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  • timing questions

    This is electrical, but very related to engine design, so.....
    Spark advance on engines in the Indian manual(s) is usually spec'd as: 1/2" before top-dead-center" on 74" chief (with 4 7/16" stroke), and 5/8" btdc on 80" with 4 13/16" stroke.
    On the drafting board the protractor reads this as approximately 32 or 33 degrees btdc. I have never been educated on spark advance beyond what I've merely picked up on the farm, or in a shop, by observation only. No one has ever elaborated on what determines ideal position for either full power at whatever high rpm, or for idle position.
    It seems that 33 degrees btdc is pretty much a maximum advance for 3500 rpm on a standard chief (pinging results from advance beyond that), but it also seems the chevy v-8's that I "farmed" with as a kid did not advance beyond that either, even when, uh, MY BROTHER rev'd it up beyond 5000.

    I'd like some discussion on this because if this is common knowledge, it isn't common to me, and I'm not sure that my contemporaries could dazzle me with brilliance either.

  • #2
    Harley Shop Dope 260G of 1 July 1947 says the 4" stroke VLs should be timed 11/32" btdc for moderate compression engines in solo service, with high compression [about 6 to 1] and sidecar engines timed at 5/16". The same advice is given for the 80 cubic inch engines of 4 1/4" stroke. Both 61 and 74 cubic inch overhead valve models are timed at 7/16" btdc. The advice for the WR racers is to set the magneto for 31 degrees btdc.

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    • #3
      Assuming that Steve is citing specs in inches of piston travel, here is a calculator to give you degrees. It will require that you know the length of the con rod.

      http://www.dansmc.com/mc_software2.htm

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