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  • 1 Post By jerry clayton

Thread: Tuning the ignition timing curve
          
   
   

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  1. #1
    Jed Rumbolz is offline CHR Member Visit my Photo Gallery
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    Tuning the ignition timing curve

     



    I have a question that involves ignition time. I am not satisfied with the timing curve on my car, I think the car could be going faster. I've got a 381 sbc stroker. with no vacuum advance 12 degree initial timing, 36 max centrifugal advance timing, and the timing max comes in at 3000rpm. I leave the line at 3000rpm and the converter flashes to 4000 before it really grabs. It just seems like that 36 degrees of timing is a lot down around 3000 to 4000rpm. If you look at timing curve of production engines it looks the timing keeps advancing until about peak torque and then is flat. I believe this is due fact that after peak torque volumetric efficiency goes down which requires less timing, but your rpms are still increasing which requires more timing so they cancel each other out making a flat spot in the timing curve. I've looked at some dyno charts and it seems like peak torque is always happens at about 75% rpm(give or take a couple percent) at which peak horsepower happens. So if peak power seems to happen at 7200rpm on my engine, peak torque is about at 5400rpm? Does that mean I want my timing to gradually increase until 5400 and then flatten out?

  2. #2
    Jed Rumbolz is offline CHR Member Visit my Photo Gallery
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    Also would a vacuum advance be beneficial if this car is pulling 2 to 3 inches in the plenum @ 6800rpm crossing the line. On the starting line at 3000rpm (foot braking) the vacuum is up to 8 in. (Not trying larger carb or more plenum spacer at this time)

  3. #3
    jerry clayton's Avatar
    jerry clayton is offline CHR Member Visit my Photo Gallery
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    Find the total timing for max power-------find the initial timing for starting the engine-adjust the curve so its at initial when shut off or turning with starter ( so it doesn't buck backwards) use spring weight kit to get the total timing in as quickly as possible ( heavy wt or weakest springs) If you get spark knock driving you probably are lugging the engine------use higher grade of fuel
    36 sedan likes this.

  4. #4
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    Welcome to the forum, Jed. Your torque curve is going to be driven by your cam specs. There's not a formula to relate peak torque to HP, but you may find comparisons to specific engines. My torque curve is almost flat from 1900rpm up to 5000rpm, and then starts falling off pretty quick; while peak HP is about 4500rpm. It all depends on the specifics of your engine, as I understand.
    Roger
    Enjoy the little things in life, and you may look back one day and realize that they were really the BIG things.

  5. #5
    Jed Rumbolz is offline CHR Member Visit my Photo Gallery
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    Everyone I talk to has the mechanical advance WO at 3000rpm, I just seems like there's to many variables between engines to have the number be the same. This is a 381 ci SBC leaves the line at 3000rpm shifts at 7200rpm. I think by playing around with that curve the 60ft and 330ft times would be way better.
    Last edited by Jed Rumbolz; 08-29-2015 at 08:27 PM.

  6. #6
    Jed Rumbolz is offline CHR Member Visit my Photo Gallery
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    What kind of engine? What does your timing curve look like

  7. #7
    36 sedan's Avatar
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    Jerry gave you the secret in post #3.

  8. #8
    Jed Rumbolz is offline CHR Member Visit my Photo Gallery
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    Quote Originally Posted by jerry clayton View Post
    Find the total timing for max power-------find the initial timing for starting the engine-adjust the curve so its at initial when shut off or turning with starter ( so it doesn't buck backwards) use spring weight kit to get the total timing in as quickly as possible ( heavy wt or weakest springs) If you get spark knock driving you probably are lugging the engine------use higher grade of fuel
    Jerry this car runs a 11.5:1 static compression ratio and is one of the few at track that runs 91 octane pump gas. Has seen hard street duty. the only problem we experienced was blowing the ceramic insulator loose on the spark plugs(you hold the hex on the electrode and turn the ceramic) but we had the max timing at 40 degrees, the car was run a while like this and we finally contacted champion about the problem and retarded the timing to 36 degrees. The car was running good, no signs of detonation, no knocking, never ran to hot. After a tear down for refurbishing there were no signs of detonation found internally.

  9. #9
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    Quote Originally Posted by Jed Rumbolz View Post
    the only problem we experienced was blowing the ceramic insulator loose on the spark plugs(you hold the hex on the electrode and turn the ceramic)
    This can be caused by over tightening your plugs, use a torque wrench.

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