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[GMCnet] More Onan ignition development [message #338536]

Thu, 08 November 2018 20:43

Jim Miller

Messages: 501
Registered: March 2008

Karma: 10

Senior Member

My experimentation continues on alternative Onan ignition systems. The two-magnet Pertronix 1121 described in one of my earlier postings seems to be a very workable solution if one wants to stick with the OEM series-secondary coil and it definitely produces far less stress on the ignition coil than the 1181 module that has traditionally been used. The electronic triggering via the Pertronix also eliminates the misery that comes with corroded breaker points and their condenser. The spark voltage with this system remains low, however, due to the series secondary of the coil and as such the plug gap must be kept at 0.020 to produce reliable ignition.

Moving forward three decades from the 1970s brings me to logic triggered one-coil-per-plug systems that integrate charging, firing and dwell-limiting circuitry right on the coil assembly - in this case using GM/Delphi coils for LS2 engines that run $20-$40/ea depending on whether you get the real Delphi ones or the knock-offs from China. These coils make life very easy since they produce a very hot spark and the system designer does not need to deal with triggering circuits, flyback voltage clamps and other real world complications. The DIY engine controls community likes these coils since they produce a big spark and are easily to interface - you raise a 5V logic signal to charge the coil and then drop it to fire the plug - and this simplicity is why I chose them. I figure Delphi knows far more about the subject than I do - and the less circuitry I have to design the better.

I selected a multi-purpose Honeywell gear tooth sensor that has found applications in the industrial control world as well as in the automotive world for crank and cam position sensing. Based on its spec sheet I machined a trigger wheel that had a notch of the correct angular size to charge the coils for an appropriate duration (4ms) and then to fire them. I am using plugs that have their gaps opened up to 0.040” which would never have worked on the OEM system.

Pictures of this work are at: http://www.jcmco.com/gallery/album31

It has amazed me how well this unit picks up instantaneous load as applied by my load bank. I can go from a no-load condition to 5kW nearly instantaneously and the engine does not stutter, hunt or otherwise stumble around AT ALL - but it sure does change the exhaust note from a nice drone to a gutteral groan when hit with nearly full load. I’m really happy with the way this system works and will continue with its development.

—Jim

Jim Miller
1977 Eleganza
1977 Royale
Hamilton, OH

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Jim Miller 1977 Eleganza II 1977 Royale Hamilton, OH