Odd Fire TBI

**fastacton** · 04-18-2016

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?

**lionelhutz** · 04-18-2016

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.

**fastacton** · 04-18-2016

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?

**1project2many** · 04-18-2016

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.

**lionelhutz** · 04-18-2016

Originally Posted by fastacton

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.

**fastacton** · 04-18-2016

Originally Posted by lionelhutz

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.

**lionelhutz** · 04-18-2016

Originally Posted by fastacton

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.

**Willys43** · 04-19-2016

Originally Posted by fastacton

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.