Musing on Ignition systems

[D Cluley]

This is an offshoot of my Distributor Wiring thread Distributor Wiring Diagrams where @Big_John & I have been discussing Pertronix wiring.

I am just working through some thoughts and questions and am looking for confirmation or corrections.

The coil is provided with positive voltage whenever the ignition is on, once the engine starts, this is battery voltage minus what the ballast resistor absorbs. The ground connection is through the points in the distributor. The points are just a mechanical switch that opens and breaks the circuit when the spark plugs fire. The coil output to the spark plugs is a proportionally larger voltage and the way to increase this (for a hotter spark) is to increase the input voltage. This is the reason the start position of the ignition switch bypasses the ballast resistor to provide the coil with full battery voltage. The Mopar electronic box and the Pertronix Ignitor I (I believe the Ignitor 2 & 3 have some other features that complicate this) use a magnetic sensor and an electronic circuit to replace the mechanical points. They do not inherently provide higher voltage or a hotter spark, they simply open and close the ground connection more precisely and consistently and without the maintenance of the points.

I have heard that the ballast resistor is there to keep the coil from overheating and/or to lengthen the life of the points by reducing arcing and pitting of the contacts. Either of those sounds reasonable, however if the resistor is there to protect the points, why did Chrysler keep it when they went to the electronic box in the '70s. If it is to protect the coil, why does Pertronix suggest that you don't need it. Specifically, their instructions say the resistor isn't needed if the coil is at least 1.5 ohms and the FSM says the stock coil is 1.65-1.79 ohms. After decades in business, I don't doubt that Pertronix knows what they are talking about, but it seems odd. Perhaps Chrysler just wanted a larger margin of error?

If I am in fact understanding all of this, a Pertronix installation that keeps the ballast resistor will not provide better performance than the points but will not in any way be worse. You are simply eliminating the points Maintenace. A Pertronix installation that eliminates the resistor will provide a hotter spark due to the higher voltage. I assume that to take advantage of the hotter spark, the spark plug gap should be increased?

 

[halifaxhops]

How do I explain this the Hotter spark is not really true it is a more Compressed time or crisper spark. The most you will get out of a stock mopar coil is 28K. Now say a 60 K V coil yes open up the gap.

Also on a points system it is the condenser that stops the points from arching not the ballast resistor.

 

[halifaxhops]

 

[Big_John]

The points are just a mechanical switch that opens and breaks the circuit when the spark plugs fire.

You have that backwards. Simplified, the points open and the plugs fire. Some reading on that subject: How Does an Ignition Coil Work? It’s a Windings Road!

And some reading on Pertronix: How It Works - Pertronix Ignition Systems

I'll bet the videos that @halifaxhops has posted will help you understand how this works. I will watch them myself a little later as I'm finishing my breakfast and headed out to "Cars and Coffee".

A couple things.... Think in terms of current rather than voltage and look at dwell times.

 

[MoPar~Man]

When the battery is cranking the starter motor, battery voltage will not be 12 volts (and certainly not 14 volts). It might be 10 or 11. I haven't measured this exactly, but it will be lower. Hence the ballast resistor is not in the circuit during starting. When running, the ballast resistor is in the circuit. I've measured the voltage across the resistor with a scope. It's 3 volts while the points are closed. My resistor is 0.5 ohms. That's 6 amps flowing through the circuit.

I'm building an electronic tach, and I'm using the signal across the ballast resistor to drive an opto-isolator. I've put 0.47 uf caps from each leg of the resistor to ground and also across the resistor to reduce the spikes on the falling and leading edge of the signal.

My voltage regulator will keep the system voltage set to 14.4 volts (as measured at the cigarette lighter) when the RPM is higher than idle, but at idle it will drop to 12 volts and even lower (if I'm idling at a stop light for a while). At what RPM the alternator kicks in to bring system voltage to 12.5 and higher I don't know, it's somewhere between 600 and 1000 rpm. My regulator is electronic - because it does not have external resistors mounted to the underside of the case?. One thing I don't like about this is how the system voltage changes quite a but during city driving, because of the low (or no) output of the alternator during braking / stopping because engine RPM falls to idle, and I like to have a low idle speed (reduce gas consumption). When driving away from a stop, I see ammeter swing far into charge to put juice back into battery (this is a new battery). I mention all this about the charging system because the ignition system will see 12 to 14.5 volts supply depending on what the battery / alternator is doing. If I could change anything it would be to have an alternator that can give 14 volts at 600 rpm idle. Maybe the 2-wire alternators do this? Mine is a 1-wire.

One other thing I've done is to put a diode across the points to reduce the flyback voltage from the coil. This eliminated some troublesome glitches I was seeing on the tach signal when the points open. They say that having a diode across the points reduces sparking and pitting at the points, increases their life. Maybe a cap does this too, but I suspect the cap is more to reduce radio interference.

 

[volksworld]

mopar used a ballast resistor...chevy used a high resistance wire from the key to the distributor...I think ford did the resistance wire too...VW and others never used a resistor, they ran full 12v to the coil...so there are coils out there that are designed to run at a lower voltage thru a resistor and coils that can handle a full 12v input...when you throw "aftermarket high performance" into the mix now you have some kind of compromise between maximum output voltage and the damn thing overheating and breaking down

 

[Big_John]

mopar used a ballast resistor...chevy used a high resistance wire from the key to the distributor...I think ford did the resistance wire too...VW and others never used a resistor, they ran full 12v to the coil...so there are coils out there that are designed to run at a lower voltage thru a resistor and coils that can handle a full 12v input...when you throw "aftermarket high performance" into the mix now you have some kind of compromise between maximum output voltage and the damn thing overheating and breaking down

I'll bet you'll find those VW coils have a higher primary resistance. Thus the same as a lower primary resistance coil and a ballast resistor in series.

 

[D Cluley]

Thank you @halifaxhops those were great videos.

 

[D Cluley]

You have that backwards. Simplified, the points open and the plugs fire. Some reading on that subject: How Does an Ignition Coil Work? It’s a Windings Road!

And some reading on Pertronix: How It Works - Pertronix Ignition Systems

I do understand the relationship between the points and the plugs, just worded it badly. How about "...opens and breaks the circuit when it is time for the spark plugs to fire"

I had seen that Motor Trend article on Pertronix earlier but got more out of it the second time through. Thanks. It does seem to confirm that the Ignitor 2 & 3 do more, but the 1 is just solid-state points.

 

[halifaxhops]

Thank you @halifaxhops those were great videos.

I like them they kind of dumb them down!

 

[MoPar~Man]

More musings on ignition systems:

- There isin't a lot written about what happens in the distributor, in terms of the spark having to jump a gap there, and what exactly the gap size is. The contacts in the cap are usually aluminum, sometimes brass, and why don't they erode or wear like the spark plug point wears. We all tend to think that the high tension wire from the coil is directly connected to the plug wire (electrically) but the plug wires are isolated pieces of wires in space. I don't see how or why the spark would jump to them inside the distributor since the other ends are not connected to ground. The rotation of the rotor inside the distributor, the size of the rotor contacts and the tower contacts, engine RPM, all affect how long the rotor will be in proximity to the tower and allow the spark to jump across it. There is a phenomena of spark duration, how much of that is governed by the geometry of the tower - rotor contacts and rotation speed?

- when you search the web or ask AI what the relationship is between spark intensity, duration, plug gap and fuel efficiency (MPG), you won't get an answer. You are never told that a higher spark voltage which allows you to have a wider gap will have this or that effect on MPG. You won't find a thing about spark duration and it's effect on MPG. All you'll find are bland statements like combustion efficiency and MPG will fall if the ignition system / plug specs fall outside OEM specs. Don't we know by now about all the dynamics of gasoline vapor and the combustion spark enough to state some definative thngs about that phenomena?

 

[Big_John]

- when you search the web or ask AI what the relationship is between spark intensity, duration, plug gap and fuel efficiency (MPG), you won't get an answer. You are never told that a higher spark voltage which allows you to have a wider gap will have this or that effect on MPG. You won't find a thing about spark duration and it's effect on MPG. All you'll find are bland statements like combustion efficiency and MPG will fall if the ignition system / plug specs fall outside OEM specs. Don't we know by now about all the dynamics of gasoline vapor and the combustion spark enough to state some definative thngs about that phenomena?

There's actually a simple answer to that. There's a point where there's no improvement left. You are starting the fuel mixture burning and that mixture really doesn't care how it was ignited.

The difference is in how reliable that spark is generated. A failing ignition system will cause decreases in MPG or make it run poorly. Once the system is corrected, and the spark is being reliably generated, the fuel mixture goes back to burning like it should. Improvements to the ignition system are really about generating that spark more reliably and no deterioration in the quality of that spark. Cue the peanut gallery comments about failing electronics that aren't part of this discussion.

If you read a lot of threads here, you'll see references to "upgrades" on many different topics besides ignitions and then how wonderful that upgrade was over "stock". What they don't tell you is their point of reference before the "upgrade" was actually broken or worn and wasn't right to begin with. Same with ignition systems. If the car was running right to begin with, there would be minimal, if any, gains in how the car runs or MPG.

There are obviously some caveats to that, and most will involve other modifications to the engine where an "improved" ignition system will help, but let's save that for another day and just talk about a basically stock engine.

I say all that, and I kind of like the different ignition systems. I got exposed to that when I was a kid and my Dad was a fan of newer electronics in about everything, including ignition systems. I have one car with an added Mopar electronic ignition (older system with USA parts) , one with stock dual points and one with a Pertronix Ignitor II. All start and run nicely. The dual point is probably going to be converted to an Ignitor II when it's time to replace points.

I've also run a couple MSD ignitions, built back when they were built in the USA and reliable. I liked those. Even ran a Heath-Kit CDI ignition my Dad had built many years ago for a while.

So, basically, if you are looking for gains in MPG, it's really not here. The basic Ignitor I is really just a way to not have to mess around with points. The Ignitor II has some additional features that I like (read the link I posted above). The Mopar aftermarket ignition was the one of choice for years, but offshore sourcing has really hurt that. I've said before, if I were starting from scratch, I would consider using an older, rebuilt Mopar electronic distributor with a GM HEI and a Ford coil, but my experience with the Pertronix has been really good too.

 

[Big_John]

Just thought of an analogy.

Let's say you have a bucket of gasoline and you have matches and a lighter and you want to set it on fire. Either one will light it up. The matches are basic and cheaper. The lighter costs more, but it's not as affected by wind or getting wet. Points are like matches. They get the job done as long as conditions are good.

 

[MoPar~Man]

There's actually a simple answer to that. There's a point where there's no improvement left. You are starting the fuel mixture burning and that mixture really doesn't care how it was ignited.

Then why did cars back 25 years ago (or maybe a little more) go to coil-on-plug with spark gaps like 50 thou instead of 35?

What is the optimal spark voltage / gap? And what about duration. Nobody talks about duration.

 

[Big_John]

Then why did cars back 25 years ago (or maybe a little more) go to coil-on-plug with spark gaps like 50 thou instead of 35?

My comments are based around the time frame of our C-bodies and not newer cars, but I think this part of my post kind of covers it.

There are obviously some caveats to that, and most will involve other modifications to the engine where an "improved" ignition system will help, but let's save that for another day and just talk about a basically stock engine.

The newer engines have a lot of other factors that we don't deal with in our older cars. Lean mixtures and variable timing for example. They want a hot, dependable spark that doesn't need much, if any maintenance. Individual coils are only going to fire once every two crank revolutions where a V8 powered older car will have a coil that fires 4 times in each crank revolution.

What is the optimal spark voltage / gap?

Good question. It depends on the capability of the system. With my Pertronix II, they say you can open the plug gap up .005". My MSD equipped cars could have up to (IIRC) .060" plug gap. The MSD would also fire a fouled plug.... and as a side note, when I was drag racing, when I started using the MSD-7, I stopped using "warm up" plugs when we bought the car to temperature in the pits. A lot less burned knuckles LOL!

 

[Big_John]

And what about duration. Nobody talks about duration.

Then I'm not going to talk about it either....