Quick single 12v coil ?

Modern low impedance coils are good for a high rpm engine, they charge quickly so the output remains acceptable at high rpm where charge time is nil.
Extremely high voltage coils are needed for engines with high cylinder pressure- like supercharged engines. The higher the air pressure -the higher the voltage needs to be to jump the gap.
The highest voltage needed for a Norton motor is when the cylinder pressure is the highest -at the torque peak -which comes at a relatively low rpm with a Norton motor.
Nortons don't benefit from a extremely high voltage or a low impedance primary.
What they do like is a long duration spark which helps avoid missfires. In most cases that means a large heavy coil -or a pair of coils.

"What they do like is a long duration spark which helps avoid missfires. In most cases that means a large heavy coil -or a pair of coils."

Along the "long duration" lines, the primary marketing hype for multiple discharge systems is how they ensure "better" ignition by generating "multiple sparks." Actually, they DON'T!

OK, OK, to be correct, they DO generate multi-sparks at low RPM but not at the range where the engine makes its maximum power. At higher RPM they provide a single spark, just like ignitions did in your 1968 Chevy, Ford, or Mopar. The multi-sparking of a typical aftermarket v8 multiple spark system stops at around 2500 RPM.

We did a bazillion (OK, maybe not quite THAT many) dyno tests of ignition systems, comparing them to stock systems. In no case did a high-zoot (scientific automotive term) ignition provide consistent, repeatable increases in max HP over the stock system on a stock engine.

I would agree that there is no increase in max HP.

But I have seen a small increase in low rpm torque when using multiple spark CD units like an MSD. They do help with flame propagation at lower rpms -particularly if the mixture isn't perfect.
You will often see the result of the faster propagation after installing an MSD ignition as an increased tendency for the engine to ping when the cylinder pressure is high. [low rpm -heavy throttle] Once you get the timing curve corrected to eliminate the ping, the most noticeable gain is a slight increase in fuel economy.
I did run an MSD-6 on my Norton for a couple years. I eventually removed it. The gains were not worth the weight of the unit and the big coils.

I also used a high frequency [plasma] ignition unit from an aircraft for a while. It drove two tiny coils with a high frequency AC voltage and had an adjustable burn time. It was complicated and hard on spark plugs and wires. I also noticed an increase in engine temperature. It made it impossible to foul a plug.
Again -an increase in max torque but no increase in max HP.
This was the only ignition system I used that made enough of a difference to notice the increased pull when in the saddle.

I am now back to a simple ignition with one large twin post coil driven by a GM- HEI module. The timing signal for the HEI is supplied by the fuel injection unit.

MexicoMike said:

"What they do like is a long duration spark which helps avoid missfires. In most cases that means a large heavy coil -or a pair of coils."

Along the "long duration" lines, the primary marketing hype for multiple discharge systems is how they ensure "better" ignition by generating "multiple sparks." Actually, they DON'T!

OK, OK, to be correct, they DO generate multi-sparks at low RPM but not at the range where the engine makes its maximum power. At higher RPM they provide a single spark, just like ignitions did in your 1968 Chevy, Ford, or Mopar. The multi-sparking of a typical aftermarket v8 multiple spark system stops at around 2500 RPM.

We did a bazillion (OK, maybe not quite THAT many) dyno tests of ignition systems, comparing them to stock systems. In no case did a high-zoot (scientific automotive term) ignition provide consistent, repeatable increases in max HP over the stock system on a stock engine.

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My classic mini has an aftermarket Pertronix EI with a low resistence primary coil (0.6 ohm I believe). Supposed to give all sorts of benefits like idle stabilization and big sparks etc. The coil failed a few months after I bought the car. It was the oil filled version. I bought the epoxy filled type as replacement and it seems ok for now. It does mount on the engine block so gets high vibration.

"They do help with flame propagation at lower rpms -particularly if the mixture isn't perfect."

Yes, we saw that regularly on the dyno. I realized later that my post may have sounded as if I didn't think MSD systems were good systems. I apologize for giving that impression. They are excellent systems and we used them extensively in high-performance/competition engines.

I was trying to make the point that installing one on a stock-engined vehicle and expecting more max HP was an unrealistic expectation. OTOH, if you used the stock ignition on a high compression competition engine, you'd essentially have no spark at full load/RPM.

acotrel said:

What is more important - form or functionality ? - with two coils there more electrical connectors which can make poor contact. If you have a perfect Commando, you would definitely want originality.

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I thought if you wanted a perfect Commando you replaced most of the original stuff?