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Thread: Odd Fire TBI

  1. #46
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    The only way a singe even fire ECM could work on an odd fire engine is by not doing timing with the ECM. I this this has been well established.

  2. #47
    Super Moderator Six_Shooter's Avatar
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    Well established by whom? There are (single) ECMs that will run an oddfire engine without issue, definitely in the aftermarket, and I'm sure the same could be done with an OEM ECM as well with the right ICM set-up... If I cared enough about injecting an oddfire engine I think I'd use an 8 cylinder reluctor (or how ever many it would take to get even pulses between firing events), so that the pulses were even and just used the dizzy cap towers that are needed for the engine to run. Boom, one ECM, even pulses for the ECM to read, and odd fire at the ignition with timing control. Yes the RPM will read a bit high, but with a simple edit of the ADX it could be made to show accurate. No sync issues between ECMs, no ECM trying to control something it can't or isn't connected to and throwing a fit.

    I'm sure I could come up with a more elegant way if I really wanted to as well. Actually that more elegant way would be an ECM that will run an oddfire engine natively without any trickery, save the headaches.
    Last edited by Six_Shooter; 04-17-2016 at 09:48 PM.
    The man who says something is impossible, is usually interrupted by the man doing it.

  3. #48
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    I'd have to agree with Six_Shooter on this. I don't see why an odd-fire HEI distributor (which I understand is based on a V8 HEI with two dummy posts and a 6 point reluctor) couldn't be used along with a 7 or 8 pin ignition module to run TBI fairly easily. I'm relatively sure I've read about this being done on another forum. If I had an odd-fire motor here, I'd certainly give it a try. Has anyone here actually tried this?
    1973 K-5 Blazer, TBI 350, TH400, 1 ton axles & 38" SSRs'
    1975 280Z, TBI 350, 700R4
    1953 M-38A1, TBI Buick 231
    1951 Ford Panel, 5.3 with 4L80E

  4. #49
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    Odd fire V6 firing angles as you go through 720 degrees of crank revolution firing the 6 cylinders
    0 90 240 330 480 570 720

    Even fire V8 crank angles as you go through 720 degrees of crank revolution firing the 8 cylinders
    0 90 180 270 360 450 540 630 720

    As you can see, only the first 2 ignition events of the V6 engine fall at the same crank angles as the V8 engine. Hence, the V8 even firing 90* angle reluctor will not work on this engine. Period. End of story. Well established by the almost 40 years history of the engine.

  5. #50
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    So, for the odd-fire HEI, they must have used a V8 sized distributor, but with a more specialized reluctor and distributor cap. What were the results when someone tried to use a 7 or 8 pin module and a TBI setup?
    1973 K-5 Blazer, TBI 350, TH400, 1 ton axles & 38" SSRs'
    1975 280Z, TBI 350, 700R4
    1953 M-38A1, TBI Buick 231
    1951 Ford Panel, 5.3 with 4L80E

  6. #51
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    The ignition module does not care what engine it is used on. It's not the problem. In bypass mode it only knows pulse out, pulse in. The problem is how the ecm interprets signals from the ICM. The code is built around the idea that the distributor pulses are equally spaced in time, and that the amount of time between pulses represents RPM. When the time is changing rapidly the ecm will calculate rapid increases and decreases in RPM. Since the odd-fire pickup coil pulses are unequally spaced to match the odd-fire firing intervals, the time between pulses will be unequal. This is not the best solution to deliver accurate ignition timing. It is possible to use a single stock ecm with odd fire. But it would require custom code and a cam position sensor. Most folks are not ready to write custom code. IMO the dual ecm solution is not a bad one.

    I know of a couple of successful dual ecm project vehicles. Way back in the DIY-EFI days an individual known as "TedSCJ" used dual 7730's on a Jag V12. Following that, a member of DIY-EFI and the SyTy community piggybacked a 7749 and a 7427 in a Syclone in order to use a 4L60E. Although both solutions seemed less than ideal, they still worked. Both applications used similar combinations of shared sensors. MAP, TPS, and REF can be shared but two wire passive resistance sensors such as CTS and IAT cannot as the resistor network it creates changes the voltage the ecm will see.

    The odd fire engine did use a V8 cap but it only had six terminals. Two of the six terminals had "long" contacts on the rotor side. They extended through the area where the missing V8 terminals would normally be. That was the only way to ensure spark reached the correct cylinder without making a completely new cap.

  7. #52
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    Quote Originally Posted by fastacton View Post
    What were the results when someone tried to use a 7 or 8 pin module and a TBI setup?
    I've never read about anyone trying it, but it wouldn't work because the ECM would have a hard time figuring out the rpm and the timing advance would be completely out of whack.
    Last edited by lionelhutz; 04-18-2016 at 11:11 AM.

  8. #53
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    Quote Originally Posted by lionelhutz View Post
    I've never read about anyone trying it, but it wouldn't work.
    Sometimes theory and math don't prove out in the real world. Based on what 1project2many explained about the time between pulses being how RPM is calculated (I didn't know that), it might not work very well. But without actually testing it, I wouldn't write it off completely.
    1973 K-5 Blazer, TBI 350, TH400, 1 ton axles & 38" SSRs'
    1975 280Z, TBI 350, 700R4
    1953 M-38A1, TBI Buick 231
    1951 Ford Panel, 5.3 with 4L80E

  9. #54
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    Quote Originally Posted by fastacton View Post
    Sometimes theory and math don't prove out in the real world. Based on what 1project2many explained about the time between pulses being how RPM is calculated (I didn't know that), it might not work very well. But without actually testing it, I wouldn't write it off completely.
    Sigh, this was discussed multiple times already...

    The ECM takes the reference pulse from the previous cylinder and adds a time equal to the correct crankshaft rotation to fire the next cylinder. All the GM even fire ECM's do this by delaying the ignition signal to the ICM by either (90*-advance*) or (120*-advance*) of delay after each reference pulse.

    I'll try to make a simple timeline of how a V8 engine works for you to understand this. Assume the base timing is set to 0* and the ECM is currently running the timing at 25* BTDC.
    0* -> ICM sends #1 reference pulse to ECM.
    65* -> ECM has taken #1 reference pulse and added the delay to send pulse to ICM to fire the coil for #8.
    90* -> ICM sends #8 reference pulse to ECM.
    155* -> ECM has taken #8 reference pulse and added the delay to send pulse to ICM to fire the coil for #4.
    180* -> ICM sends #4 reference pulse to ECM.
    245* -> ECM has taken #4 reference pulse and added the delay to send pulse to ICM to fire the coil for #3.
    270* -> ICM sends #3 reference pulse to ECM.
    335* -> ECM has taken #3 reference pulse and added the delay to send pulse to ICM to fire the coil for #6.
    etc, etc

    See how the firing of each cylinder is based off the reference pulse for the previous cylinder? Now, can you get why it won't work? Even if the ECM can somehow interpret the ICM pulses and calculate rpm correctly, the delayed pulses it sends back to the ICM to fire the coil will result in completely wrong timing advance. The odd fire with a combination of 90* and 150* will simply be completely off.

  10. #55
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    Quote Originally Posted by fastacton View Post
    Sometimes theory and math don't prove out in the real world. Based on what 1project2many explained about the time between pulses being how RPM is calculated (I didn't know that), it might not work very well. But without actually testing it, I wouldn't write it off completely.
    At least 1 megasquirt system has a software provision for "degrees of offset" for odd fire engines, GM never fuel injected an odd fire engine, Tunerpro doesn't have any provision for offsetting ignition firing. I'm kinda cheating using a second ECM using the Base timing in tunerpro to offset the ignition for 3 cylinders by 90*. The Ecms are using some shared sensors only to try and provide the same signals to both ECMs in order to produce similar output. I could have used two complete sets of sensors, I just didn't see the need. If need be I can change 1 wire and run the all fuel off of one ECM. If this works I will have gained tremendously over an aftermarket system, simply because I have GM's programming for a 90*V6 engine. The aftermarket requires borrowing someone elses programming or starting from scratch. I'm using 6 cylinder odd fire HEI with three pickups ground off, feeding two 8 pin modules. Each module supplies 1 ECM with distributor pulses. 1 ECM is set to 0* base timing, 1 is advanced 90* base timing, which creates the odd fire timing. It ought to work, time will tell.

  11. #56
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    At least 1 megasquirt system has a software provision for "degrees of offset" for odd fire engines
    Is there a cam sensor or synchronizing sensor involved?

  12. #57
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    Quote Originally Posted by 1project2many View Post
    Is there a cam sensor or synchronizing sensor involved?
    Crank trigger, cam sensor, either or both, it looks to be software selectable, it is software selectable for 2 or 4 stroke, or rotary, 2,3,4,5,6,8,12,16 Cyl, 7 or 9 Cyl radial. It looks to be able to work with any brand vehicle while leaving vehicle OEM ECM in place, tons of features, it does have some base programs. It has a 300 hundred page manual. Specified not for street, probably because of emissions. It looks really sweet, I'd love to play with one. List price $1200. That price would total the Willys.

  13. #58
    Super Moderator Six_Shooter's Avatar
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    That sounds like the MS3Pro, which while it is a great system, is the most expensive version and is WAY overkill for any TBI set-up. An MS2 will have more than you need, but I think is the cheaapest MS that will run odd-fire. MS2 systems can be had for about $300 from diyautotune.com.
    The man who says something is impossible, is usually interrupted by the man doing it.

  14. #59
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    I learned that Maserati's approach to odd fire was to use dual coils connected to a single distributor. The 225 will likely never see the same RPM as the Maserati and will likely never have a dwell issue. But we have a solution should there be a need for 5k+ rpm revs in the old Willys.

    It appears the MS2 code can use an option bit for odd fire engines and a user entered value for "small angle" to help differentiate the pulses. However, this solution is not universal. Several posters on the msextra board used dual pickups in a single distributor and a special input mode in MS that took the input from the dual pickups. One poster even ran dual MS systems.

    GMECM's actually average the crank pulse signal some. It may be possible to rewrite the GM code so it will do the same. There are also changes needed to the dwell computation to account for small / large time variations. I suppose we could determine a window of time representing maximum possible engine acceleration and determine that any substantial change in time less than that window must represent the switch from "large" to "small." It would follow that the next pulse is a "small" to "large" transition. I really wish I had time to play with this stuff. It would be an interesting excercise.

  15. #60
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    Quote Originally Posted by Six_Shooter View Post
    That sounds like the MS3Pro, which while it is a great system, is the most expensive version and is WAY overkill for any TBI set-up. An MS2 will have more than you need, but I think is the cheaapest MS that will run odd-fire. MS2 systems can be had for about $300 from diyautotune.com.
    It is the Ms3Pro, I didn't see any documentation for odd fire in the other models, but that doesn't mean it isn't supported. I think the Ms3 pro is primarily directed at racing. Cool system.

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