senior Member ugean's Avatar
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    Older cars you can hear it. I used to have an old 302 that if I didn't run good fuel in you would hear it. It is more of a "ping" than a "knock". Newer cars catch it before it gets bad enough for you to hear. Or at least they should.

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    Senior Member Bargeld's Avatar
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    I'm surprised that such an in-depth article didn't touch on the effect of spark plugs/coil packs and how they can be changed to assist in the problem. In high compression engines (especially if you are performance tuning), the 'spark' is one of the the easiest ways to make minor adjustments based on each specific application. As an example from my previous Audi TT, once I had a SRI and stage 1 APR tune, it was 'highly recommended' that I replace the stock plugs in the coil packs with a slightly smaller gap and one temperature setting colder.
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    Senior Member JukieMcJukerson's Avatar
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    Epic post. Great job!

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    Quote Originally Posted by Bargeld View Post
    I'm surprised that such an in-depth article didn't touch on the effect of spark plugs/coil packs and how they can be changed to assist in the problem. In high compression engines (especially if you are performance tuning), the 'spark' is one of the the easiest ways to make minor adjustments based on each specific application. As an example from my previous Audi TT, once I had a SRI and stage 1 APR tune, it was 'highly recommended' that I replace the stock plugs in the coil packs with a slightly smaller gap and one temperature setting colder.
    You are so right about the Colder spark plugs. I might have deleted it while copying pasting it.


    Sent from my Autoguide iPad app

  10. #9
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    Hi all
    I have very long thread for knock effect performance but i want share with short summery for you i hope you unerstand 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.

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