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Thread: Knock Retard

  1. #1
    Fuel Injected!
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    Knock Retard

    Hello,
    i wonder what is the theorie about knock retard from knock retard-Map vs tachometer vs Knockretard table and spark advance table. Should i decrease spark advance where knock retard counts are very high or should I decrease spark advance over the whole range and knock retard counts are that high at some places because i drove in that region most of the time?

  2. #2
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    It usually takes several degrees of retard to stop knock once it begins, and this reduces power. Additionally once knock begins there is residual heat in the chamber and piston top which takes time to cool to allow maximum advance after retard. Finding the best rate at which to advance timing after knock retard can be tricky. Factory strategy is to reduce spark advance quickly when knock is detected then increase advance slowly while checking for knock recurrence.

    You are better off preventing knock before it begins than letting it begin and retarding timing to stop it.

    When you are viewing the data with spark knock remember to think of cause and effect over time. When knock occurs ask what has happened to cause it. Did the air:fuel ratio go slightly lean just before knock began? Was there a slight increase in spark advance? Is there possibility of part of the chamber or piston heating enough to ignite the intake charge? Work in reverse from the time when knock begins to try and locate the source of chamber heating. If you are experiencing knock during acceleration you may find that AFR leans slightly just prior to onset of knock, and that richening the mixture slightly prevents knock without changing timing. Or you may find that reducing advance before the onset of knock, at a lower rpm, is better than reducing advance at the rpm where it occurs.

    In the case where you are seeing knock in a less transient state, for example when you are maintaining a vehicle speed, think about what may be introducing heat into the chamber to cause knock to begin. Heat energy has to travel through the heads into the coolant coolant but if you are introducing more heat than can be removed, knock will occur. In this case slightly reducing spark advance at the rpm where knock occurs may be the best answer. But be sure to watch for slight throttle changes and slight changes in load as indicators that some operating condition needs to be addressed. If you tap the throttle and knock begins, you may want to increase acceleration enrichment instead of reducing spark advance.

  3. #3
    Fuel Injected!
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    Thank you for your answer. Seems to be a difficult and theoretical topic!

  4. #4
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    I don't believe the subject is difficult. But I believe most people think about how to treat knock after it begins instead of trying to prevent it from happening.

    The goal of tuning is to create peak cylinder pressure at the point where about 50% of the heat from combustion has been released. For pump gasoline this appears to be around 7-8 degrees ATDC. If you have to reduce spark advance 4 to 6 dgrees to stop knock then peak pressure will occur after the optimum crank angle and you will lose efficiency. But if you can prevent knock by adding fuel or reducing spark advance by one or two degrees prior to the onset of knock then you will maintain greater efficiency.

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