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Why are coils 3 ohms?

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pinkyponk Avatar
pinkyponk Gold Member Adrian Page
Berwick, Nova Scotia, Canada   CAN
Why not 6 or 50? What's the dealio?

Adrian



Home built Eaton M62 Supercharger with 9psi boost, "stock" high ratio rocker arms, 8:1 compression, Piper 270 cam, ported head, matched manifolds, CB Performance computerized ignition.



Edited 1 time(s). Last edit at 2017-05-14 07:40 PM by pinkyponk.

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Fred Winterburn Avatar
Ripley, Ontario, Canada   CAN
For points ignitions if the current exceeds 4 amps through the point contacts, they will burn exponentially faster. 3 ohms limits the current so that it is only briefly higher than 4 amps at low rpm. It also keeps the coil from cooking itself to death when the points are closed continuously such as with the ignition on and the engine not running and the points just happening to be closed. With points, the primary side resistance should be a minimum of 3 ohms which includes any separate ballast resistor for a lower ohms coil. The two should add up to a minimum of 3 ohms. Exceeding 3.5 ohms starts to reduce spark energy more than it should. Fred

In reply to # 3511515 by pinkyponk Why not 6 or 50? What's the dealio?

Adrian



'Anyone who likes liver, can't taste it'

benhutcherson Avatar
benhutcherson Gold Member Ben Hutcherson
Louisville/Frankfort, Kentucky, USA   USA
1970 MG MGB
In reply to # 3511605 by Fred Winterburn For points ignitions if the current exceeds 4 amps through the point contacts, they will burn exponentially faster. 3 ohms limits the current so that it is only briefly higher than 4 amps at low rpm.

To add to that, most of the modern high energy computerized systems I've worked with have very low primary resistance.

The last one I changed was on a '97 Geo Prizm(Toyota Corolla), which had a single coil and a distributor but used the ECM/CPS to trigger the coil. My friend reported that the car would crank but not start. When I went digging(Toyota wants you to replace the whole distributor, but my friend didn't want spend $250 on a car worth about that much) I found the coil open. The replacement checked out at around .75Ω, and the car is still more or less running with that coil.

I think the new COPs I've installed have been in that same ballpark.

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rocannon Avatar
rocannon Platinum Member Frank L
Inconveniently located in the wilderness, Oklahoma, USA   USA
1967 MG MGB GT "GT From Hell"
1980 MG MGB "Restored By Photoshop Inc."
The stock CEI coil was in the 0.8 ohm range.

In reply to # 3511609 by benhutcherson
In reply to # 3511605 by Fred Winterburn For points ignitions if the current exceeds 4 amps through the point contacts, they will burn exponentially faster. 3 ohms limits the current so that it is only briefly higher than 4 amps at low rpm.

To add to that, most of the modern high energy computerized systems I've worked with have very low primary resistance.

The last one I changed was on a '97 Geo Prizm(Toyota Corolla), which had a single coil and a distributor but used the ECM/CPS to trigger the coil. My friend reported that the car would crank but not start. When I went digging(Toyota wants you to replace the whole distributor, but my friend didn't want spend $250 on a car worth about that much) I found the coil open. The replacement checked out at around .75Ω, and the car is still more or less running with that coil.

I think the new COPs I've installed have been in that same ballpark.



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benhutcherson Avatar
benhutcherson Gold Member Ben Hutcherson
Louisville/Frankfort, Kentucky, USA   USA
1970 MG MGB
In reply to # 3511615 by rocannon The stock CEI coil was in the 0.8 ohm range.

I thought the Lucas CEI system on MGBs used a 1.5Ω coil that-in normal running-was supplied power via a 1.5Ω resistance wire.

course2kid Jeffrey Johnson
Fountain Valley, CA, USA   USA
1979 MG MGB "Lucy (Lucifer)"
Deleted post as it was accidentally posted before complete (fat-finger typing error).



Edited 1 time(s). Last edit at 2017-05-14 11:02 PM by course2kid.

course2kid Jeffrey Johnson
Fountain Valley, CA, USA   USA
1979 MG MGB "Lucy (Lucifer)"
EDIT: i had to make some corrections per Fred's comment that followed my original post.

A coil designer is probably needed to get a real-world practical answer, but, I can attempt an academic answer...

A coil is basically a transformer. Two coils, primary and secondary wrapped around a common magnetic path so that they are both coupled to the same magnetic field. The primary will have both resistance and inductance and will have a time constant of L/R. This time constant determines how quickly current builds up in the coil to it's maximum value of V/R. The magnetic field strength is generally proportional to the current in the primary (unless the soft magnetic materials get saturated, but, I wouldn't think the designers would push the flux that high simply because doing so makes the design process more challenging).

When the current path of the primary is interrupted by the points, the magnetic field collapses and induces a voltage on the secondary coil. That voltage is dependent on the number of turns on that coil and the rate of change in the magnetic field strength. The output voltage on the secondary must be plenty high enough to jump the gap between the rotor and distributer cap (at atmospheric pressure) and must also be high enough to jump the spark plug gap (at 150 psi or whatever compression your engine achieves). Also, the voltage cannot be so high that it can cause sparks along unintended paths (e.g. from the plug electrode to the top of the piston or from the rotor to the distributor body).

Additionally, the total energy stored in the magnetic field must be designed such that the energy in the spark at the plug is sufficient to properly ignite the A/F mixture. The ratio of turns between secondary and primary determines the peak secondary coil voltage and, I think that the total spark energy is related to the time constant of the secondary (L/R), the spark gap in the plug, the spark gap from rotor to cap, and the length of time that the current to the primary is interrupted.

Once the engine designers have defined how much spark energy they need for proper ignition of the A/F mixture at the designed compression and the engine rpm range (which determines the time period of a charge/discharge cycle), the distributor designers have to consider practicalities like the arc lengths of the rotor and cap, the range of desired vacuum advance, mechanical tolerances, and the number of cylinders to determine the actual minimum charge discharge period.

The coil designers would use that information along with the spark voltage requirements to determine what (L/R) ratios they need for the primary and secondary coils. Then they'd consider the currents they need to carry in each coil along with heat transfer considerations as well as size, weight, and packaging constraints to select appropriate wire gages.

Sorry, this explanation is getting pretty long and boring without much practical insight, so, I'll do my best at that ... If we tried to design a 6 ohm in place of a 3 ohm coil, we could either choose to use an equal length wire with half the cross section of the original wire or a double length wire of the same cross section (or other combination of wire length and cross section that yields 6 ohms resistance).

If we did the first, the time constant would be half that of the original coil because it would have the same number of turns, but twice the resistance (L stays the same, R doubles, and L/R is halved). In addition, the coil size and weight would be halved. On the downside, the steady state current V/R would be halved, so, the fully-charged energy stored in the magnetic field would be 1/4 of the original coil (magnetic energy is 1/2 L i squared) and the secondary coil might not be able to pull enough energy from this weakened field to provide adequate spark energy.

If we tried the second way (twice as many coils of the same gage wire), L would be quadrupled (L is proportional to number of turns squared), R would also be doubled, so (L/R) would be double that of the original coil. Unfortunately, the size and weight of the primary coil would be doubled. Since R is doubled the steady state current is halved, but, the magnetic field energy remains the same since there are twice as many turns on the primary coil (energy is 1/2 L i*i where L has been quadrupled, but i has been halved, so energy stays the same).

Since R is doubled and current is halved, and heat generation (i squared x R) is 1/2 that of the 3 ohm coil. So, this would probably work, but the coil would be larger, heavier, and more costly to produce).



Edited 2 time(s). Last edit at 2017-05-15 09:29 PM by course2kid.

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glbishop Avatar
glbishop Gary Bishop
Spring Hill, FL, USA   USA
In reply to # 3511625 by benhutcherson
In reply to # 3511615 by rocannon The stock CEI coil was in the 0.8 ohm range.

I thought the Lucas CEI system on MGBs used a 1.5Ω coil that-in normal running-was supplied power via a 1.5Ω resistance wire.

Lucas did use a 1.5 ohm coil and resistor wire to 'externally' limit current for the Opus system. The CEI replacement Lucas box has the GM 4 pin module inside. It has internal current limiting capability and was designed to be used with coils as low as .6 ohm. No external current limiting devices are needed however considerable heat is generated when the module is doing all the limiting. I've run it that way without any problem but I did use a big heat sync on top of the Lucas box.

Right now I'm using a 1.5 ohm std coil and an external ballast resistor to prevent the module's current limiting circuit from activating. Much less heat for the module to deal with and the ignition works just as well.

Fred Winterburn Avatar
Ripley, Ontario, Canada   CAN
Actually, if you halve the current, you will make the energy storage 1/4 of what it was with double the current, since the energy stored in an inductor is proportional to the current squared times the inductance. I'll bet Charles Kettering experimented long enough to find out what worked and what didn't. Fred

In reply to # 3511647 by course2kid A coil designer is probably needed to get a real-world practical answer, but, I can attempt an academic answer...

A coil is basically a transformer. Two coils, primary and secondary wrapped around a common magnetic path so that they are both coupled to the same magnetic field. The primary will have both resistance and inductance and will have a time constant of L/R. This time constant determines how quickly current builds up in the coil to it's maximum value of V/R. The magnetic field strength is generally proportional to the current in the primary (unless the soft magnetic materials get saturated, but, I wouldn't think the designers would push the flux that high simply because doing so makes the design process more challenging).

When the current path of the primary is interrupted by the points, the magnetic field collapses and induces a voltage on the secondary coil. That voltage is dependent on the number of turns on that coil and the rate of change in the magnetic field strength. The output voltage on the secondary must be plenty high enough to jump the gap between the rotor and distributer cap (at atmospheric pressure) and must also be high enough to jump the spark plug gap (at 150 psi or whatever compression your engine achieves). Also, the voltage cannot be so high that it can cause sparks along unintended paths (e.g. from the plug electrode to the top of the piston or from the rotor to the distributor body).

Additionally, the total energy stored in the magnetic field must be designed such that the energy in the spark at the plug is sufficient to properly ignite the A/F mixture. The ratio of turns between secondary and primary determines the peak secondary coil voltage and, I think that the total spark energy is related to the time constant of the secondary (L/R), the spark gap in the plug, the spark gap from rotor to cap, and the length of time that the current to the primary is interrupted.

Once the engine designers have defined how much spark energy they need for proper ignition of the A/F mixture at the designed compression and the engine rpm range (which determines the time period of a charge/discharge cycle), the distributor designers have to consider practicalities like the arc lengths of the rotor and cap, the range of desired vacuum advance, mechanical tolerances, and the number of cylinders to determine the actual minimum charge discharge period.

The coil designers would use that information along with the spark voltage requirements to determine what (L/R) ratios they need for the primary and secondary coils. Then they'd consider the currents they need to carry in each coil along with heat transfer considerations as well as size, weight, and packaging constraints to select appropriate wire gages.

Sorry, this explanation is getting pretty long and boring without much practical insight, so, I'll do my best at that ... If we tried to design a 6 ohm in place of a 3 ohm coil, we could either choose to use an equal length wire with half the cross section of the original wire or a double length wire of the same cross section (or other combination of wire length and cross section that yields 6 ohms resistance).

If we did the first, the time constant would be half that of the original coil because it would have the same number of turns, but twice the resistance (L stays the same, R doubles, and L/R is halved). In addition, the coil size and weight would be halved. On the downside, the steady state current V/R would also be halved, so, the fully-charged energy stored in the magnetic field would also be halved and the secondary coil might not be able to pull enough energy from this weakened field to provide adequate spark energy.

If we tried the second way (twice as many coils of the same gage wire), L would be doubled, R would also be doubled, so (L/R) would remain unchanged. Unfortunately, the size and weight of the primary coil would be doubled. Also, snce R is doubled the steady state current is halved, but, the magnetic field energy remains the same since there are twice as many turns on the primary coil. Since R is doubled and current is halved, and heat generation (i squared x R) is 1/2 that of the 3 ohm coil. So, this would probably work, but the coil would be larger, heavier, and more costly to produce).



'Anyone who likes liver, can't taste it'

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Steve S. Stephen Strange
Harrisonburg, Virginia, USA   USA
1972 MG MGB MkII "The Mouse Trap"
Jeffrey-
Nicely put!

ingoldsb Avatar
ingoldsb Silver Member Terry Ingoldsby
Calgary, Alberta, Canada   CAN
1971 MG MGB
And, if you don't believe Fred, try running a ballasted coil setup without a ballast resistor - it doesn't take long for the points to burn. A common failure in the era of points-based ignition systems was for the ballast resistor to burn out. It would drive people crazy - the engine would start while they were cranking (the ballast resistor is bypassed) and die as soon as they released the key. The quick cure to get home was to put a piece of wire across the ballast resistor - which doubled the current through the points (and coil). However, you couldn't go far like that or you would fry the points (and possibly the coil).

I suppose the question might be, why didn't they manufacturers go to heftier points (or dual points)? I suspect it was a question of optimization. The 3 to 4 amp current provided enough energy to do the job and the benefits of higher current were small compared to the cost of a larger and more expensive breaker point system.

Of course, when semiconductors came into the picture, lower resistance (higher energy) coils became possible. Tighter emission requirements were also a driver to ensure that the cylinder fired on every turn (er, 2nd turn) of the crank.



Terry Ingoldsby
terry.ingoldsby@DCExperts.com

29desoto Avatar
29desoto Gold Member Wayne Sanders
Outside Otis, OR - in the woods!, USA   USA
1953 MG TD
1965 MG MGB V6 Conversion "Carmine"
1977 MG MGB "Ole Yeller"
1979 MG MGB "Kermit"    & more
OK, let's assume for a moment we are talking about a late model "B".

Fred is saying to measure the overall resistance of the circuit up to and through the coil. I'm thinking I may be fighting the same thing as some of the current discussions going on, so I'm curious.

Where would I measure from/to to measure overall resistance of this circuit? Can someone give me some wire connections and colors to research?

TIA as always.



Wayne Sanders
Rose Lodge, OR
(TD 4288)

"I don't care to belong to any club that accepts people like me"-joined Willamette Valley Club in 2011

77 MGB - Four tone paint -Pertronix - hood props - cheap tires - new springs all around - mixture of poly and rubber bushings- fuzzy seatcovers - stereo out of old F150 pickup -runs flawlessly and drives nice.

78 MGB - Done right by a good PO, and I only bought it. Tell Jim how nice it is. But I did put in a Datsun 5 speed.

53 TD Mark II - a work in progress.

79/65 MGB - Carmine- V-6 - T-5
This car is now very nearly completely done. Sure to find something else, but not now.........

pinkyponk Avatar
pinkyponk Gold Member Adrian Page
Berwick, Nova Scotia, Canada   CAN
In reply to # 3511605 by Fred Winterburn For points ignitions if the current exceeds 4 amps through the point contacts, they will burn exponentially faster. 3 ohms limits the current so that it is only briefly higher than 4 amps at low rpm. It also keeps the coil from cooking itself to death when the points are closed continuously such as with the ignition on and the engine not running and the points just happening to be closed. With points, the primary side resistance should be a minimum of 3 ohms which includes any separate ballast resistor for a lower ohms coil. The two should add up to a minimum of 3 ohms. Exceeding 3.5 ohms starts to reduce spark energy more than it should. Fred

In reply to # 3511515 by pinkyponk Why not 6 or 50? What's the dealio?

Adrian

Thanks Fred. That's what I figured.

Adrian



Home built Eaton M62 Supercharger with 9psi boost, "stock" high ratio rocker arms, 8:1 compression, Piper 270 cam, ported head, matched manifolds, CB Performance computerized ignition.

tvrgeek Avatar
tvrgeek Silver Member Scott S
Bowie, Maryland, USA   USA
1965 MG MGB
1974 MG MGB GT ~ For Sale ! ~
CEI used the GM HEI transistor module, designed for 1.5 Ohms 4 cylinder, or .75 on V8s. No points to burn. Much better ignition. If the distributor had a decent curve, it would be an excellent system.

Points are limited to the 4 Amps. So, 3 Ohm coil ( or with the brilliant 1.5 Ohm/ballast resistor for starting boost) If we had not invented electronic ignitions, I would be wondering if iridium contacts would let us run 2 Ohms or less with a conventional system. Maybe some of the other tricks like bifurcated contacts.

As clever as the start boost system is, it does provide considerably less energy running as the primary inductance is a lot less. I would not run it on a 4-cylinder. Of course, I prefer high energy electronic systems anyway.



Cogito ergo sum periculoso

Steve S. Stephen Strange
Harrisonburg, Virginia, USA   USA
1972 MG MGB MkII "The Mouse Trap"
Scott-
Although I'm not a specialist in ignitions systems as you are, I do know that that while unballasted ignition coils have the virtue of rapid charging times, they also release their stored energy faster, producing an ignition spark of shorter duration. For combustion to be triggered efficiently within the combustion chamber, the pressure wave that is produced by the squish (quench) of the fuel-air charge between the piston and the edges of the combustion chamber must arrive in the vicinity of the spark plug either before or as the spark plug fires. However, due to the shorter-duration ignition spark produced by an unballasted ignition coil, the accuracy of the ignition timing is of more critical importance. 6 Volt Ballasted ignition coils produce a longer-duration ignition spark, therefore this is rarely a problem and the engine is less sensitive to small errors of ignition timing. Another advantageous factor of the 6 Volt ignition coil is that, having half the number of primary windings as a 12 Volt ignition coil, it also has half of the inductance of a 12 Volt ignition coil, but the condenser (capacitor) value is the same for both. This lower inductance that is inherent to the 6 Volt ignition coil means that it recharges more quickly when the contact breaker points close again (inductance in a component has the effect of causing the electrical current to build more slowly than in a pure resistance), therefore it can be used at higher engine speeds without a loss of High Tension (HT) output. So, why do you dislike that ballasted system? If it's energy that you like (and who doesn't on a cold winter's morning?), you can always install a Lucas Sport Coil for the ballasted system and get 40K Volts, no problem. This is what I run, and my compression ratio is 10:1. That's kinda high for today's 93-Octane pump fuels. I do, of course, have to pay closer attention to the ignition timing and the carburetion, but I have no ignition shortcomings.



Edited 1 time(s). Last edit at 2017-05-15 07:59 PM by Steve S..

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