Odd Fire TBI

[Willys43]

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.

[1project2many]

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?

[Willys43]

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.

[Six_Shooter]

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.

[1project2many]

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.

[Willys43]

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.

It's not even a 225, it's a 198 Fireball, just too damn cool to throw out. I checked my gear ratio chart and the highest RPM I used was 4000 to get 82 mph, which it will never see with this nut behind the wheel. I'm going to try to use two ign modules and 1 HEI coil and see if it will work. Do you think I'll have a dwell issue? I thought dwell was adjust in respect to fuel burn time in each Cyl rather than when the Cyl burns it(odd fire). The Odd fire HEI distributor didn't have any provision for adjusting the dwell,(outside of the initial setting). Please shed some light on this. While reading in the MsPro manual I found a very good explaination of IAC stepper motor operation, I understand the 4 wire system now.

[Six_Shooter]

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.

Yes it is, I have one sitting here to go into a friend's car where we're replacing the FAST system that never worked right. The engine in s a built Pontiac 496. We're also going from TBI to MPFI.

It's not even a 225, it's a 198 Fireball, just too damn cool to throw out. I checked my gear ratio chart and the highest RPM I used was 4000 to get 82 mph, which it will never see with this nut behind the wheel. I'm going to try to use two ign modules and 1 HEI coil and see if it will work. Do you think I'll have a dwell issue? I thought dwell was adjust in respect to fuel burn time in each Cyl rather than when the Cyl burns it(odd fire). The Odd fire HEI distributor didn't have any provision for adjusting the dwell,(outside of the initial setting). Please shed some light on this. While reading in the MsPro manual I found a very good explaination of IAC stepper motor operation, I understand the 4 wire system now.

Dual ICMs and single coil will not work. Keep in mind how a coil is triggered. The negative side of the primary is held low to charge the coil and sparks on the secondary when that signal is removed. Using dual ICMs in the way you propose will cause the negative to always be held low or possibly spark at the wrong time. Shortening dwell (which is not something normally supported in tuning software for GM ECMs) MIGHT make it work ok, but as the RPM rises I see it being more of an issue. This is one of the big reasons why I've been saying stay away from the dual ECM route. You need two completely separate ignition systems, and isolate the fuel and air intake of each sets of cylinders COMPLETELY to make this work, like I said treating it like two completely independent engines that happen to share a common crank.

To even give it a chance of working you will need two coils and a dual coil isolator (usually found in endurance and NASCAR racing}. The isolators are usually used to switch from one ignition system to another in the case of an ignition failure situation, but I don't see why it can't be used with two active ignitions systems.

[Willys43]

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.