Technical Advice on electronic distributors

These CDI box ignitions were very expensive & were mainly used on Corvettes or 409 powered full size Chevys ..

 

I think that still was inductive systems, just with transistor(s) switching the current going through the coil. The ballast resistor in series with the coil would not be needed for CD ignition, and besides, as it's among the first electronic ignition systems it's much more likely to be a modified version of what was already around (inductive), rather than a completely new tech (CDI).

Either way, no matter if it's CDI or just a transistor amplifier, the points will absolutely last much longer as it only handles a small current to trigger the electronics, a fraction of the total load of the coil.

I agree completely about the time related upsides of CDI vs. inductive. A high rpm V8 needs lots of sparks in a minute, a single coil points system will struggle to provide high energy sparks in that situation. An old low rpm (and probably low compression) V8 demands a lot less from the ignition, an engine with four cylinders is a walk in the park by comparison.

The transistor based amplifier would also make stronger sparks possible compared to points only, as there's a limit to how much current you can send through the points - not a hard limit, but more current reduces the points life and eventually it would just be too short to be practical. The transistor amplifier lets you use a lower resistance coil that will draw more current = more power (electrical power, get your mind out of the horsepower gutter!).

The system still needs a fair bit of dwell time to reach maximum spark energy so it doesn't solve it completely, but the higher current may let you get enough spark energy at a higher rpm, i.e. you can run higher rpm before the sparks runs out of energy.

 

How many timer per second do the points have to open in a V8 at say 8,000 rpm? If I have the math right, it's amazing they can do that!

 

I'm just heading for bed so let's go for 6000rpm. Divides nicely into 100 revolutions per second.
The engine fire all cylinders every two revolutions, so thats 100/2*8 =400 sparks per second, if my sleepy brain is cooperating.
8000rpm would be 1/3 more, so 533 sparks per second.

The spark from an inductive ignition system under load (i.e. not idling with very low cylinder pressures) lasts for around 1 millisecond. In theory, at 8000rpm the spark would be lit more time than it's out... assuming the coil had enough time to build enough spark energy between sparks, and I doubt it would. Spark duration is one thing that goes down when there is less energy.
CDI systems produce sparks with way shorter duration. So they have their downsides too, the spark needs to actually light the fuel on fire, not just make a quick visit in the cylinder.

 

Thanks, that was my answer as well. I guess I never really thought about it but it's truly amazes me, more so than electronics, that a spring loaded points can do that, and accurately. 500 times a second. Try touching your thumb and finger together, maybe 5 times a second.

Ah well, back to the subject at hand. Thanks!

 

LOL I am with you.

Ben

 

I remember these very well, although my first personal experience was with a 1964. I had a Corvette with points, buddy had a GTO with TI. These things lasted just a wee bit longer than a snowball in Miami

The local racers laughed at them.

The only folks that I knew that had them were the college rich kids, whose Daddy bought them the most expensive of everything. In 1964, these dudes were playing the Parker Brothers Monopoly game with real cash.

Jon

 

I think the point to recognize is not that they weren't the greatest ignition when compared to susequent ignition systems, but the fact that they were a move to electronics which could produce a better spark/and more sparks eventually. MSD built a business on the idea of multiple spark discharge.
Look at virtually anything that was made prior to 1965 and for the most part they were pretty rudimentary. How many hot rodders out there are looking for original Fordomatics or Powerglides rather than 700R4s and T5s ? Even the venerable and often sought 700R4 went thru an evolutionary process as the first iterations were not as good as the later ones.
I'm simply saying that "something" other than the conventional points ignition was available prior to 1965 and it looks like an "electronic" ignition to an electrically challenged individual like myself.