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  • jim cear

    Duke

    Duke , in an archieved post you described three types of total timing, at idle ,simulated WOT, (both with vacuum advance disconnected) and finally at +/- 2300 rpm, 50 degrees with vacuum connected. This has helped solve a debate between myself and another individual over the advance curve of my 340shp ( with an LT-1 cam),however I do not understand how this is possible without detonation. (and it obviously is)
  • Duke W.
    Beyond Control Poster
    • January 1, 1993
    • 15610

    #2
    Re: Duke

    Detonation tendency increases with increasing fresh charge density, so when the engine is throttled, there is less tendency to detonate. Also, flame propagation speed increases with increasing fresh charge density (or decreases with decreasing density) so more timing is required at cruise when the engine is throttled than at WOT.

    Exhaust gas dilution of the fresh charge both slows flame propagation speed, which is why SHP engines need more total idle timing (26-32 degrees) than medium performance engines (20-26 degrees) and reduces detonation tendency. The latter can be illustrated by disconnecting EGR on engines so equipped. If you do, the engine will often detonate at part throttle acceleration.

    There is also signficant exhaust gas dilution at part throttle cruise due to valve overlap, especially with SHP cams. This combined with low inlet density increases the timing requirement, which is why SHP engines can handle (and need) total cruise timing as high as the low 50s.

    The 30-30 cam engines, which have the highest overlap of all OE cams have the most aggressive OE timing maps with the centrifugal advance all in at 2350 and the vacuum advance all in at 8". With 10-14 degrees initial, 24 centrifugal, and 16 vacuum the total cruise timing at over 2350 is in the range of 50-54 degrees. This map yields total WOT timing of 34-38, and total idle timing is usually about 32 since the centrifugal starts at about 700, which is below the 900 idle speed needed for acceptable (IMO) idle quality.

    Though they have less overlap (and therefore less exhaust gas dilution at idle and part throttle), the Duntov and LT-1 cams respond well (good power,throttle response, fuel economy, and less tendency to overheat at idle and low speed) to this timing map if fuel octane is sufficient to prevent detonation.

    Between the extremes of idle and WOT peak revs there are an infinite number of operating conditions that can be uniquely specified by manifold vacuum and revs. Each of these conditions has an ideal timing and this can be supplied by an advance mechanism that adjusts timing by a combination of revs and vacuum. This is what centrifugal and vacuum advance do. They are pretty simple and crude, but quite effective at providing near the ideal spark timing for all engine operating conditions as long as they are set up to each engine configuration's manifold vacuum and exhaust gas dilution characteristics and fuel octane quality is sufficient to allow ideal timing under all operating conditions without detonation.

    Duke

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    • jim cear

      #3
      Thanks Duke *NM*

      Comment

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