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Let's talk ignition advance curves......

Discussion in 'The Garage' started by Beast388, Aug 16, 2005.

  1. Beast388

    Beast388 1/2 ton status

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    To start, here is a bit of background. Six years ago, when I built my 388, I plopped in my then new Summit HEI and ran it since then without much thought about the advance curve.

    This past weekend I decided to check and see what kind of total/vacuum advance I had and see if it was where it should be.

    I was a bit suprised to see that I only had 10* of mechanical advance and another 18* of vacuum. With an intial advance of 10*, I was only getting 20* total at WOT. Of course this is well below the 32-36* that SBC's like.

    I went an bought a Mr Gasket advance curve kit with the weights and springs. Of course most of these types of kits come with 3 different types of springs and after reading the instructions, in order to get all the advance in by 3000rpm or so, I would need to use the lightest(black) springs.

    Now this goes against everthing I had learned before, since my understanding was that the medium and heavy springs were for vehicles that did not accelerate very quickly......like my 6000lb Suburban. :D The heavier springs would not only bring the advance in slower, but wouldn't even allow it to advance fully. The graph for my kit can he seen here.

    So, my question is......For those of you who have reworked your advance curve, what springs did you end up using?

    The obvious answer is for me to drive the truck and see if it pings. I have done that. I went out and flogged her hard and with a noisy gear drive, the hum of the tires, and dual 40 series Flowmasters, I can't tell if it is pinging or not.

    The butt dyno tells me that I have made some improvement, but constant detonation would be a bad thing. :D

    What experiences had you guys had with adjusting the advance curve?
     
  2. ntsqd

    ntsqd 1/2 ton status

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    What do the plugs look like after a hard romp? That will tell you if it's detonating.

    There is no rule that say you can't mix and match those springs. Or alter the loops on the ends to get some initial slop or change the pre-load.

    Some day we'll be able to use one of those fancy computers to change some numbers and alter the advance curve..... :rolleyes:
     
  3. Beast388

    Beast388 1/2 ton status

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    Hmm.....not much interest in this subject. :doah: :dunno:

    Anyways, I had the wife help me plot my advance curve by holding the engine at certain RPMs while I checked the advance with the timing light.

    With 10* initial advance and the lightest springs in the kit:

    1000rpm = 12*
    1500rpm = 20*
    2000rpm = 22*
    2500rpm = 24*
    3000rpm = 26*
    3500rpm = 28*

    These readings were taken with the vacuum advance unplugged.

    I bumped up the initial to 14* to get 32* total at 3500rpm and took it for a drive. WOW! What a difference that made! My engine obviously likes having a good advance curve! :bow:

    Now I just need to make SURE I am not detonating at all by testing it with different loads on the engine. :thinking:
     
  4. 77crewcab

    77crewcab 1/2 ton status

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    You mentioned optimum advance range for the sbc is 32-36 any idea what it would be for a BBC?
     

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