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Thread: Ignition Timing for Modified Engines

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    Bodger

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    Re: Ignition Timing for Modified Engines

    Arr those were the days ...... adding weld on the bob weights and filing back to the desired angle then consulting
    my box of various length and strength of dizzy springs ...... Worked very well with x/flows and Pinto,s once id got my
    head round how it all worked ....

    A chap called Graham Hickman of RATTS shown me the ropes with the rollers and dizzy mods ......

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    Pit Crew Decade Plus User MK1_Oz's Avatar

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    Re: Ignition Timing for Modified Engines

    This is a very old thread but the topic is still relevant.

    I fully understand the article by DA. The one thing that I cannot get my head around is having total advance in by 3500rpm. Given that the time available for the spark to full ignite the fuel mix gets less with rpm, having advance all in by 3500rpm does not address this. At 8000rpm there is less than half the time for the spark to do its thing but no additional advance. I would have thought a more gradual advance curve up to max revs would be better? Obviously not else people would be doing that.

    Anybody care to explain?
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    Re: Ignition Timing for Modified Engines

    2D spark curves aren't exactly sophisticated and I'm guessing when the arsetickle was written EFI and computer accurate timing wasn't widely available or used. Straight lines are a replication of what bob weights did in distributors. Cylinders will fill at different rates based on rpm and cylinder head efficiency - generally they fill better at low rpm peaking anywhere between 3 - 5000 rpm ie. peak torque. After that cylinder fill will be less so advance actually needs to be retarded to start the burn later and achieve the same cylinder pressure. Obviously with bob weights they can't do this but look at a modern EFI timing map - usually 3D using rpm / Angle / Throttle position to dial in exactly what an engine requires throughout the operating range - see map below and compare with the articles straight lines!
    Click image for larger version Name:	Ignition Map 05 Duratec.JPG Views:	144 Size:	70.1 KB ID:	81052

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    Re: Ignition Timing for Modified Engines

    That map is doing the same thing as the distributor - for a given throttle position, advance all in by 2800rpm.

    Even a distributor could be changed to allow a more gentle advancing of timing up to a higher rpm. I still don't really get why the same advance works from 3500ish rpm all the way to 8000+ rpm...
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    Re: Ignition Timing for Modified Engines

    i agree that map doesnt look right to me, a most of the maps i see in the ecus i use end up with more advance either side of peak torque, although as we can only see figure upto 6,000 rpm there might be differences higher up the rev range

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    Re: Ignition Timing for Modified Engines

    A distributor is set-up to give the proper advance at the rpm at (and around) which max. torque happens.
    If you would advance it more at higher rpm the piston would have to 'fight' against the burning mixture longer plus cylinder-pressure would become so high that detonation will occur.

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    Re: Ignition Timing for Modified Engines

    The map is just an example of a 3D map - I just found it online to show the changes.
    MK1 OZ did you read my explanation as you are just repeating yourself or are you not understanding it?

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    Re: Ignition Timing for Modified Engines

    The map is just an example of a 3D map - I just found it online to show the changes.
    MK1 OZ did you read my explanation as you are just repeating yourself or are you not understanding it?

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    Spanner Monkey

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    Re: Ignition Timing for Modified Engines

    Priceless information. I modified my Fomoco distributor with this info, it did involve a lot of trial and error but the end results were better than I could have hoped. Cheers DV Andrews, and keep your lady away from the kit car, you know what I’m talking about
    Are we there yet?

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    Bodger

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    Re: Ignition Timing for Modified Engines

    My electronic Sierra dissy has two posts that the bob weights hit against when max advance is reached but there is also two slots that limit the rotation of two pins on the top shaft. It really looks like the posts would need to be bent a lot to reduce the advance the amount that is required so adding material looks a much better solution. As it is from the factory the weights contact the posts and the pins also reach the ends of the slots At the same time. As it is now with material added to the posts the bob weights hit the the post but if you turn the top shaft by hand while holding the bottom shaft stationery it will still advance more due to there still being clearance in the slots So my question is do the slots need filling as well

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