Preferred ignition module (distributor)

[CamB]

 

 

 

I have a twin spark engine (Porsche flat six), which has a twin distributor (and twin coils) firing 12 plugs. The pair of plugs per cylinder fire simultaneously and I believe Porsche used a Bosch 124 module per coil on the earlier cars and a single Bosch 200 Module (2 channel) with two coils on the later cars. 

Mine is more complicated, as it currently has a vintage Motec with Bosch 008 smart modules (which control dwell). I am in the process of upgrading to a G4+ and for various reasons don't believe the 008 modules are a smart choice (refer here if interested - http://dtec.net.au/Ignition Coil Dwell Calibration.htm) for the G4+. I am looking for advice on what to use instead:

• 6 cylinder distributed, two coils firing at once, max RPM about 7200rpm
• coils are Bosch MEC718 - dwell is about 3ms
• I understand I can connect two igniters (or two channels of a 2 channel igniter) to a single G4+ ignition output
• I see the options as:
  • Bosch 124 modules. I think these are ok, but possibly older tech?
  • Bosch 037 / J121 modules - simple 3 pin igniter
  • Bosch 200 module - only one - I have some slight reservation about how much it will like firing both channels simultaneously (and at high RPM) although as I said, it looks like Porsche might have done that themselves
  • Stick with what I have (I believe it is fundamentally compatible with the G4+ - please confirm(!)), populate the dwell table sensibly, as assume this will largely overcome the problem discussed in the link, which is:

Quote

     The Hall Effect triggered module type internally converts the square wave input signal (from your ECU now) into a saw tooth voltage and uses this new sloping signal in their dwell setting process. The limit they can adjust their dwell will be restricted If you do not give them a long enough pulse.

To expand on this; these modules fire when the input signal goes from Hi to Lo, so if you have a Hi period of the triggering pulse say 3 ms long before you wish to fire then this will be the maximum dwell time that can achieved. Set up your ECU so the pulse Hi time is always longer than the greatest dwell time needed.

Thanks in advance.

 

** Supplementary question:

- "Spark duration" appears to reduce dwell at RPM above which its not possible to achieve both, yes? How does one determine an appropriate duration?

[Adamw]

I think any of your suggestions would work fine and there would be little difference between them.  I'd personally probably shy away from the 008 just because you have no control over dwell and they are very hard to get replacements for nowadays.

1 hour ago, CamB said:

• Bosch 124 modules. I think these are ok, but possibly older tech?
• Bosch 037 / J121 modules - simple 3 pin igniter

These are both capable of about 7-8A and are well proven/reliable.

 

1 hour ago, CamB said:

Bosch 200 module - only one - I have some slight reservation about how much it will like firing both channels simultaneously (and at high RPM) although as I said, it looks like Porsche might have done that themselves

I havent seen a 200 fired parallel before but with his 4 Channel sister, (the 211) it is quite common to use those in wasted spark set ups firing 2chs together and they dont seem to give any drama.

 

1 hour ago, CamB said:

Stick with what I have (I believe it is fundamentally compatible with the G4+ - please confirm(!)), populate the dwell table sensibly, as assume this will largely overcome the problem discussed in the link, which is:

You could calculate out a ms based dwell table if you really wanted too but you dont even need to get that technical, you just set the "dwell mode" to "duty cycle" and for the dwell table you just set a single fixed DC%.  For the 008 the recommended duty cycle was always 70% from memory but you might be able to bump that up a little more depending on the RPM you need to run.  With a distributor you quickly run out of dwell at high RPM, 70%DC at 7000RPM on a 6 cylinder distributor system will give you a max dwell of 2ms,  80%DC will give you a max of 2.3ms. 

[CamB]

Thanks Adam, very helpful. Will look at the 124/J121 options I think.

For some reason I had thought duty cycle was locked to 50%. Clearly I was completely wrong....