My car is eating coils

[FrankH]

My 100M has been stranded by three Pertronix Flame-Thrower high voltage coils. One went around 150 miles, one about 50, and one only 15 miles. Has anyone else had problems with these coils? Are they not compatitble with my stock ignition system? Is this a symptom of some other problem? The car now has the 55 year old, original coil and is running great after about 60 miles.

Any ideas? I'm getting insecure about taking the car very far from home.

Thanks,
Frank

 

[Roger]

What do Pertronix say about it? It does seem peculiar!

 

[Keoke]

FrankH


Get the Lucas sport coil and the problem will go away.--Keoke--

 

[57_BN4]

Hi Frank,

Check the primary winding resistance if you still have a functioning coil, it should be equal to or greater than 3 Ohms to work reliably with stock points system. If it is 1.5 Ohms you can run it with a 1.5 Ohm ballast resistor in the supply wire but this doesn't really achieve any improvement over the 3 ohm coil on a low revving four cylinder engine.

The Flame Thrower II and III coils (the ones they call High Voltage) are only 0.6 and 0.32 Ohms which requires electronic dwell control as they will draw 20~40 Amps if connected straight to a battery. Check the label on the coil, if it says Flame Thrower II or III then they must be used with an electronic dwell controlled distributor or ignitor.

If you have been running a coil with less than 3 Ohms, change the points as they will have had too much current through them and may strand you again some time.

Andy.

 

[andrea]

Hi Andy
I see that you are very prepared on this argument-

I came to see around for a good 40000 volts coils
to joint to my 123 distributor on my AH100/6 BN4
actually I have the Standard LUCAS Coil
any suggestion?

Same problems with my MG TD I should change the stock
points with Petronix electronic dwell control also here
I have a Standard Lucas Coil
Cheers
Andrea

 

[Keoke]

Hi Andy
I see that you are very prepared on this argument-

I came to see around for a good 40000 volts coils
to joint to my 123 distributor on my AH100/6 BN4
actually I have the Standard LUCAS Coil
any suggestion?

Yes get a lucas sport coil and open your points up to 0.035 or 0.040.--Keoke

Same problems with my MG TD I should change the stock
points with Petronix electronic dwell control also here
I have a Standard Lucas Coil
Cheers
Andrea

 

[RAC68]

Hi Frank,

Over the past 5 years of installation, the 40K Pertronix coils installed on my TR7 (with Pertronix ignition) and Healey (with Crane CD ignition) have caused no issues. As Andy mentioned, for coils to perform properly, either they are equipped with an internal resistor or require the installation of an external ballast resistor. In my case, the Healey coil has the appropriate level of internal resistance. Since your original 25K Lucas coil did not require a ballast resister, I would expect the Healey-specified Pertronix coil would also be internally ballasted.

As Andy also indicated, high performance coils are often non-ballasted and approach the line of incompatibility with original ’50s/’60s ignition technology. Although many feel they are getting greater engine output when adding a substantially hotter coil, in most situation I have observed, the improvements are marginal at best and open the gates to rotor and cap burn issues. I would double check you are using the correct coil specified by Pertronix and then follow all ignition connections to make sure all is connected and secure. I would also recommend you call or e-mail Pertronix and present them with the situation along with a returned faulty coil for their investigation.

Good luck,
Ray (64BJ8P1)

 

[BLU_HLY]

Frank,
Apparently the Pertronix coils do have a probem. I installed a Pertronix coil specified for the 100 with 3 ohms resistance.
After less than 1000 mi. it died. This happened while driving thru the Navajo Nation with no cell service. Rescued by a worker in the oil fields! No receipt, no warranty was their response and the coil was dead when they checked it.
Went with the Lucas Sport coil - no problems for more than 12000 mi.
CGH

 

[Keoke]

Frank,
Apparently the Pertronix coils do have a probem.

----------------------------YEP!!!




I installed a Pertronix coil specified for the 100 with 3 ohms resistance.
After less than 1000 mi. it died. This happened while driving thru the Navajo Nation with no cell service. Rescued by a worker in the oil fields! No receipt, no warranty was their response and the coil was dead when they checked it.

Went with the Lucas Sport coil - no problems for more than 12000 mi.----------------- :thumbsup:
CGH

------------------------Keoke

 

[Donny_L]

Dumb question I'm sue but what is the benefits of upgrading to the Lucas sport coil anyway ?

 

[elrey]

Simply put; surer spark, better burn. But for a guy on a budget, if your coil is still working, you might wish to spend your funds elsewhere for now. --elrey

 

[57_BN4]

Probably the difference between a 'sports' coil and a regular one is the sticker is different... Or perhaps the internal design is more robust, I don't really know. If the resistance is the same then the HV output is almost certainly much the same.

The problem is that most engines can outrev a conventional coil. The coil takes time to charge from the battery voltage and some time to discharge into the spark plug. Typical charging time would be 5ms and discharge 1ms for a conventional coil, which is equal to about 3300rpm on a six cylinder. At higher rpm the coil cannot charge fully and performance starts to drop off gradually. This is why CDI is a good upgrade, it maintains full spark power way past the redline, like up to 10000 or something because it charges a capacitor instead of a coil.

The sole purpose of the low resistance (technically impedance) coil is to speed up the charging time to allow higher rpm. Halving the resistance halves the charging time and hence doubles peak output rpm, but also the current draw doubles. This is where the 1.5 ohm coil has to exist with a 1.5 ohm resistor to limit the current or the points/module will burn out.

Later model engines need more capability than a fixed dwell (on vs off time) system can provide and so the coil charging time on >1990s engines is controlled electronically to about 2.5ms or even less for those ultra low resistance coils. This is why the basic Pertronix Ignitor or any "points replacement" electronic ignition must still have a total of about 3 ohms resistance to work unless it states otherwise. The more complex Pertronix II and III ignitors have the smarts inside to handle the fancy low impedance coils but unfortunately I think they only make the basic Ignitor to fit the Healey. You'd need to upgrade the wiring too with a relay as the key switch and wiring is average with the standard system.

I have fitted a Ignitor II into a 25D4 distributor but the issue is that it is quite a bit taller than the Ignitor and requires the base plate lowering by about 4mm and the rotor grinding away underneath to clear. Not the happiest setup, would be better to use an external unit from another maker.

My choice for a bog standard engine is Pertronix Ignitor and new standard/sports coil. The electronic switch gives a considerably better spark simply because it cuts off instantly compared to the points which must physically open on the cam and is therefore much slower = less spark energy [edit- actually it = less firing voltage].

For a race car it would be an external dwell controlled module or ECU and matched coil, leads, plugs, wiring loom, fuses etc or a CDI setup.

Andy.

 

[Bob_Spidell]

Some of this does not agree with my understanding of spark ignition systems (someone here will correct me if I'm wrong):

- capacitors will not change voltage--you still need a transformer (coil) somewhere in the ignition system. Modern cars have one coil per cylinder, whose primary voltage (12V) is controlled electronically instead of with points. Capacitors can only store energy, in a CDI the stored energy is probably used to provide a 'sharper' spark with less rise time and, possibly, multiple sparks (I'm speculating here; don't know that much about CDIs)

- re: "fixed dwell (on vs off time)" - dwell is fixed (and measured in camshaft degrees), but the time varies with engine speed. At slow RPMs dwell (points closed) time is longer than at high RPMs. But, coil saturation time remains the same and that is why spark can drop off at higher RPMs (usually, above 6,000RPM, not a factor for most of our Healeys). That's why your coil will heat up if you sit at idle a long time. Electronic systems can maintain--or tweak--the time constant (TC) as required.

- resistance does not change the coil saturation (charging) time significantly (we're probably talking nanoseconds). Some coils have less internal resistance because they are meant to be used with some sort of external resistance (ballast resistor or wire, or Pertronix?)

- if you put a 1.5ohm resistor in series with a 1.5ohm coil you will have the same resistance as a 3.0ohm coil; hence same 'charging' time

I've never heard of coil resistance being used to modulate the charging time, but I suppose it's conceivable.

I have a stock (original, 165K miles) coil with a Pertronix. I put the Pertronix in so I wouldn't have to mess with points, but was pleasantly surprised when the engine seemed to start easier and run a bit smoother across-the-board.

 

[Keoke]

have a stock (original, 165K miles) coil with a Pertronix. I put the Pertronix in so I wouldn't have to mess with points, but was pleasantly surprised when the engine seemed to start easier and run a bit smoother across-the-board.

Yep BOB, and that is because the high voltage output even from your "standard coil" is higher than was produced with points. This is because the time rate of change of current through the coil's primary is higher due to the faster switching time of the electronic system.---Keoke

 

[57_BN4]

Hi Bob,

The capacitor in a CDI system is charged to 400V by an internal DC-DC inverter. The coil in a CDI is actually a transformer which steps up the 400V to many kV by the turns ratio. There is virtually no firing time for a CDI, it all happens in one big whack as the capacitor dumps into the transformer whereas a conventional coil will spark for up to 120 crank degrees at high rpm so much of the spark energy is not utilised.

Both capacitors and inductors/coils are AC devices, we just use the term resistance to rate coils because it is easy to measure. a 1.5 ohm resistor and a 1.5 ohm coil are the same thing in DC terms but the coil behaves very differently to the resistor in AC terms, hence the correct term 'impedance' which is the measure of AC resistance.

Dwell is a percentage so the relative amount of on vs off doesn't change but the actual times obviously do as you mention. I probably should have said "on vs off percentage".

Resistance does change the charging time (about 5ms) because it affects impedance which is actually what controls the charging time. Coils have less primary winding impedance to get over the shortcomings of conventional coils- they don't rev very high because the charging time becomes longer than the time-to-next-spark.

The increase in starting and running performance you get from a Pertronix is because the dV/dT in greatly improved (how quickly the current turns off) and this directly controls peak firing voltage. This alone allows bigger plug gaps as the higher voltage can jump the bigger gap. Once the spark is initiated the voltage falls back for the 1ms 'burn' just as with points. As recommended by Pertronix, it is good to replace high mileage coils as the ignitor unit makes them work harder to produce the better spark and this will find any weakness inside, although given the reason this thread started I'm not entirely convinced any more. Perhaps fitting the new coil to the boot is a better strategy.

Andy.

 

[Keoke]

The sole purpose of the low resistance (technically impedance).


NO Andy
Resistance and Impedance are two different quantities For example:

.A coils design will always have a fixed DC resistance of"X" Ohms but its impedance is a variable quantity that changes with the applied frequency.

1] Resistance is basically a DC component.

2] Impedance is an AC component .

Both are expressed in terms of OHMS but Impedance is a frequency dependant quantity.

------------------------------Keoke

 

[Bob_Spidell]

Thanks Andy, that broadens my understanding. You are, of course, correct about the effect of resistance in the TC (hadn't thought about it that way before). AFAIK, DC-DC converters use an internal coil as well (I don't know of any other way to raise DC voltage).

Another point is that some coils--notably the stock Healey coils--have the primary and secondary windings wired in series. That way, the primary windings are 'added' to the secondary windings when the field collapses and greater output is produced. If you have the coil wired backwards you lose something like 5-10% of your spark energy.

My dad--old-time mechanic--has always maintained that if you have enough spark to ignite the mixture that's enough (the flame front takes care of the rest). I'm also not convinced a 'stronger' spark--like the gimmick plug companies (Splitfire, Diamondfire, etc.) advertise--adds much benefit under normal conditions in an otherwise well-tuned car. A stronger, 'crisper' spark helps at high RPM, with leaner mixtures and also if the secondary circuit is marginal for some reason.

Another thing occurred to me (again, speculation). The original Ignitor supposedly could burn out if the key was left on and the unit happened to be in the 'fire' (forward biased) state (you can burn points this way, too). Maybe Pertronix added some resistance to their coils to keep from having a dead short if the key was left on and the Ignitor was 'on.'

 

[Keoke]

The increase in starting and running performance you get from a Pertronix is because the dV/dT

Given E is a constant voltage did you mean di/dt

 

[Patrick67BJ8]

I bought a "coil tester" in the '70s and I'm sure it was a few years old at that time. There's no name on it and only a serial number on the backside. Does anyone have an idea of who the manufacturer is? I would be intersted in obtaining an instruction sheet for it. I'd post a photo of it but like so many people on the forum, we can't figure out how to do it.
Thanks,
Patrick

 

[Bob_Spidell]

Sun made coil and distributor testers:

https://www.thegaragegazette.com/index.php?topic=347.0