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Thread: Ignition timing erratic

  1. #1
    Registered Member chevynut's Avatar
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    Ignition timing erratic

    I was getting ready to remove my headers to send them off for coating but decided to do one more timing check to make sure the timing was synced up on the Ramjet 502 so I don't burn the headers up when they get back.

    I'm running the engine with a Holley Commander 950 PRO ECU without the GM Spark Control Module and knock sensor wired up. So the ECU is controlling the distributor directly. The COMP double roller timing chain and GM distributor are brand new as is the MSD coil. Cranking ignition timing advance is set at 15 degrees and 18 degrees at idle in the spark map.

    With the engine running and timing light hooked up the timing is wandering quite a bit. It's so bad I can't really tell where it is, but it's somewhere between 10 and 20 degrees I think. The engine idles a little rough with the cam I'm using, especially before it warms up because it's running open-loop under 120 degrees and runs on the rich side. But I believe the timing is set at the same at all RPMs I achieve at idle and a wide range of MAP signal. The next step is 20 degrees but I don't think I get to that RPM at idle. Warm idle is about 850-900 and cold idle starts at 1100, slowly dropping with temperature. It seems like no matter what the RPM does, the ignition timing should be steady when idling.

    I don't know if the problem is the ECU, the distributor, or my timing light. I'm running the engine off of a different battery than the timing light, but both batteries are grounded to the engine. This is just for convenience, nothing else. The alternator is not hooked up but I have a charger on the battery. The timing light seems to be functioning correctly or at least it fires repeatedly like its supposed to.

    I don't recall it doing this when I checked timing after firing it up for the first time, but maybe it did and I didn't notice it. It's a little hard to see the timing mark down there.

    What's the most likely cause? Is there anything else I should be looking at?
    Last edited by chevynut; 04-14-2018 at 05:36 AM.
    56 Nomad, Ramjet 502, Viper 6-speed T56, C4 Corvette front and rear suspension


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  2. #2
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    How’s the idle rpm Laszlo? Not familiar with your specific ignition box, but could the advance be changing because the rpm is? Is the rpm stable when the engine is warming up? How does the advance vary when the engine is warmed up?

  3. #3
    Registered Member chevynut's Avatar
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    Dick, like I said the engine idles a little rough due to the cam I'm using that has 108 degree lobe centerlines. It's just a little "lopey". I'll check the spark map again but I think almost all of the cells in the spark map have 18 degrees in them at idle conditions and I think that's up to 1200 RPM which I don't think it reaches at idle. I did find one weird cell value in the map when I downloaded it from the ECU. I've seen this before and I don't know if it's my PC, the interface cable, USB to Serial adapter, or the ECU causing it. I corrected the cell value and thought it worked but I probably should download it again and look to see if it's right. I still need to get the other PC I have working so I can replace the one I'm using that seems to have some issues. However, the engine should run without the PC anyhow as long as the map is loaded correctly.
    56 Nomad, Ramjet 502, Viper 6-speed T56, C4 Corvette front and rear suspension


    Other vehicles:

    56 Chevy 2-door BelAir sedan
    56 Chevy 210 4-door sedan
    57 Chevy 210 4-door sedan
    1962 327/340HP Corvette
    1961 Willys CJ3B Jeep
    2001 Porsche Boxster S
    2003 Chevy Silverado 2500 HD Duramax
    2019 GMC Sierra Denali Duramax

  4. #4
    Registered Member chevynut's Avatar
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    I think maybe I see at least part of the problem. I just downloaded the spark map in the ECU and looked at it. It has the same weird value of 63.75 degrees in the cell corresponding to a MAP of 76 and RPM of 1000 that I saw yesterday. The Cell at 76/800 has 20 degrees. All of the cells from 400 to 1000 RPM and 12-137 MAP should have 18 degrees. I input the correct values yesterday and I thought they were entered into the ECU correctly but apparently not. Advance goes to 23 degrees for all MAP values at 1200 RPM which it might see when idling cold.

    I'm running a spark map provided by Holley, but I don't know if it's right. I don't know why I even need a 400 RPM cell column since the engine will never operate there.
    56 Nomad, Ramjet 502, Viper 6-speed T56, C4 Corvette front and rear suspension


    Other vehicles:

    56 Chevy 2-door BelAir sedan
    56 Chevy 210 4-door sedan
    57 Chevy 210 4-door sedan
    1962 327/340HP Corvette
    1961 Willys CJ3B Jeep
    2001 Porsche Boxster S
    2003 Chevy Silverado 2500 HD Duramax
    2019 GMC Sierra Denali Duramax

  5. #5
    Registered Member chevynut's Avatar
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    One more thing.....I found out that the ECU interpolates between cells for spark advance. In other words if the 1000 RPM cell says 18 degrees and the 1200 RPM cell is 23 degrees, if the engine is at 1100 it will run 20.5 degrees advance. Maybe that's why it's looking erratic. I also think that 63.75 value is screwing with the timing if it's really what's in the ECU. The 76 MAP value and 1000 RPM is right where it idles until it warms up. Not sure why that value is corrupted.

    I guess I need to start it up and watch the PC to see what advance it's running at, then check it with the timing light when it warms up and stabilizes.
    56 Nomad, Ramjet 502, Viper 6-speed T56, C4 Corvette front and rear suspension


    Other vehicles:

    56 Chevy 2-door BelAir sedan
    56 Chevy 210 4-door sedan
    57 Chevy 210 4-door sedan
    1962 327/340HP Corvette
    1961 Willys CJ3B Jeep
    2001 Porsche Boxster S
    2003 Chevy Silverado 2500 HD Duramax
    2019 GMC Sierra Denali Duramax

  6. #6
    Registered Member chevynut's Avatar
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    Well I think I fixed it. I tried several times to change those bogus numbers in the spark map to no avail. I could change them, but when I saved the file and retrieved it again it was still messed up. I don't know if I'm doing it wrong or what.

    So I decided to start fresh with a 502 map from Holley that I had to modify for my TPS settings and WB O2 stuff. Once I got it set up I saved it and loaded it into the ECU and started up the engine. Sure enough, the timing is solid now and the engine idles a lot differently. Before it was "lopey" but now it's more "choppy" and sounds better to me.

    I don't know when or how this changed, but I looked at my original Nomad map and it also has the bogus numbers in it. So somewhere along the line there must have been a glitch that I saved in the file. I was using a serial port but changed to a USB to serial adapter somewhere along the way. Anyhow, I hope it's fixed now.
    56 Nomad, Ramjet 502, Viper 6-speed T56, C4 Corvette front and rear suspension


    Other vehicles:

    56 Chevy 2-door BelAir sedan
    56 Chevy 210 4-door sedan
    57 Chevy 210 4-door sedan
    1962 327/340HP Corvette
    1961 Willys CJ3B Jeep
    2001 Porsche Boxster S
    2003 Chevy Silverado 2500 HD Duramax
    2019 GMC Sierra Denali Duramax

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    Cool, sounds like you found/ fixed the problem Laszlo. The 66 Chevy II I built not too long ago has a FAST ECU and distributor controlling a mark iv LS-6 454 with aluminum heads, hydraulic roller, electronic hilborn individual runner fuel injection (near 600hp - a good street machine). It’s amazing how well it runs with the ecu controls.

  8. #8
    Registered Member chevynut's Avatar
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    Now that I resolved the erratic timing problem and have consistent spark advance, I want to make sure I have the idle timing set where it should be. The Holley 502 spark map has 18 degrees at idle as I mentioned. However, in the Holley C950 Pro manual they say....

    "The timing an engine needs at idle varies widely depending on its specifications. Typically, the bigger the cam, the more advance the engine needs for the best idle. It is not uncommon that an engine with a big cam likes 25-32° of timing at idle. Stock engines may only like 4-12° at idle. A camshaft with more duration will require more initial advance. Some aggressive camshaft profiles may require as much as 24-28° initial timing."

    Some articles I found on the web say that with vacuum advance connected, idle spark advance can go into the high 20s and they specify that "initial timing" is without any vacuum advance. I found a pretty good article on the web:

    http://www.autospeed.com/cms/article...Right&A=109132

    "Most engines will idle happily with an ignition advance of 15 - 32 degrees. This is a very wide range - some engines will certainly not be happy at 32 degrees and others won't be at 15 degrees! An overly high amount of ignition advance for a given engine will result in lumpiness at idle, excessive hydrocarbon emissions and sometimes exhaust popping, while too little advance will also cause lumpiness. If the engine runs closed loop fuel control at idle, too much idle timing advance can disrupt the oxygen sensor reading, causing the self-learning process to overly enrich the idle mixture. Setting the optimal ignition timing can therefore best be done by trial and error variations."

    What does "best" and "like" mean? What does "happy" mean? This all seems very ambiguous to me so how do I tell when my engine idle advance is set right? I know too much advance on startup will cause the engine to kick back, making starting hard. But since I have a separate "cranking spark advance" built in that's irrelevant at idle. So what is it that you look for to get the "best" idle advance setting? They talk about a "lumpy" idle but I'll have that with the "right" amount of timing too.

    Also, as I understand it, "base timing" is pretty much irrelevant when you've synced the actual engine timing with the spark map setting. If that's the case I don't understand why you would want to disconnect the computer control of the distributor to set timing once the engine is running. Holley actually sells an adapter to allow you to do that and I don't see the purpose of it. There is a location in the ECU program to enter "timing reference" which is the same as "base timing". How does the ECU use that? I know mine uses that for cranking advance but I think that's all. So what will it hurt to change it?

    So it seems like with the engine running I could just turn the distributor until I get what I want (whatever that is ), then read the idle advance using a timing light. Then re-sync the distributor with the ECU and set the map to the reading I got with the timing light.
    56 Nomad, Ramjet 502, Viper 6-speed T56, C4 Corvette front and rear suspension


    Other vehicles:

    56 Chevy 2-door BelAir sedan
    56 Chevy 210 4-door sedan
    57 Chevy 210 4-door sedan
    1962 327/340HP Corvette
    1961 Willys CJ3B Jeep
    2001 Porsche Boxster S
    2003 Chevy Silverado 2500 HD Duramax
    2019 GMC Sierra Denali Duramax

  9. #9
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    What does "best" and "like" mean? What does "happy" mean?
    My definition of that, and I think the author's too, would be that the engine is idling the smoothest possible with the least variation in idle speed, and that the idle speed is the lowest that is stable, with the most vacuum (lowest MAP). All this knowing that a big cam will make this more difficult, by its nature. An extension of that is that the engine will take more throttle opening smoothly. Proof that you're there is that changing things makes any of those characteristics worse. To determine all this, part of it is by ear, part is going to be by looking at the scanner data, looking mostly at idle speed and MAP.

    Don't make this hard, you'd aim for the same thing with a mechanical/vacuum distributor and a carb. The nice thing with EFI is that you can add idle timing without affecting timing under other conditions. I.e., you have separate cranking timing and can have any timing you need above idle, as long as the transition is smooth.

  10. #10
    Registered Member chevynut's Avatar
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    Thanks for the input. With the broad range of "acceptable" idle advance it's not clear to me what's good and what's not. I've move the distributor at least 15 degrees and nothing much seems to change as far as how the engine sounds. Moving the distributor is a lot easier than changing the spark map and I can re-sync when I get it set where I want it. I was just concerned about burning up the header coating, but perhaps the header temperature at idle isn't what I should be worried about.

    Looking at idle speed doesn't seem like a good metric because the IAC tries to compensate for the choppy idle as it bounces around due to the cam. I'll see if I can see any difference in the MAP or not. Sounds like I may want as much idle advance as I can get as long as it starts and runs good.
    56 Nomad, Ramjet 502, Viper 6-speed T56, C4 Corvette front and rear suspension


    Other vehicles:

    56 Chevy 2-door BelAir sedan
    56 Chevy 210 4-door sedan
    57 Chevy 210 4-door sedan
    1962 327/340HP Corvette
    1961 Willys CJ3B Jeep
    2001 Porsche Boxster S
    2003 Chevy Silverado 2500 HD Duramax
    2019 GMC Sierra Denali Duramax

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