Coils
Posted: 22 Mar 2001 8:41--------------------------------------------------------------------------------
By MoparKev (Kev) on Friday, March 02, 2001 - 12:15 am:
So, Dave, as a bloke after cheap horses, what coil should I buy to replace a standard coil? I have never seen a coil advertised as "Variable Magnetic, Automatically Controlled or Demand Output." I run side gapped plugs at the mo but have a brand new set of NGKs for the Nats. I hope you run a tech piece on plugs. These tech notes are great as I know quite a bit but clarification of the basics is great.
By David Robson (Dave) on Friday, March 02, 2001 - 11:03 am:
I know the Jacobs coil I use is that type but they didn't invent it so there are others. I will look in my new Summit Catalog tonight to see what is on offer.
By David Robson (Dave) on Friday, March 02, 2001 - 02:52 pm:
I will re-do that coil stuff tonight and see if I can get the image to work. I will also add ways to test the coil.
I will cover the Chrysler Electronic Ignition in a separate piece.
By David Robson (Dave) on Friday, March 02, 2001 - 09:22 pm:
THE IGNITION COIL
The amount of energy supplied to your spark plugs is determined by your distributor and the design of the coil.
A coil is actually made of two coils. A primary and a secondary. The primary and secondary are wrapped one inside the other around a core which can be magnetised. The primary is connected to the +ve and -ve terminals and the secondary to the -ve and the main H.T. lead.
Coils generate their high voltages when the current supplied to the primary winding is suddenly interrupted.
Electricity is generated in the secondary winding by an increasing or decreasing magnetic field in the coil core. Its the rate of change of this magnetic field that determines the level of output. Fast rate of change = big output.
Our teacher at college (before things like health and safety) made a few of us stand in a row holding hands. Each guy at each end held one end of a secondary winding on a large transformer. The teacher put a 1.5 volt AA battery onto the primary. Nothing happened. He then disconnected the battery. The shock threw us all apart and gave us a hell of a jump!
When the points close (battery on) and current builds in the primary coil, the magnetic field of the core slowly increases which generates a small voltage in the secondary winding. However this is not large enough to jump the spark plug gap so no current flows.
When the points open (battery off) the current to the primary is cut off, the magnetic field of the core suddenly collapses which generates a huge voltage in the secondary and across the spark plug gap.
Remember, it is the rate of magnetic field change that determines the output. If the magnetic field rose as fast as it collapses then you would get a spark at the plug but current would be flowing in the opposite direction to normal. I mention this because there are ignitions which work this way and I might cover these at a later date.
The amount of change in the magnetic field also has an effect, i.e. if the primary current has not been there long enough for the field to build to its maximum (length of dwell) then the output will be lower as a result. So the more dwell you can get from your points the better the spark. Up to a point.
It is current (AMPs) that ignites fuel (some might argue with that). Voltage is there only to jump the plug gap. A higher voltage than that which is necessary to jump the plug gap will only reduce the amount of current available. Basic Ohm's law.
For street driven cars the standard coil will, in most cases, be fine. What I have seen in some peoples cars though are "High Performance" coils that are actually high voltage coils and some can produce 50,000 volts or more. A coil that produces such high voltages must also reduce its spark duration and current strength proportionately (Ohm's Law again). At idle and normal RPM levels, your spark plugs need a long duration burn as well as solid spark power to burn off contaminants and ignite the air/fuel mixture. The weak, low current, high voltage coil has too low a current to burn off the contaminants. This results in fouled plugs, misfires and poor mileage. However, as rpms increase then the amount of actual dwell time decreases. This results in a weaker spark at high RPMs with a stock coil. These high voltage coils can help in this situation.
I would leave these high voltage coils for high RPM race cars only. This is what they were really designed for.
Things to look for in a coil
There are coils which are better than your stock units. The best ones are coils which have a variable permeability core. The permeability is the cores ability to magnetise. Basicaly this means that the voltage does not drop off as much as RPM increases. These make the best all round street/strip coils. They are advertised with names like Variable Magnetic, Automatically Controlled or Demand Output.
A metal case filled with fluid is an important feature as the fluid pulls heat off the windings and into the metal case which then conducts the heat away. Some coils have plastic cases to contain the high voltages they put out. Nothing wrong with that but all things being equal, a metal cased coil will last much longer as a plastic and a cool metal case coil can put out as much as 35% more power than it would snuggled up in plastic. Remember. Heat = higher wire resistance = less current flow.
About 28,000-30,000 volts out is the right balance for general applications, as well as racing, as long as the spark gap dosen't get too large. Remember that it is current, not voltage, that ignites fuel. Look for a coil which emphasises its ability to fire through fouling contaminants and recommends a larger plug gap. Stay away from coils that operate with a reduced gap.
There are ways to increase the voltage and current output of your coil but I will (try to) cover this topic with Electronic Ignitions, Spark Amplifiers and Computers, in the near future.
Checking The Coil
The coil should first be checked visualy for any damage. Cracks anywhere on it mean it is useless.
Dirt anywhere on it will attract moisture and conduct power away down to earth making your spark weaker.
The coil primary winding can be checked with an Ohm meter. First disconnect any wires to the coil. Place the meter across the +ve and -ve terminals on the coil. Resistance should only be about one ohm. If resistance is much higher than this or zero then put it in the bin.
The secondary winding is checked much the same way only this time place the meter between the +ve terminal and the connector in the coil HT tower. Resistance should be somewhere between 4,000 and 10,000 ohms on a normal duty coil.
Again, if the value is much above or below these limits, replace the coil.
I don't know what a high performance coil would measure so if you have one maybe you could make a note of what it is.
By MoparKev (Kev) on Friday, March 02, 2001 - 12:15 am:
So, Dave, as a bloke after cheap horses, what coil should I buy to replace a standard coil? I have never seen a coil advertised as "Variable Magnetic, Automatically Controlled or Demand Output." I run side gapped plugs at the mo but have a brand new set of NGKs for the Nats. I hope you run a tech piece on plugs. These tech notes are great as I know quite a bit but clarification of the basics is great.
By David Robson (Dave) on Friday, March 02, 2001 - 11:03 am:
I know the Jacobs coil I use is that type but they didn't invent it so there are others. I will look in my new Summit Catalog tonight to see what is on offer.
By David Robson (Dave) on Friday, March 02, 2001 - 02:52 pm:
I will re-do that coil stuff tonight and see if I can get the image to work. I will also add ways to test the coil.
I will cover the Chrysler Electronic Ignition in a separate piece.
By David Robson (Dave) on Friday, March 02, 2001 - 09:22 pm:
THE IGNITION COIL
The amount of energy supplied to your spark plugs is determined by your distributor and the design of the coil.
A coil is actually made of two coils. A primary and a secondary. The primary and secondary are wrapped one inside the other around a core which can be magnetised. The primary is connected to the +ve and -ve terminals and the secondary to the -ve and the main H.T. lead.
Coils generate their high voltages when the current supplied to the primary winding is suddenly interrupted.
Electricity is generated in the secondary winding by an increasing or decreasing magnetic field in the coil core. Its the rate of change of this magnetic field that determines the level of output. Fast rate of change = big output.
Our teacher at college (before things like health and safety) made a few of us stand in a row holding hands. Each guy at each end held one end of a secondary winding on a large transformer. The teacher put a 1.5 volt AA battery onto the primary. Nothing happened. He then disconnected the battery. The shock threw us all apart and gave us a hell of a jump!
When the points close (battery on) and current builds in the primary coil, the magnetic field of the core slowly increases which generates a small voltage in the secondary winding. However this is not large enough to jump the spark plug gap so no current flows.
When the points open (battery off) the current to the primary is cut off, the magnetic field of the core suddenly collapses which generates a huge voltage in the secondary and across the spark plug gap.
Remember, it is the rate of magnetic field change that determines the output. If the magnetic field rose as fast as it collapses then you would get a spark at the plug but current would be flowing in the opposite direction to normal. I mention this because there are ignitions which work this way and I might cover these at a later date.
The amount of change in the magnetic field also has an effect, i.e. if the primary current has not been there long enough for the field to build to its maximum (length of dwell) then the output will be lower as a result. So the more dwell you can get from your points the better the spark. Up to a point.
It is current (AMPs) that ignites fuel (some might argue with that). Voltage is there only to jump the plug gap. A higher voltage than that which is necessary to jump the plug gap will only reduce the amount of current available. Basic Ohm's law.
For street driven cars the standard coil will, in most cases, be fine. What I have seen in some peoples cars though are "High Performance" coils that are actually high voltage coils and some can produce 50,000 volts or more. A coil that produces such high voltages must also reduce its spark duration and current strength proportionately (Ohm's Law again). At idle and normal RPM levels, your spark plugs need a long duration burn as well as solid spark power to burn off contaminants and ignite the air/fuel mixture. The weak, low current, high voltage coil has too low a current to burn off the contaminants. This results in fouled plugs, misfires and poor mileage. However, as rpms increase then the amount of actual dwell time decreases. This results in a weaker spark at high RPMs with a stock coil. These high voltage coils can help in this situation.
I would leave these high voltage coils for high RPM race cars only. This is what they were really designed for.
Things to look for in a coil
There are coils which are better than your stock units. The best ones are coils which have a variable permeability core. The permeability is the cores ability to magnetise. Basicaly this means that the voltage does not drop off as much as RPM increases. These make the best all round street/strip coils. They are advertised with names like Variable Magnetic, Automatically Controlled or Demand Output.
A metal case filled with fluid is an important feature as the fluid pulls heat off the windings and into the metal case which then conducts the heat away. Some coils have plastic cases to contain the high voltages they put out. Nothing wrong with that but all things being equal, a metal cased coil will last much longer as a plastic and a cool metal case coil can put out as much as 35% more power than it would snuggled up in plastic. Remember. Heat = higher wire resistance = less current flow.
About 28,000-30,000 volts out is the right balance for general applications, as well as racing, as long as the spark gap dosen't get too large. Remember that it is current, not voltage, that ignites fuel. Look for a coil which emphasises its ability to fire through fouling contaminants and recommends a larger plug gap. Stay away from coils that operate with a reduced gap.
There are ways to increase the voltage and current output of your coil but I will (try to) cover this topic with Electronic Ignitions, Spark Amplifiers and Computers, in the near future.
Checking The Coil
The coil should first be checked visualy for any damage. Cracks anywhere on it mean it is useless.
Dirt anywhere on it will attract moisture and conduct power away down to earth making your spark weaker.
The coil primary winding can be checked with an Ohm meter. First disconnect any wires to the coil. Place the meter across the +ve and -ve terminals on the coil. Resistance should only be about one ohm. If resistance is much higher than this or zero then put it in the bin.
The secondary winding is checked much the same way only this time place the meter between the +ve terminal and the connector in the coil HT tower. Resistance should be somewhere between 4,000 and 10,000 ohms on a normal duty coil.
Again, if the value is much above or below these limits, replace the coil.
I don't know what a high performance coil would measure so if you have one maybe you could make a note of what it is.