Hot Rods CDI....who’s running ‘em on their hot rod?

Well guys this old dude is running MSD 6AL box and an electronic Mopar distributer on my old 392,,,, It looks like an old points style and the box is hidden under the dash....Sorry but I do like it....

 

I built the heathkit version way back in high school (1976). Installed on my 69 Camaro ss. Switched to CD and it ran like poo, popping and misfires. Had no way to diagnose, so removed it. Always wondered what was wrong, was there an underlying problem with the car, or the box. Pretty sure it got deep six when my parents moved.

 

A little bit different explanation for how the CDI works. Yes as Bob described, a battery powered CDI makes its own voltage with its internal power supply to charge the discharge capacitor. Some produce a really high voltage (550V or more) and some produce lower voltages but not too many nowadays. The voltage impressed on the coil primary winding when the CDI discharges is that voltage times the turns ratio of the coil. The coil will see that voltage for a very brief moment until the gap breaks down and limits the total voltage. This is different than the Kettering system that uses the coil to produce 300V or more on the coil primary from a supply voltage of 12V. The Kettering system doesn't need a power supply to produce the voltage because it does so by means of self induction which occurs as the points open. It's the electrical equivalent to water hammer and that's how all inductive systems work to make the voltage necessary out of a meagre 12V and a turns ratio of 1:100 (typically). Inductive systems are self regulating in that the voltage will continue to rise if the plug gap does not break down. There is always a voltage overshoot which is commonly 1.5 times the gap breakdown voltage or even more for a weak inductive ignition. So if it takes 20kV to fire the plug, the peak voltage seen on a scope would be 30kV or more. The overshoot is always the same percentage over and above the gap firing voltage so with an inductive ignition, be aware that the total voltage can climb quite high when opening up plug gaps. CDI, is different. It gives what it gives based on the voltage to which the discharge capacitor has been charged. There is still a voltage overshoot with CDI at the firing point, but the total voltage impressed on the coil very briefly will be higher yet based on the design voltage of the power supply. So, if the plug gap only needs 20kV to fire, the voltage impressed on the secondary windings (albeit briefly) could be 55kV assuming a coil with 1:100 turns ratio. That is hard on insulation. If the CDI is reduced to 300V, the total voltage can't be higher than 30kV and relatively wide plug gaps will still fire with a CDI. Most CDIs charge the capacitor to a very high voltage because it's an easy way to pack more energy into the capacitor. Some, like the old Deltas, used a high voltage because the power supply would run out of steam as the rpm increased so to keep enough available voltage at high rpm, the voltage was much higher than needed (or desired) at low rpm. Fred

 

I noticed the engines I take apart that I used the CD / points system, the heads seemed very clean...

I'm going to try the new Vertex Z-6 CD box with a 1962 iron points distributor I have. I'm still looking for a iron distributor with the tach cable connector and no vacuum advance. Talking to a local engine builder, he highly recommends this ignition system.. He also recommended the Mallory #28675 coil.

 

Also ran a triple 500 with Demco's and just let it go a year or so ago.

 

What engine are you going to run this on?

 

That’s what I started this thread with.

 

Were you using carbon core wires or solid copper core wires. I tried using packard 440 solid copper core wires with the 10 box and a single points distributor and it did the same thing. Put carbon core wires on it and walla!!!

 

Pretty sure just regular carbon core yellow wires. Had to have yellow wires, they added 20 hp.