Contact Breaker Cams

[Roger B]

As the end of a long chain of thought about various ignition problems I wondered if the rate of opening of the contact on a 'conventional' ignition system is important?   

If the contacts open too slowly is more of the spark energy lost in arcing and less goes to the spark plug? 

I have experienced the opposite problem when there is too much spark energy and parasitic sparks everywhere 

[Admiral_dk]

Very interesting question and it seems logical that there must be an optimal curve / accelleration curve ....

That said - I believe that you always will want the electric part to happen as quick as possible .... (to avoid too much Arching off the Points).
I agree that this also will create the biggest amount off energy out of the coil (any not too much on the Primay side) .....

If you have problems with sparks jumping where you don't want them .... I would say that You just might be the best qualified here (by far) as it is your field of Work ....
I can think of (on the Seconday Side) - Insulation, Primary Voltage, Winding Ratio and Distance (a problem on a Model).

Per

[stevehuckss396]

If you have trouble with spark flying everywhere try reducing the gap at the sparkplug. I have .012 inch gap on my v8 and it runs well and went a long way to getting that spark through the distributor  and where it is supposed to be. Also less stress on the coil and other ignition parts.

[Roger B]

I know the cause of my sparks everywhere problem, applying 12V to a 6V coil   

[Jo]

  At least you found it Roger

Jo

[stevehuckss396]

That'll do it. I put 12 volt to a cdi that requires 3-6 volt and had the same result.

[bobh]

You want the contacts to open and close as quickly and as CLEANLY as possible. No bounce. Contacts that don't open and close smartly will cause arcing and will mess with your timing as the coil won't discharge until the arc on opening breaks, a variable thing on each cycle. Bob

[Admiral_dk]

I forgot to mention that the cam profile should have a certain Dwell Time -> the profile will be somewhat different with different RPM Ranges ...!
The trick is just to reach the time required for the Primary Current to reach Saturation @ Max. RPM's.

You can add a Power Resistor in Series with the Primary - both to protect the Primary and to lower the Voltage.

Per

[stevehuckss396]

I forgot to mention that the cam profile should have a certain Dwell Time -> the profile will be somewhat different with different RPM Ranges ...!
The trick is just to reach the time required for the Primary Current to reach Saturation @ Max. RPM's.

You can add a Power Resistor in Series with the Primary - both to protect the Primary and to lower the Voltage.

Per

I remember Chrysler called it a Ballest resistor. General Motors was built into the hot wire going to the coil. When the key was in the start position the Resistor was bypassed full voltage was delivered to the coil. When the engine started and the key went back to the run position the Resistor was back in the circuit. Guys who didn't know better would move the coil and shorten the wire up cutting down or out the Resistor on the chevys. That brings back memories.  Still have 2 cars with points so I also still have a Dwell meter. When you only drive them a few hundred miles a year points last a looooong time.

[Admiral_dk]

Memories - well I haven't seen a Dwell meter since my Teens, when I had an after Scool job @ VAG (VW, Audi) .... 

Funny that Crysler shorted the Resistor during Cranking as it gets more and more important to have the Resistor in Circuit the lower the RPM.
For the slightly more electricly inclined : Any Coil has a reluctance to change the Current through it (measured in Henries) - the higher the Value, the longer it takes to 'Charge the Coil' for the next spark - but it also means a stronger spark needed for higher Powered Engines. As this has a negative sideeffect in a much higher amount of Power wasted in the Coil @ lower RPM's - somtimes to the point of destruction of the Coil - especially at a stopped Engine with the Ignition applied or at Cranking speeds .... 
Some smart Engineer realized that if you needed to use a High Power Ignition Coil optimized for high RPM's - just add a simple Power Resistor in series with the Coil, as the Resistor absorbs the extra Power @ low RPM's and don't have any influence @ high RPM's.

I know it sounds Counter Intuitive - and I felt the same way as those you describe Steve (short the Resistor) - but it works perfectly with it if all the Values are correct.
In my case it was not until working with my first CNC Router, that I read a great explanation on how it worked - both with graphich and Math examples - Google is your friend again here .... 

A modern ECU uses either 'Racing Coils' as it calculates when it needs to turn on the Coil, so it just reaches Saturation @ the time when it Fires the Spark.
Some newer versions even has an ability (+ sensor in the Coil) to just use as much Power as needed (but not more) to safely fire the Plug - so it adjust the Current in the Coil continuesly.

Per

[Roger B]

Lots of interesting thoughts  I remember some conventional car engine ignition cams having asymmetrical profiles I guess to better control the opening and closing of the points. I tend to make mine with a suitable off centre arc to give the profile, good or bad I don't know  Here are a couple of examples, a 180° and a 360° cam from the same engine.

This is the one where I also had 12V on a 6V coil due to failure of the 7806 regulator I was using.

There are still some running problems so as an experiment I am trying my power resistor bank as a ballast, 6R8 down to 1R0.