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Thread: Revisiting spark advance to optimize ...NEED SMART EYEBALLS....

  1. #1
    Back to the Omnipresent! Spittybug's Avatar
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    Revisiting spark advance to optimize ...NEED SMART EYEBALLS....

    I'm not sure I've ever shared this file with the group. I made a spreadsheet to mimic the 3 components of spark advance; static, mechanical and vacuum. In my EFI setup I run two advance tables that are additive. One is based on static + mechanical, the other is vacuum.

    In my spreadsheet I've taken the values from our manual and labeled them "spec" and highlighted in yellow. If I've extracted stock data correctly, the static advance is 15*, the mechanical advance goes from 0* to 20* starting at 1000 rpm and getting all in by 3000 rpm (actually this isn't actually stated, but seems to be consensus) and the vacuum advance goes from 20* to 0* from 15 in Mg down to 0 in Mg (manifold vacuum).

    The model allows for you to change the shape of the mechanical advance curve by simulating either one or two springs in a distributor and the start/end RPMs that they work over (simulates spring stiffness). The vacuum advance is not adjustable and the curve is based on the manual data.

    The stock setup DEACTIVATES the vacuum advance at idle through the use of the little micro switch on the throttle. I believe this is to prevent too much advance when cranking so that it isn't too difficult to start the car. But, that means at idle a stock car is at ~15* advance. If it weren't for this cutout, the advance would be 15* static plus another 20* vacuum since at idle the engine is creating the most vacuum. If you have your foot on the gas when cranking, does this make the timing immediately jump to 35*?

    One question that begs asking is what is the optimal advance for idle? The theoretical answer is the one that creates the greatest manifold vacuum (lowest kPa numbers in Megasquirt) and that's what they recommend when tuning in Megasquirt. What values have you other EFI guys determined?

    The model also lets you put in a maximum amount of advance. 15* + 20* + 20* = 55*. Does this sound too much for our stock engines? It would occur at high RPM throttle lift and deceleration. In a stock setup, the micro switch "pulls out" the 20* of vacuum advance, so I need to figure out how to model this.... Megasquirt has a decel fuel adjustment, but not one for spark timing. Practically speaking, just lowering vacuum advance values in higher RPM, higher vacuum areas of the table (lower right) does the trick.

    I would love to look at some of your advance tables that you are running on your cars to see how my model compares to them. My goal has been to mimic the stock system based on the assumption that some good engineering went into making it as good as they could in the first place (maybe too much credit?). Given the 100% flexibility that software gives us, I'd like to tweek!

    Comments? Please post tables so that I can compare. Thanks.

    Megasquirt timing tool.xlsx
    Owen
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  2. #2
    Senior Member Bitsyncmaster's Avatar
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    When you start the car the engine is turning around 500 RPM. If you step on the gas at that RPM I don't think you would get much if any vacuum. If you have to much advance during starting you may get a backfire. Also high advance at idle makes the RPM higher and hard to control idle RPM.
    Dave M vin 03572
    http://dm-eng.weebly.com/

  3. #3
    Administrator Ron's Avatar
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    $.02

    Generally speaking a good engine will have ~5 in. hg. vacuum while cranking, so it would add ~3 advance, if you press the accelerator while cranking (which you shouldn't do with most any non-carbed vehicle)...and the engine temp is >40C/104F.

    The optimal static timing is "13" (+/- 2, for high vacuum and smoothness) BTDC @ 750 RPM (+/- 50 RPM, for smoothness and auto v/s manual trans).

    The stock setup deactivates the vacuum advance at idle to prevent "over advancement during idle and deceleration".

    13 + (2) + 20 + 20 = 55 @ 4000 RPM is maximum advance spec for a stock engine.
    (FWIW, many set stock timing by revving the engine to 4000 RPM and setting the timing to 55, then double check static is at 13 (+/- 2) at idle. This helps verify all three factors are present and gives max top end performance...)

    Note:
    The manual uses "hg. in."
    Your spreadsheet table uses "Kpa" (?kPa Absolute?) and "Hg.gauge" (Inches Mercury Gauge).
    hg. in. absolute and hg. in. gauge are ~inverses.
    => ~20 in. hg. Absolute = ~10 in. hg. Gauge = ~67 kPa (Absolute).
    Conversion table HERE.

  4. #4
    '82 T3 Turbo FABombjoy's Avatar
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    It looks like a lot of advance but seems to run well with low compression / slow heads. The highest advance cells are part throttle anyway.

    I plugged the service manual values in and used Megasquirt to interpolate the equivalent table (and minus .75 degree/lb boost). Never any issues with misfire/ping/etc. Rev limit at 6000. I suspect there is room for improvement though and I'd like to schedule some time on a load cell dyno and get the table optimized.

    The vac advance solenoid is goofy - teeing it in to the throttle tap should do the same thing as the idle switch/solenoid. That's all present to help speed up catalytic warmup. It's also a great generator of electrical noise if you have an early car without the filter caps.
    Luke S :: 10270 :: 82 Grey 5-Speed :: Single Watercooled T3 .60/.48 :: Borla Exhaust :: MSD Ignition :: MS3X Fully SFI Odd-fire EFI :: DevilsOwn Methanol Injection

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  5. #5
    Administrator Ron's Avatar
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    Quote Originally Posted by FABombjoy View Post
    It's also a great generator of electrical noise if you have an early car without the filter caps.
    LOL!

    ====

    Never played with Megasquirt... Since the vacuum would skyrocket when you let the throttle plates snap shut at high rpm you would want the advance to reduce quickly, so, you make the throttle tap say the converter is cold?

  6. #6
    '82 T3 Turbo FABombjoy's Avatar
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    If you pull a lot of advance on decel you may end up with a burbly/poppy exhaust or exessive engine braking. If you have overrun fuel-cut tuned most of those timing areas are moot since there's no fuel to burn. You can also make the car feel jerky by pulling timing on decel. Imagine that situation where stuck behind a tractor while you're in a lower gear waiting for oncoming traffic to pass. So you're modulating the throttle to keep speed and during this you're bouncing up and down the ignition load table. If you're crossing a wide range of timing you'll feel it oscillate.

    If you're running fuel-only MS, you could emulate the stock cold-advance behavior and have the advance solenoid controlled by MS based on coolant temp and use the throttle edge vacuum so the solenoid is just triggered on once, instead of on and off constantly. I'd probably drop the stock solenoid and use a three port instead so it can be set up on-when-cold and off-when-warm which is a better failsafe.

    You can even leave K-jet but pull all the ECUs and have Megasquirt control ignition, idle, and the frequency valve. But that's like running 25 miles instead of finishing the marathon.
    Luke S :: 10270 :: 82 Grey 5-Speed :: Single Watercooled T3 .60/.48 :: Borla Exhaust :: MSD Ignition :: MS3X Fully SFI Odd-fire EFI :: DevilsOwn Methanol Injection

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  7. #7
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    huh?

  8. #8
    Senior Member Bitsyncmaster's Avatar
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    Quote Originally Posted by FABombjoy View Post
    The vac advance solenoid is goofy - teeing it in to the throttle tap should do the same thing as the idle switch/solenoid. That's all present to help speed up catalytic warmup. It's also a great generator of electrical noise if you have an early car without the filter caps.
    Yes the solenoid will produce noise when it switches off. I've seen the noise fire the ignition ECU. That causes the RPM relay to prime if the engine is not running and you hit the gas.
    Dave M vin 03572
    http://dm-eng.weebly.com/

  9. #9
    Administrator Ron's Avatar
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    Oops. I was already with you on the why...didn't mean to imply cutting the advance off totally.
    I was wondering about the how using Megasquirt -- I got ya with the emulating... Thanks!

    =============

    Maybe this will help-
    This is a from the first warm run on a thoroughly beefed up engine in son's street car.
    (Definitely not a PRV, but if something can go wrong, it will ;-).
    Note the advance climbs after the accelerator is released and the advance drops sharper (for the first half).
    It turns high 10s and is very well behaved (no exhaust/engine braking problems).

    wot-idle.jpg

  10. #10
    Senior Member Josh's Avatar
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    It seems to me you are REALLY overthinking this.
    Keep in mind you will need to tune your VE table to support modifications to your spark table.

    It is apparent to me you need to retard the timing across the board. Nowhere should it be in the 50s even at low load high RPM. I would exercise caution entering the 40s honestly. Unless you are really keeping an eye on things like Luke and I do.

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