Interesting info here.

But perhaps wait for others to confirm.

At high rpm breaker points may 'bounce' failing to follow the cam profile exactly. Two sets of points helps to minimize unintended disruptions of the primary current. The coil is not intended to "fire twice".

http://www.imperialclub.com/Repair/Lit/Master/044/...

Why do you think the coil would fire twice? There is still just one switching event just that the two points events are staggered a bit.
Like the article says the points share the load so to say.

Typically a distributor has a lobe for each cylinder, correct? So how does the distributor fire all the cylinders with half as many lobes? What is the action which creates this ability?

Thank you for the links! I had read the MGA Guru piece previously but hadnt seen the 2nd document. Very helpful!

Josh

is this Marelli dual points distributor for an inline 6, or a V6? If V6, what angle between the banks?

I think you're going off on a tangent there, Tmoose. The moving parts of the distributor don't know or care what is arranged or going on outside. All they do is fire sparks at regular intervals. Where the sparks go is down to the distributor cap and secondary wiring.

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

In this case each distributor is running an inline 6cyl firing at 120 intervals. However, since it is synchronized with a second bank of the same configuration, it alternates firing bewtween the 2 banks. A colleague has told me it works because each both points fire in sequence at all times like a waste spark ignition, one pulse on compression and the next on exhaust.

So if this is the case, when the leading point opens the coil discharges and continues to discharge as the trailing point opens and doesn't stop until the leading point closes and disrupts the coil's magnetic field. Am I right?

JoshHill, as I mentioned to Tmoose, the moving parts have no clue what is arranged outside. I assume your distributor cap has 6 terminals, and your distributor rotor turns once per every 2 engine revolutions. In that case, there is no possibility of a waste spark, in the accepted sense.

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

Thanks Hemi. Yea, the distributors run directly off the back of the intake camshafts for speed of half of the crankshaft and the cap has 6 contacts.

The 2 banks are 60deg apart.

British motorcycle twins used a single lobe cam and twin points, the Trident and Rocket III triple points. You set timing for each cylinder individually. Other engines were similar. Or single points and cam lobe on a waste spark like BMW twins.

Thanks LadaTrouble. Do those points setups have overlap between the two sets as well?

And it sounds like you're saying this setup works because it is actually a waste spark setup?

Josh

The Brit bike ignitions (by Lucas) had a dedicated coil and set of points for each cylinder, running on a common ignition cam. Not a dual-point (or triple) system in the context of the OP's system.

If the distributor has but 3 lobes, my hunch is the sets of contact points do not have overlapping open periods to extend the dwell.
Rather, each points set makes 3 individual sparks, with NO extended dwell advantage for the coil provided by one of the "typical" type of dual points set-up.

The OP's query as to "Why" Marelli chose this set up over a 6 lobed points distributor is an un-necessary puzzle caused by a lack of "what" (up-front info) from the OP.
My frivolous hunch is the required firing order is not every 120 crank degrees/60 distributor degrees like an inline 6 would be, or a 90° or other even-fire six. Instead of making a distributor cam with 6 lobes at 50-70-50-70 etc, they //may have opted for a simpler symmetrical 3 lobed cam, and offset one set of points to create the asymmetrical firing necessary for one bank of cylinders.

Am I close ?