12 replies to this topic

[croc7]

I could use some help with some phraseology regarding ignition coils.  I think that I heard AC Dodd refer to a 'hi power' coil on one of his tuning videos, usually a 0.8  ohm coil and why its called that?  Does the each coil differ in it's discharge voltage?  What I don't know is the applicability and why there is a choice of .8, 1.5, and 3.0 ohm coils.  'The .8 ohm coil works with the 65D but increase the plug gap to .035 , a 1.5 will work with a 65D but the plug gap should be .028, the 3.0 ohm coil is fine with a point/condenser system but reduce the plug gap to .025...... and then there is the ballast wire thrown in to add a little variety to the mix.

Thanks

[Ethel]

I can have a go..

 

With points distributors you need around 3 ohms total resistance or the points tend to melt because of the amount of current they are fed. With all ignition, the voltage of the secondary winding increases until it's high enough to jump the plug gap. The coil won't affect that, but different coils will be able to keep producing the voltage needed longer than others as the revs rise & the dwell period reduces. Resistance  is the potential to produce heat, so a coil with lower internal resistance will produce less of it & suffer less increase in resistance, thus heat and so on.

 

Our A Series only have 4 plugs & aren't that revvy. So the ignition is less demanding than some supercar with 3 times the cylinders & maybe twice the rpm.

[Spider]

Hey mate :)

While it might be tempting to go for something that's not far off lightning, I would caution against that. Something in the 'medium' power class is all that's needed for easy cold starting and smooth running. Going to a higher voltage coil than what's needed is unhelpful I've found. The caps on these are on the small side and the rotor buttons don't have a lot of insulation in them, so tracking and even puncturing the insulation is fairly easy to do.

The resistance of the 12V winding comes down to a few factors and isn't the only factor that determines their output. Generally, the lower the resistance here increases usually increases their output voltage, but as Ethel mentioned, it also makes them run hotter and also puts more load on the rest of the ignition system LT components and the wiring in the car, including the ignition switch.

 

Off hand, the coil I have been running faithfully for a very long time is about 28 kV and 3.0 ohms. It's a Bosch GT40 but sadly, no longer available.

[68+86auto]

If you stick to the factory configuration then you know it will work. If they needed higher output then the factory would've done it. They don't need higher output and it actually causes issue like already mentioned. When they did fit high energy systems they had special rotors and an insulating shield.

Once you change from the factory configuration then the required plug gap is a guess without test equipment.


Consider the ballast ignition system a 3ohm system, that's basically how it performs except for starting. The resistor does do more than just "increasing" spark while cranking. It reduces the coil current when sitting at idle as the resistor heats up.




A lower resistance coil generally provides higher maximum output voltage. Why does the firing voltage matter you ask? The short version is that it makes little difference on a mini.

On a typical old engine (like a mini), at idle the ignition system needs to produce around 7kV (7000v) to jump the gap and ignite the mixture. Under open throttle conditions that voltage requirements is increased to lets say 15-20kV as the mixture is initially leaner. I was going to write a lesson on plug firing voltage but it is easier to just copy a book.

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Edited by 68+86auto, 22 April 2024 - 02:44 AM.

[timmy850]

There’s a few nuances to working out the coil power, as things like the dwell time and the effect of the resistor can change with rpm

A few examples using round numbers
0.8ohm coil at 12V draws 15amps
1.5ohm coil at 12V draws 8amps
1.5ohm coil at 6V draws 4amps
3.0ohm coil at 12V draws 4amps
The more amps, the higher the coil and spark energy (assuming the remainder of the system stays the same). On a 65D a 0.8ohm coil would theoretically make 3.75 times more power than a 3.0ohm.

A 3.0ohm coil will work fine on a conventional points system as they use 50-60 degrees of dwell and this will charge up the coil sufficiently for combustion. However if you use a 3.0ohm coil on a 65D, at idle the module commands 20 degrees of dwell. 20 degrees is sufficient for a higher power coil, but would make a weak spark with a 3.0ohm coil

It’s best to have the whole system running as a matched system to the module or points in the distributor

[68+86auto]

However if you use a 3.0ohm coil on a 65D, at idle the module commands 20 degrees of dwell. 20 degrees is sufficient for a higher power coil, but would make a weak spark with a 3.0ohm coil

Is the 65d really that crude? Most modules will increase the dwell as required.

[ACDodd]

No the 65d is not that crude, again an example of people not understanding how things work by taking what people
Say too literally. The 65D is a variable dwell unit, 20 at idle and around 50 @ 5000rpm.

Factory fitted a high power system in 1984 to 1275 metros via the 65d dizzy, and kept the same power system till 1996 including carb 1275 minis and SPI minis.
The factory fitted this system to ensure they could light off a lean cruise mixture. They also never suffered with reliability problems, and it was much better than the contact breaker systems it replaced.

I use it because it gives me the same advantage, higher compression engines need more spark power than lower compression engines for the same lean air fuel ratio, there for later factory engines had to deal with both high compression
And lean cruise fueling. If you fit anything else you just won’t be able to tune the engine for lean cruise unless you use high power ignition. In reality for road driving, there is no place for a 3 ohm coil, when a 0.8 version will allow more efficient running. Of course just changing the coil and then not tuning th engine properly won’t make any difference. You have to be able to tune the engine properly, which alas is some that 95%of forum users do not know how to achieve.

Ac

Edited by ACDodd, 22 April 2024 - 07:40 AM.

[timmy850]

 

However if you use a 3.0ohm coil on a 65D, at idle the module commands 20 degrees of dwell. 20 degrees is sufficient for a higher power coil, but would make a weak spark with a 3.0ohm coil

Is the 65d really that crude? Most modules will increase the dwell as required.

 

Of course they will increase dwell vs rpm. However I don't know if the 65D has a load sensing capability to service different ohm coils (like some aftermarket ones do). Seeing as it's a matched system from the factory it could quite well be set up only for a 0.8ohm coil??

 

But if you set it up as per factory it should all work fine

[Ethel]

 

However if you use a 3.0ohm coil on a 65D, at idle the module commands 20 degrees of dwell. 20 degrees is sufficient for a higher power coil, but would make a weak spark with a 3.0ohm coil

Is the 65d really that crude? Most modules will increase the dwell as required.

 

 I don't think Timmy said that. To me he was just illustrating his point that the ignition is the sum of its parts & you can't just swap coils - a high energy 3 ohm coil won't be high energy on a system designed for 0.8 ohms & under half the dwell.

[Spider]

The amount of Dwell any particular coil needs is dependent of the coil construction and in particular the magnetic properties of the magnetic core and how quick that charges (magnetically) and saturates. This becomes a double edge sword though as we'd like a core that charges (and hopefully saturates) quickly, conversely, such a core also discharges quickly, which would make the spark period or time that the actual spark last for short too.

 

I mentioned in my post above a 'medium class' voltage coil. These are needed with unleaded fuels as these have a greater tendency to foul spark plugs and something is needed with a bit more punch than the earlier ignition systems had to burn through any fouling as it starts. They are also needed for leaner mixtures that are demanded by modern standards. For the more mature generation among us, you'll recall it was around the time of unleaded fuels starting to appear on the market that car makers were fitting electronic ignition systems of one type or another. I recall in particular Chrysler making a big marketing deal out of theirs. Ultimately here too, while I've spoken against having too high a output voltage from any particular coil, in realty, this is in fact limited by the spark gap. The bigger the gap, the bigger the voltage needed to jump that gap and also the higher the voltage that is actually made in the system before being limited by that gap. Pretty much, once a spark is initiated, the voltage in the HT system won't rise much over that initiation voltage.

One huge advantage electronic systems have over points is that as they switch, they are either ON or OFF, clean and sharp. This alone makes for a fatter and better spark over points or any other mechanical switch as these simply can't switch as clean and as sharply as an electronic switch can.

[68+86auto]

 

 

However if you use a 3.0ohm coil on a 65D, at idle the module commands 20 degrees of dwell. 20 degrees is sufficient for a higher power coil, but would make a weak spark with a 3.0ohm coil

Is the 65d really that crude? Most modules will increase the dwell as required.

 

Of course they will increase dwell vs rpm. However I don't know if the 65D has a load sensing capability to service different ohm coils (like some aftermarket ones do). Seeing as it's a matched system from the factory it could quite well be set up only for a 0.8ohm coil??

 

But if you set it up as per factory it should all work fine

 

 

I expect that a 65d would adjust the dwell for different coils, I haven't tested it though. I have one on a car but the engine is currently apart.

I would guess that any ignition module designed for low resistance coils like 0.8ohm would have a current limiter and vary the dwell.

[Ethel]

Points are mechanical, isn't it just that?

 

... They give two timing events, based on the angular rotation of the cam, from the points closing to reopening. That's why we state dwell in degrees. An electronic trigger effectively gives one event per spark so, unless it can sense the coil discharging via its connection to the primary, must work off elapsed time from the trigger event for dwell. It will be constant by time, but variable by angle - an advantage for producing constant sparks, provided the time interval stays within the bounds of the corresponding angular interval between the coil firing subsequent cylinders.

[68+86auto]

Points are mechanical, isn't it just that?

 

... They give two timing events, based on the angular rotation of the cam, from the points closing to reopening. That's why we state dwell in degrees. An electronic trigger effectively gives one event per spark so, unless it can sense the coil discharging via its connection to the primary, must work off elapsed time from the trigger event for dwell. It will be constant by time, but variable by angle - an advantage for producing constant sparks, provided the time interval stays within the bounds of the corresponding angular interval between the coil firing subsequent cylinders.

 

It's complicated how it works but yes the time should stay roughly fixed. GM HEI uses the rise/shape of the curve from the magnetic trigger to determine the dwell time, a lot of manufacturers copied this. The current limiter also determines dwell.