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ignition timing on a TBI

This is a discussion on ignition timing on a TBI within the Performance forums, part of the General Discussion category; would apprecicate any help on this one... my 94 chevy truck with 350tbi has been experiencing some slight pre-ignition/detonation. the ...

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    ignition timing on a TBI


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    would apprecicate any help on this one... my 94 chevy truck with 350tbi has been experiencing some slight pre-ignition/detonation. the only mods are a K&N stock style filter and cat back exhaust. So today, I double checked the timing. With the engine at full operating temp, I bypass the EST (disconnect the brown wire with the black stripe) and check the timing. It is dead on 0 degrees BTC just like the book says. But when I plug in the wire (re-connect the EST) the computer bumps my timing to 20 degrees BTC. I have a timing light with an advance on it, so I know its 20 degrees. Does this shound like a tad much? does anyone know what the correct advance should be when the EST is engaged? If this is too far advanced, what do I need to replace?

    Thanks in advance.
    70 chevrolet

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    Re: ignition timing on a TBI

    check for codes stored in the computer. for instructions do a search on this site for obdi codes. code 12 should be the only code you have.
    The computer uses information from the Map and Coolant temp sensors in additon to engine RPM's. A diagnostic scan will be helpfull in troubleshooting sensor information.
    Low MAP sensor output voltage= more spark advance.
    Cold engine= more spark advance(normal)
    High MAP sensor output voltage= less spark advance
    Hot engine = less spark advance.
    Detonation can be caused by low MAP sensor output or high resistance in the coolant sensor circuit.
    poor performance can be caused by high MAP sensor output or low resistance in the coolant temp circuit.
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    Re: ignition timing on a TBI

    no codes (only 12)... But does 20 deg of advance sound like too much? I will check the map sensor though...
    Last edited by 70chevrolet; 05-10-2006 at 09:27 AM.

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    Re: ignition timing on a TBI

    The factory programming is designed to allow Electronic Spark Control (ESC) to advance the timing near the point where it senses too much knock. That system was designed with the idea that the engine had high-swirl fast-burn heads, the stock compression ratio and the stock cam (so the cylinder pressure is in the acceptable designed range), AND that you don't have excess carbon in the cylinder chambers (which would raise the CR) AND assuming that the EGR is functional and providing the proper flow of EGR.

    I'll duck all of the above and skip to the exhaust: you've altered it which has lowered the exhaust pressure in the manifold/Ypipe, so that means when the EGR valve opens that you won't get as much EGR flow as you did previously... so that would lead to increase in knock.

    You could also get the exact same behavior with a completely stock exhaust and stock engine, but with restricted EGR (or blocked) plumbing, or from a failed EGR valve, or from a failed EGR solenoid.

    HTH.

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    Re: ignition timing on a TBI

    Damn... Thats good stuff... So what your saying is... they computer will continue to advance until the knock sensor tells it to stop. And becuase of the exhaust mod, even if the engine is functioning perfectly, I may still get a tiny bit of spark knock.. I know the egr is good. so I may (tell me if I'm out of line here) retard the base timing just a cuny hair. Or should I just learn to live with a tad of knock....
    Last edited by 70chevrolet; 05-10-2006 at 10:03 AM.

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    Re: ignition timing on a TBI

    you could try it.
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    Re: ignition timing on a TBI


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    Re: ignition timing on a TBI

    Quote Originally Posted by 70chevrolet
    Damn... Thats good stuff... So what your saying is... they computer will continue to advance until the knock sensor tells it to stop. ...
    Yes --- where the limit is set by the ignition advance limits stored as a table in the EPROM (chip) for the: rpm, throttle position, and load (MAP sensor) that you are at.

    Using an aftermarket EGR valves (such as the one sold by Wells) that has replacable orificed washers would allow you to add more EGR (using a washer with a bigger hole, or orifice), and that might solve your problem. The "right" way is to alter the tables that command EGR in the EPROM but you might find the adjustable EGR valve a lot easier to try.

    Backing off on the static timing (distributor rotation) will also work but it will cause some loss in torque at all rpms and loads, so you might lose some power and mpgs. HTH.
    Last edited by kdrolt; 05-11-2006 at 06:09 AM.

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    Re: ignition timing on a TBI

    Quote Originally Posted by kdrolt
    Backing off on the static timing (distributor rotation) will also work but it will cause some loss in torque at all rpms and loads, so you might lose some power and mpgs. HTH.
    Lose torque yes, but I would think that retarding the timing and thereby decreasing the compression ratio, I may pick up a few MPG? Nay?

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    Re: ignition timing on a TBI

    ok, a stock engine will run with the correct 0-2 inital timing
    lead. yes the first 12-16 timing cells are 20 so the "advance"
    is in proper working order.
    weak or low fuel pressure will develop a detonation issue.
    currently im battling the same issue with a tip in and lockup
    "ping". checked my fuel pressure and im at 10psi from 14psi.
    power is down 30hp/25ft/lb torque at the rear wheels and mpg
    is down 5mpg hiway. i got a new pressure reg from GM ($75)
    tweaked the tamper-resistant torx back to 14psi and no ping
    or detonation, powers back and mileage is comming around.
    you some info you might wanna have.

    airdeano
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    ? horsepower
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    will find out soon!

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    Re: ignition timing on a TBI

    Quote Originally Posted by kdrolt
    The factory programming is designed to allow Electronic Spark Control (ESC) to advance the timing near the point where it senses too much knock. That system was designed with the idea that the engine had high-swirl fast-burn heads, the stock compression ratio and the stock cam (so the cylinder pressure is in the acceptable designed range), AND that you don't have excess carbon in the cylinder chambers (which would raise the CR) AND assuming that the EGR is functional and providing the proper flow of EGR.

    I'll duck all of the above and skip to the exhaust: you've altered it which has lowered the exhaust pressure in the manifold/Ypipe, so that means when the EGR valve opens that you won't get as much EGR flow as you did previously... so that would lead to increase in knock.

    You could also get the exact same behavior with a completely stock exhaust and stock engine, but with restricted EGR (or blocked) plumbing, or from a failed EGR valve, or from a failed EGR solenoid.

    HTH.
    What? Ummm, o.k. I'll start by saying that I normally stay out of threads like this but I can't on this one. First of all, that is not how the ECM controls spark. An engine that was constantly running up against the knock sensor would never last through the warranty period without leaving parts in the driveway. Secondly, if there were enough carbon in the cylinders to effect compression ratio, every fire department in the county would be knocking on your door from the black cloud you left behind. Third, restricted EGR, now that's a possibility but it is more than likely the intake manifold EGR passage bieng clogged which TBI trucks are notoriously known for. A stock truck with a cat-back only suffering from low exhaust back pressure, no way. With a big cam and long tubes, maybe the backpressure is an issue but the self EGR'ing of the cam is more of an issue. Lastly, to the original poster, 20deg. of idle timing is perfectly normal. Look at a carbed setup with the vacuum advance hooked up, 22 to 26 deg idle timing is the norm. For your detonation issue, I would look at the obvious like an exhaust or vacuum leak, or as Airdeano mentioned a fuel issue like filter, pump, regulator, etc. Don't forget the EGR passage. HTH

    P.S.: kdrolt, from your previous posts, you seem pretty knowledgable. Why put out bad information? No flame intended.
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    Re: ignition timing on a TBI

    Quote Originally Posted by airdeano
    ok, a stock engine will run with the correct 0-2 inital timing
    lead. yes the first 12-16 timing cells are 20 so the "advance"
    is in proper working order.
    weak or low fuel pressure will develop a detonation issue.
    currently im battling the same issue with a tip in and lockup
    "ping". checked my fuel pressure and im at 10psi from 14psi.
    power is down 30hp/25ft/lb torque at the rear wheels and mpg
    is down 5mpg hiway. i got a new pressure reg from GM ($75)
    tweaked the tamper-resistant torx back to 14psi and no ping
    or detonation, powers back and mileage is comming around.
    you some info you might wanna have.

    airdeano
    well if you-all say that 20 deg BTDC is completely normal, than I'm not going to monkey with the timing. Recently I installed a cfm-tech fuel pressure gauge, I am currently running at 12.5 psi. I have the AFPR but havent installed it yet. I wasnt sure if a bone stock (less k&n and flowmaster cat back) would warrant more fuel pressure. Also, I wasnt sure of the quality of these things... I dont want to be on the highway and have it rattle my fuel pressure down to nothing... I know my EGR is good becuase about 6 mos ago it threw a code, so I removed, checked, cleaned and re-installed it with a new vaccum line. The truck runs awesome but when I punch it, I can feel it boot down to 2nd gear and I can hear knock. What do you think about bumping the fuel pressure?
    Last edited by 70chevrolet; 05-11-2006 at 05:51 AM.

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    Re: ignition timing on a TBI

    Quote Originally Posted by HaulnA$$
    kdrolt, from your previous posts, you seem pretty knowledgable. Why put out bad information?
    Bad? None of it was bad. Maybe you ought to read the FSM more often, and hey, no flames intended either.

    As for the above "bad info" topics:

    How ESC works, and why

    http://experts.about.com/q/GMC-Repai...rk-Control.htm

    and remember that I said (above) that the spark will be advanced (by the ESC system) until it reaches the prestored (table) spark limits in the PROM. The ESC system doesn't increase timing advance indiscriminantly. It increases the advance ONLY until it reaches the limits of what's stored in the PROM. 20 degs advance would be one of the many limits, in this case for idle and no load. At WOT in a TBI truck, the max advance is probably more like 28 to 30 degs BTC at 3500 rpm to redline.

    Carbon buildup
    http://www.chevron.com/products/prod...ate_plus.shtml
    http://www.motorwatch.com/automotive...bonRemoval.pdf

    EGR buildup
    The EGR valve is the most likely failure point because it's continually exposed to severe heat; the solenoid that control the vacuum feed to the EGR would be next most-likley failure point, and the EGR passages in the head and intake being clogged would be the least likely. The passages cast within the intake manifold are fairly large and they exit (into the bores beneath the TBI) through two wide slots. The slots are probably the narrowest point in the EGR path, aside from there the EGR valve seats onto the intake manifold. So if the slots are open, and the EGR in/out paths at the EGR valve flange are open (can you reach into them with your finger with the EGR valve removed?) then the internal EGR passages are probably clear as well.

    I also agree with the above comments on low fuel pressure (clogged filter, weak pump or both) and vacuum leaks -- both lead to lean conditions and that's when knock is most likely.
    Last edited by kdrolt; 05-11-2006 at 06:39 AM.

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    Re: ignition timing on a TBI

    I'd say almost every engine with a knock sensor adjusts timing in this way. All the performance vehicles I've owned do so anyway. They have a base timing value which is then either advanced or retarded up to a maximum deviation as dictated by the knock sensor input.

    Usually the carbon buildup causes preignition not because of the increased compression but because its sharp areas (vs. a smooth piston surface) build up heat which can cause the mixture to ignite before the spark plug fires.

    70chevrolet -

    Yep, massive advance at idle is normal. After all, there's no load on the engine. Are you sure what you're hearing is knock? I've got a tiny exhaust leak that ticks almost like knock. Just a thought.

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    Re: ignition timing on a TBI

    Quote Originally Posted by John M
    Are you sure what you're hearing is knock? I've got a tiny exhaust leak that ticks almost like knock. Just a thought.
    yup, no doubt its knock. I think I'll try to bump the fuel pressure. Im at 12.5 now. Ill move it to 13.5 and see what happens. when I changed the plugs last time, they were a tad on the white side, Im thinking that she's running lean. Could be carbon also..... whats the fix for carbon? pull the plugs and shoot carb cleaner in there?

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    Re: ignition timing on a TBI

    Quote Originally Posted by kdrolt
    Bad? None of it was bad. Maybe you ought to read the FSM more often, and hey, no flames intended either.

    As for the above "bad info" topics:

    How ESC works, and why

    http://experts.about.com/q/GMC-Repai...rk-Control.htm

    and remember that I said (above) that the spark will be advanced (by the ESC system) until it reaches the prestored (table) spark limits in the PROM. The ESC system doesn't increase timing advance indiscriminantly. It increases the advance ONLY until it reaches the limits of what's stored in the PROM. 20 degs advance would be one of the many limits, in this case for idle and no load. At WOT in a TBI truck, the max advance is probably more like 28 to 30 degs BTC at 3500 rpm to redline.
    Sorry, but that is patently false. Read my spark tuning write-up in the "Eprom Tuning Software" sticky. The ESC module has absolutely nothing to do with "advancing" or "increasing" spark timing. You could, and I have many times, completely bypass the ESC module and the timing would operate as normal with the only difference bieng that the ECM would be deaf to knock. That is the sole function of the ESC module, to filter and massage the signal from the knock sensor and route it to the ECM in a usable format. It is a fail-safe device. Timing is controlled entirely by the ECM via the EST (aka ignition) module. The timing calibration (not limit) tables in the Prom, indexed by RPM and load, are set to provide the engine with optimum (not maximum) spark advance over a wide range of conditions. The main spark table provides a basis from which timing derived. Timing can be adanced slightly or retarded considerably, as conditions warrant, from the calibration setting by spark modifiers such as coolant temp, EGR, knock, and WOT to name a few but it was never intended to push the limits until knock is detected. I have attached a stock TBI truck spark table and as you can see it is very conservative with idle timing set to 20deg. and max timing at WOT is 16.2 deg., not 28 to 30.

    The fact that carbon buildup can cause detonation is not what is bieng disputed, the fact that you say the detonation from carbon buildup is due to the compression increase from said buildup is what is bieng disputed. As pointed out in your link, carbon provides a hot spot in the chamber that ignites an alternate flame front and therefore causes detonation. This is due to the fact that carbon absorbs heat. In the rare extreme case where excessive carbon buildup decreases chamber volume, there would be many signs of the condition before it reached this point like burning thick black smoke and carbon fouled plugs and possibly even, heaven forbid, engine killing preignition.

    Quote Originally Posted by kdrolt
    EGR buildup
    The EGR valve is the most likely failure point because it's continually exposed to severe heat; the solenoid that control the vacuum feed to the EGR would be next most-likley failure point, and the EGR passages in the head and intake being clogged would be the least likely. The passages cast within the intake manifold are fairly large and they exit (into the bores beneath the TBI) through two wide slots. The slots are probably the narrowest point in the EGR path, aside from there the EGR valve seats onto the intake manifold. So if the slots are open, and the EGR in/out paths at the EGR valve flange are open (can you reach into them with your finger with the EGR valve removed?) then the internal EGR passages are probably clear as well.
    Agreed, The EGR valve is a common failure but if you search this forum you will see time and time again where people have replaced a faulty EGR valve only to have the new one fail a short time later due to the passages bieng clogged. Every single TBI intake I have removed, which is more than I care to admit, has had the EGR passages clogged to one extent or another especially those with bad valve stem seals. It is very common and a check item in the service bullitins pertaining to EGR failure.

    Quote Originally Posted by kdrolt
    I also agree with the above comments on low fuel pressure (clogged filter, weak pump or both) and vacuum leaks -- both lead to lean conditions and that's when knock is most likely.
    At least we agree on something.
    Quote Originally Posted by 70chevrolet
    Damn... Thats good stuff... So what your saying is... they computer will continue to advance until the knock sensor tells it to stop....
    This is the reason I say bad information was posted. The original poster now believes this is how ECM timing works, which is wrong. This gets posted in other threads then before you know it, people are preaching it as gospel. Unfortuneately this happens alot. I knew I should have stayed out of this thread.
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    Re: ignition timing on a TBI

    Haulin,
    The table you have posted has a 9.81* bias on it. Meaning the whole table would be about +10* in real life. Then add in the PE timing advance and you are to 28-30* @ WOT.

    A stock TBI engine will idle around 25-30* timing with the EST connected and the initial timing at 0*.

    I have both an ASDU (350 Van) and ATKX (4.3 Astro) .bin file in front of me.

    The ASDU calibration delivers 28* of total timing @ WOT(3,600 RPM) + the rpm adder.



    The ATKX delivers 30* of total timing @ WOT (3,600 rpm)+ the RPM adder. The ATKX timing table will run great in a mild, swirl port headed 350 as it is slightly more aggressive.



    Both engines idle at 26* of timing advance.

    Try logging the spark advance of a TBI engine sometime, you might be suprised at what you see. WOT of about 30*, idle around 25-30, and cruise between 25-40 depending on load and rpm.
    Last edited by Fast305; 05-12-2006 at 04:04 PM.

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    Re: ignition timing on a TBI

    Quote Originally Posted by Fast305
    Haulin,
    The table you have posted has a 9.81* bias on it. Meaning the whole table would be about +10* in real life. Then add in the PE timing advance and you are to 28-30* @ WOT.

    A stock TBI engine will idle around 25-30* timing with the EST connected and the initial timing at 0*.

    I have both an ASDU (350 Van) and ATKX (4.3 Astro) .bin file in front of me.

    The ASDU calibration delivers 28* of total timing @ WOT(3,600 RPM) + the rpm adder.



    The ATKX delivers 30* of total timing @ WOT (3,600 rpm)+ the RPM adder. The ATKX timing table will run great in a mild, swirl port headed 350 as it is slightly more aggressive.



    Both engines idle at 26* of timing advance.

    Try logging the spark advance of a TBI engine sometime, you might be suprised at what you see. WOT of about 30*, idle around 25-30, and cruise between 25-40 depending on load and rpm.
    Yes there is a bias but it is subtracted out by TunerCat and the timing table posted is actual timing. The main spark bias is subtracted from the actual table entry in the .bin file. It works like this; Table entry - main spark bias = actual timing which is the number displayed in the ALDL datastream. If there is an initial spark advance as well this number is then subtracted from the actual timing number and that timing value is sent to the EST module for correct ignition timing. The purpose of the main spark bias ito keep the ECM from trying to deal with negative numbers which cannot be stored in the Eprom. I have logged hundreds if not thousands of hours of TBI truck data and verified timing with a timing light. I have attached a log shot and assosciated .bin shot. As you can see, they match up. Note that this was before the truck was tuned.
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    Re: ignition timing on a TBI

    Quote Originally Posted by HaulnA$$
    Yes there is a bias but it is subtracted out by TunerCat and the timing table posted is actual timing. The main spark bias is subtracted from the actual table entry in the .bin file. It works like this; Table entry - main spark bias = actual timing which is the number displayed in the ALDL datastream. If there is an initial spark advance as well this number is then subtracted from the actual timing number and that timing value is sent to the EST module for correct ignition timing. The purpose of the main spark bias ito keep the ECM from trying to deal with negative numbers which cannot be stored in the Eprom. I have logged hundreds if not thousands of hours of TBI truck data and verified timing with a timing light. I have attached a log shot and assosciated .bin shot. As you can see, they match up. Note that this was before the truck was tuned.
    I don't know about the ones you have been tuning, but the ones I see have 28-30* of timing in PE mode above 3,200 rpm or so. The TunerPro timing tables are what you get at the distributer. I have verified them with both a dial-back timing light, and timing tape with a regular old timing light.

    I transposed the stock LB4 4.3 timing curve, to run my L05. I have been using that timing curve with mild 305/350 engines for a long time. It gives 30* of timing advance at the crankshaft. There is NO way a stock L05 could even get out of its own way at 16* timing advance! GM is not stupid enough to have negative timing numbers even at lower RPMs and WOT. They just don't happen unless the knock sensor is being triggered. I have as near as I can get to the stock LB4 ATKX spark tables in my EBL setup. The advance is almost always within +2/-5 of what the timing map shows. The variations are due to temperature compensation, IAT compensation, PE timing, Knock Retard, and idle spark compensation.















    Furthermore, here is something to prove that a stock L05 has more than 16* timing advance. Disconnect the EST, set the distributer at 16*, then drive it, EST disconnected. It will be a DOG, everywhere. I had mine set at 20* once upon a time, while fighting an EST problem, and it was still a dog. When I got the time, I traced the problem back to a faulty connection in the EST connector, replaced the connection, brought the timing back to 0*, suddenly my power/economy came back.

    As far as the timing curve in the newer PCMs, you've got me. I do know that the two .Bins that I work with have the best timing curves I have found for TBI applications and give 28-30* of timing advance, not 16.

    PS-with the 1227747 you have to rework the ALDL output in order to get PW and SA into the datastream. I have done both.
    Last edited by Fast305; 05-12-2006 at 08:40 PM.

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    Re: ignition timing on a TBI

    Quote Originally Posted by Fast305
    I don't know about the ones you have been tuning, but the ones I see have 28-30* of timing in PE mode above 3,200 rpm or so. The TunerPro timing tables are what you get at the distributer. I have verified them with both a dial-back timing light, and timing tape with a regular old timing light.

    I transposed the stock LB4 4.3 timing curve, to run my L05. I have been using that timing curve with mild 305/350 engines for a long time. It gives 30* of timing advance at the crankshaft. There is NO way a stock L05 could even get out of its own way at 16* timing advance! GM is not stupid enough to have negative timing numbers even at lower RPMs and WOT. They just don't happen unless the knock sensor is being triggered. I have as near as I can get to the stock LB4 ATKX spark tables in my EBL setup. The advance is almost always within +2/-5 of what the timing map shows. The variations are due to temperature compensation, IAT compensation, PE timing, Knock Retard, and idle spark compensation.....

    .....Furthermore, here is something to prove that a stock L05 has more than 16* timing advance. Disconnect the EST, set the distributer at 16*, then drive it, EST disconnected. It will be a DOG, everywhere. I had mine set at 20* once upon a time, while fighting an EST problem, and it was still a dog. When I got the time, I traced the problem back to a faulty connection in the EST connector, replaced the connection, brought the timing back to 0*, suddenly my power/economy came back.

    As far as the timing curve in the newer PCMs, you've got me. I do know that the two .Bins that I work with have the best timing curves I have found for TBI applications and give 28-30* of timing advance, not 16.

    PS-with the 1227747 you have to rework the ALDL output in order to get PW and SA into the datastream. I have done both.
    The timing table I posted is typical for all the stock TBI trucks I have tuned from '88 to '95. I'll address the '7747, $42 stuff since that is what you are familiar with:

    From the $42 hac. Calibration valuesfrom the ARJU.bin
    .....
    LD014 FCB 28 ; 10 Deg Main SA BIAS
    LD015 FCB 57 ; 20 Deg COOL SA BIAS
    .....

    ;---------------------------------------------
    ; MAIN SPARK Vs. MAP Vs. RPM
    ;
    ; 01-24-2000 Dissassemby of ARJU
    ; 15 COL x 14 BLOCKS = 210 BYTES
    ;
    ; TBL = 2.8444 * (Spk Adv + 10 Bias)
    ;---------------------------------------------
    ORG $0032 ;
    LD032 FCB 0 ; MIN MAP Val
    LD033 FCB 32 ; Min RPM Val
    LD034 FCB 15 ; LINES/BLOCK
    ;---------------------------------------------
    ; 400 RPM
    ; Spk Adv MAP
    ;---------------------------------------------
    LD035 FCB 74 ; 16.0 30
    LD036 FCB 74 ; 16.0 35
    LD037 FCB 74 ; 16.0 40
    LD038 FCB 74 ; 16.0 45
    LD039 FCB 74 ; 16.0 50
    LD03A FCB 68 ; 13.9 55
    LD03B FCB 63 ; 12.1 60
    LD03C FCB 54 ; 9.0 65
    LD03D FCB 46 ; 6.2 70
    LD03E FCB 40 ; 4.1 75
    LD03F FCB 34 ; 2.0 80
    LD040 FCB 28 ; -0.2 85
    LD041 FCB 23 ; -1.9 90
    LD042 FCB 20 ; -3.0 95
    LD043 FCB 17 ; -4.0 100
    --------------------------------->
    ;---------------------------------------------
    ; 3600 RPM
    ; Spk Adv MAP
    ;---------------------------------------------
    LD0F8 FCB 111 ; 29.0 30
    LD0F9 FCB 105 ; 26.9 35
    LD0FA FCB 102 ; 25.9 40
    LD0FB FCB 97 ; 24.1 45
    LD0FC FCB 94 ; 23.0 50
    LD0FD FCB 91 ; 22.0 55
    LD0FE FCB 91 ; 22.0 60
    LD0FF FCB 91 ; 22.0 65
    LD100 FCB 91 ; 22.0 70
    LD101 FCB 91 ; 22.0 75
    LD102 FCB 74 ; 16.0 80
    LD103 FCB 71 ; 15.0 85
    LD104 FCB 65 ; 12.9 90
    LD105 FCB 65 ; 12.9 95
    LD106 FCB 65 ; 12.9 100
    ;---------------------------------------------
    .....

    Now for the assosciated code.

    .....

    ;=====================================
    ; MAIN SA TABLE, (14x15)
    ;
    ; ECM 1227747 SA table ASDZ
    ;=====================================
    E759: LDX #$D032 ; MAIN SA TABLE
    ;
    E75C: LDAB L0026 ; S/D MAP
    E75E: LDAA L001B ;
    ;
    E760: JSR LFB67 ; 3d LK UP
    ;---------------------------------------------

    E763: LDAB L001C ; RPM/25
    E765: CMPB #144 ; 3600 RPM
    E767: BLS LE77E ; BR IF LT 3600 RPM
    ;
    E769: CMPB LD00C ; 4200 RPM, MAIN SPK HI EXTEND B.P. RPM/25
    E76C: BLS LE771 ;
    ;
    E76E: LDAB LD00C ; 4200 RPM, MAIN SPK HI EXTEND B.P. RPM/25
    E771: LE771 SUBB #144 ; 3600 RPM

    E773: ASLB ; Difference x2
    E774: LDAA LD00D ; DEG/1K RPM SLOPE (5 DEG/1K RPM) USED TILL RPM G.T. LD00C
    E777: MUL ;
    E778: ADDA L0051 ;
    E77A: BCC LE77E ;
    ;
    E77C: LDAA #255 ; FORCE MAX LIMIT
    E77E: LE77E TAB ;
    E77F: CLRA ;
    E780: STD L0057 ;

    ;---------------------------------------------
    ; COOLANT COMP SPARK Vs. LOAD Vs. Deg c COOL
    ;
    ; (12x5)
    ; Val's show in ENG units include 20 deg bias
    ; Added SA = tab-57/(256/90)
    ;
    ; Dissassemby of ASDZ 07-19-1998 14:21:16
    ;
    ;
    ; TBL = (256/90) x SPK + 20
    ;---------------------------------------------
    E782: LDAA L0021 ; COOLANT 5 - 115c
    E784: CMPA #160 ; 115 deg c
    E786: BLS LE78D ; BR IF COOL LT 115c
    ;
    E788: LSRA ;
    E789: ADCA #$00 ;
    E78B: ADDA #80 ;

    E78D: LE78D LDX #$D107 ; COOLANT COMP SPARK Vs. LOAD Vs. COOL
    ;
    E790: LDAB L0031 ; VACUUM 40 - 0 Kpa
    E792: LSRB ; VAC/2
    E793: JSR LFB67 ; 3d LK UP
    ;---------------------------------------------

    E796: PSHA ;
    E797: CLRA ;

    E798: LDAB LD1BA ; 0.9960 SPK COEF FOR EGR OFF SPK FILTER

    E79B: TST L0006 ; STATUS
    E79E: BPL LE7AB ; BR IF EGR OFF
    ;
    ;*********************************************
    ; EGR SPK ADV vs EGR CORR FACTOR
    ;
    ; IF EGR ON THESE VAL'S SUB'ED FM SPK ADV
    ;
    ; VAL = SPK ADV * (256/90)
    ;*********************************************
    E7A0: LDAA L007F ;
    E7A2: LDX #$D146 ; EGR SPK ADV vs EGR CORR FACTOR (17 LINES)

    E7A5: JSR LFB49 ; 2d LOOK UP ROUTINE
    ;---------------------------------------------

    E7A8: LDAB LD1BB ; 0.0625 SPK COEF FOR EGR ON SPK FILTER
    E7AB: LE7AB LDX L006A ; OLD FILT SPARK VALUE
    E7AD: JSR LFB12 ; LAG FILTER
    ;---------------------------------------------

    E7B0: STD L006A ; FILTERED SPK ADV
    ;
    E7B2: ASLB ; x2
    ;
    E7B3: ADCA #$00 ; Round
    E7B5: PSHA ;
    ;
    E7B6: LDAB L000A ;
    E7B8: BPL LE7D7 ; BR IF NOT b7,
    ;
    E7BA: BITB #$60 ; b6,b5
    E7BC: BNE LE7D7 ; BR IF b6,b5
    ;
    E7BE: LDAB L000D ;
    E7C0: BITB #$20 ; b5, PWR ENRICH ENABLED
    E7C2: BNE LE7C9 ; BR IF b5,
    ;
    E7C4: JSR LFDCB ; PWR ENRICH SPARK TIMER ROUTINE
    E7C7: BRA LE7D7 ;
    ;---------------------------------------------

    ;*********************************************
    ; PWR ENRICH SA CORRECTION Vs. RPM TBL
    ;
    ; INCR SA IF PWR ENRICH IS ON
    ;
    ; VAL = SPK ADV * (256/90)
    ;*********************************************
    E7C9: LE7C9 LDX #$D15D ; PWR ENRICH SA CORRECTION TBL
    E7CC: LDAA L001C ; RPM/25
    E7CE: LDAB #32 ; 800 RPM ARGUMENT MINIMUM VALUE
    ;
    E7D0: JSR LFB37 ; 2d LK UP, WITH LOWER ARG LIMIT
    ;---------------------------------------------

    E7D3: SUBA L006E ; PWR ENR SPK
    E7D5: BHI LE7D8 ; BR IF PWR ENR SPK GT OLD VAL
    ;
    E7D7: LE7D7 CLRA ;
    E7D8: LE7D8 PSHA ;
    E7D9: CLRB ;

    E7DA: LDAA L0002 ; STATUS FLAG WD
    E7DC: BITA #$20 ; b5, CLOSED LOOP
    E7DE: BEQ LE7E3 ; BR IF NOT b5
    ;
    E7E0: LDAB LD023 ; SA Diff for ALDL (8 Deg ?
    E7E3: LE7E3 LDX L0057 ;
    E7E5: ABX ; ADD IN ALDL SPARK (IF IN ALDL MODE)
    ;
    E7E6: PULB ;
    E7E7: ABX ; ADD IN POWER ENRICH SPARK
    ;
    E7E8: PULB ;
    E7E9: ABX ; ADD IN EGR SPARK (FILTERED)
    ;
    E7EA: PULB ;
    E7EB: ABX ; ADD IN COOLANT SPARK
    ;
    E7EC: LDAB L00E7 ; TIME OUT SPARK
    E7EE: ABX ; ADD IN TIME OUT SPARK
    E7EF: STX L0057 ; FINAL SPARK
    ;
    E7F1: CLRB ;
    ;
    E7F2: LDAA L0036 ; MPH/3.2
    E7F4: CMPA LD024 ; 3 MPH, HWAY Spk Adv QUAL Thresh MPH, MIN
    E7F7: BLS LE80B ; BR IF Vss LT THRESH
    ;
    E7F9: LDAA L000A ;
    E7FB: BPL LE80B ; BR IF NOT b7
    ;
    E7FD: LDAB L0078 ; HIWAY Spk Adv TIMER
    E7FF: CMPB LD025 ; 40 Sec's HWAY Spk Adv QUAL MIN TIME Thresh (Sec/1)
    E802: BCC LE810 ; BR IF TIMER GT THESH
    ;
    E804: LDAA L0070 ;
    E806: CMPA #$08 ; b3
    E808: BNE LE80B ; BR IF b3
    ;
    E80A: INCB ; INCR HWAY Spk Adv TIMER
    E80B: LE80B STAB L0078 ; HWAY Spk Adv TIMER
    E80D: CLRA ; CLR TIMER

    E80E: BRA LE81B ; GO SUB OFF SPARK BIAS
    ;---------------------------------------------

    ;---------------------------------------------
    ; HIWAY SPK ADV vs VACUUM
    ;
    ; ADDED SPK IF IN HIWAY MODE
    ; (See LD024 & LD025 FOR QUAL'S)
    ;
    ; VAL = SPK ADV * (256/90)
    ;---------------------------------------------
    E810: LE810 LDAA L0031 ; VACUUM
    E812: LSRA ; VAC/2 FOR LK UP SCALEING
    ;
    E813: LDAB #48 ; LK UP ARG LIMITER, (50 Kpa VAC)
    ;
    E815: LDX #$D157 ; HIWAY SPK ADV vs VACUUM, (6 LINES)
    ;
    E818: JSR LFB45 ; 2d LOOK UP ROUTINE
    ;---------------------------------------------
    ;
    ; SUB OFF SPARK BIAS
    ;
    E81B: LE81B TAB ; A to B Reg.
    E81C: CLRA ;
    E81D: ADDD L0057 ;
    E81F: SUBB LD014 ; Main SA BIAS, (10 Deg)
    E822: SBCA #$00 ; ROUND
    E824: SUBB LD015 ; COOL SA BIAS, (20 Deg)
    E827: SBCA #$00 ; ROUND
    E829: STD L0051 ; SAVE ADJUSTED SPARK ADVANCE (Temp)
    .....

    From Robert Rauscher's (Rbob, someone I know you are familiar with) paper on diy-efi.org:

    Ignition Control and the '747 ecm

    Robert Rauscher, 5/26/99, Updated 4/9/00
    .....

    0x014: Warm spark bias. This value is subtracted from the
    Total SA. It's purpose is to bias the main SA table,
    so that negative advance values can be in the table
    as a positive number.
    .....
    As you can see, GM did indeed use negative timing numbers in their calibrations as well as conservatine WOT timing and the code does not lie. The TBI code assumes a 0 deg. initial spark advance so the only caveat is that if you add initial base advance, it is not taken into account in the value reported in the datastream if you are outputting that value so you must add it to the value reported. With TunerPro, depending on the definition file you are using, the main spark bias may or may not be subtracted therefore the values displayed for timing may not represent actual values. As for EBL, I can't say what Rbob has in the code so it could be different. I have been doing this for a long time and i have done my homework and verified all this info on my ECM/PCM bench. Verify it for yourself or ask Rbob.
    '94 ECSB 305 TPI running on TBI PCM in PFI Mode. See the conversion here.
    '03 Tahoe 5.3 FFV. Beer for me, 105 octane E85 for her.

    My latest Project

    1972 C10 LWB. Fuel injected? Of course!

 

 
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