Willys43

[Willys43]

I don't seem to have either option. The system does tell me all cookies are cleared, could it be a cookie problem?

[1project2many]

Possibly. You should set your browser to accept cookies from GH.

[Willys43]

Ok, Tell what you think about this. Two ecms, two ignition mods, shared sensors except for timing, then use 2 crank triggers offset 30* in order to create the odd fire. each ecm running 3 cyl, 1,3,5, and 2,4,6

[1project2many]

That's it! Each ecm will drive (1) TBI injector or (3) port injectors, and you'll need to tie the two coil wires together through high voltage diodes before they connect to the distributor.

[fastacton]

I could be wrong, but I thought there was a year or two where GM put the 7 pin module in odd fire HEI distributors?

[1project2many]

I don't recall that. Odd fire went away after '77. The first 7 pin modules I remember seeing were in the early 80's. But I'm open to suggestions...

[Willys43]

My 75 HEI distributor had a 4 pin module stock. The seven pin module wasn't used until computer controlled spark came about in the eighties.

[Willys43]

What would be the difference between two ECMs using two pickup staggard 30*(12 pulses feeding two ECM) and 1 ECM Reading 12 pulses?

[fastacton]

Yeah, I'm probably wrong on that now that I think about it. But I don't see any reason why you wouldn't be able to swap a 7 pin or 8 pin module in place of the 4 pin in the odd-fire distributor. The module doesn't care about odd spacing between firing events, it just advances or retards as commanded by the ECM based on when the pickup coil triggers. Am I missing anything? I'm pretty sure I read somewhere where it's been done before. I would have tried it myself, but I haven't gotten hold of an odd-fire block. TBI works great on my even fire, though.

[Willys43]

I have already swapped a 7 pin into the odd fire distributor, just had to drill 1 hole and tap it. I think the only real question is 3 or 6 timing pulses coming off of the crank trigger and whether it will require 1 or 2 ECMs. Any way I do it I'll be firing the plugs at TDC and at the end of the exhaust stroke.





Ill

[lionelhutz]

The module doesn't care about odd spacing between firing events, it just advances or retards as commanded by the ECM based on when the pickup coil triggers. Am I missing anything?

Yes, it's not that simple. The ECM doesn't just send a signal telling the module how much to vary the timing compared to the base timing. That really doesn't even make sense since the module couldn't just fire the ignition advanced of a pickup coil trigger signal. The ECM uses the pulses from the module and then does timing calculations to send the firing events at the proper times. And, the ECM timing is based on an even firing pattern.

[Willys43]

Either module will work with odd fire.
The difference is the 4 pin module works with mechanical advance where the 7 or 8 pin module works with computer controlled spark advance. The computer can only can only function properly with an even pulse from the 7 or 8 pin module.

[1project2many]

What would be the difference between two ECMs using two pickup staggard 30*(12 pulses feeding two ECM) and 1 ECM Reading 12 pulses?

I think the only real question is 3 or 6 timing pulses coming off of the crank trigger and whether it will require 1 or 2 ECMs. Any way I do it I'll be firing the plugs at TDC and at the end of the exhaust stroke.

You know, the question is easy but the answer takes a bit more work.

If your sensor is in the distributor then there is a difference. A 12 pulse sensor in the distributor sends six pulses every crank rotation, each one 60 degrees apart. But two 6 pulse sensors with the first at 0 deg and the second offset by 30 degrees will send pulses at 0, 30, 120, 150, 240, 270, etc. Now if you look at the firing pattern and compare to the Buick engine you'll see an issue.

odd fire firing pattern: 0, 150, 240, 390, 480, 630, 720/0
Twin 6 pulse sensors: 0, 30, 120, 150, 240, 270, 360, 390, 480, 510, 600, 630, 720/0

There are twice as many pulses per revolution as required! This would create the following unintended issues:
1) Coils firing on both compression and exhaust stroke.
2) ECM triggering injectors twelve times for six compression events.

Problem 2 can be avoided by using a single injector driver with ecms and software expecting dual injectors. IOW, if each computer is expecting to control two injectors, and you only let it control one, then the fuel should be close to correct.

Problem 1 is the only one that I can't overcome easily. Firing the spark plug while the piston is on the exhaust stroke isn't an issue. Waste spark DIS systems do this all the time. The problem is that you're using a distributor. Rotor position determines where the spark goes regardless of what cylinder is at TDC. If the extra spark pulse occurs while a piston is coming up to TDC the rotor may direct spark to that cylinder too early. If the rotor is far enough from a terminal so it won't direct spark to a cylinder, then the spark is going to try and get to ground through another route which could lead to burn through in the rotor, coil, or cap. It may be possible to install ground straps inside the cap to direct unwanted spark to ground but this just seems like a path to problems to me.

With the distributor in play I suggest using twin three pulse sensors with dual ecms and configuring the ecms for three cylinder operation.

Yeah, I'm probably wrong on that now that I think about it. But I don't see any reason why you wouldn't be able to swap a 7 pin or 8 pin module in place of the 4 pin in the odd-fire distributor. The module doesn't care about odd spacing between firing events, it just advances or retards as commanded by the ECM based on when the pickup coil triggers. Am I missing anything?

The module could be replaced easily. The problem is that a stock computer and program designed for an even fire engine cannot accurately control the timing on the odd fire. The ecm is programmed to think each cylinder is the same number of crank degrees apart. It uses the pulse from the module to start a counter. Based on current rpm and the number of degrees of desired advance the ecm will fire the next cylinder when the counter reaches a certain point. This is how it can create advance. You can think of the ecm as "storing" the pulse and delivering it just before the next piston reaches TDC. Now imagine if the pulses are spaced unevenly. One spark would be early, one would be late.

Edit: Neat little reference page with a GIF of how the odd fire firing order works.
https://web.archive.org/web/20060306...e/oddfire.html

[Willys43]

New Question. This link shows #1 cyl fires is 90*. I thought TDC #1 was 0*. Truly odd fire. What am I missing?

[Willys43]

Three pulse timing isn't possible with a crank trigger. An odd fire pickup coil with three points ground off could provide three even pulses. If two ecms were receiving this signal could the base initial spark advance be set to 0* and to 90* to create 2 ECMs firing 90* offset from each another?