Does anyone know why Porsche decided to use this odd dual rotor, dual cap distributor setup on the 964/993?

Perfectly good 12 wire distributors were available on several other marques.

I don't recall reading anything about this in Frere or other books...

 

Good question. Porsche had their own 12 plug distributors.

Could be a way to alter the timing of the upper vs. lower plugs?

 

Just speculation:
- production cost of 6-point vs 12-point
- production tolerances
- electrical load and reliability
- excessive ozone creation

 

The dual setup fits nicely in the tight space. Just my guess - Cap kept to the tradition of the engine.

 

A 12 cylinder cap allows one ignition coil to fire one of 12 plugs at a time. Not what our engines need.

Our engines require a means to have two coils each fire one of a group of 6 plugs at a time ... there are two separate ignition channels.

... Gregg

 

ok, when people put 12 wire distributors in a twin-plugged 911 motor, how are those distributors changed?

 

I would imagine the contacts inside are connected so both say #1 plugs fire off at the same time. But that is just a guess

 

You get the idea John, but connecting two plugs in parallel like that can't work, because only the plug with the 'easiest' path will fire ... that's why two parallel ignition systems are used in our cars.

For a single 12 lead cap to work, it would require two coil connections and two rotors which would create two distinct ignition channels; one for each bank of plugs. I've never seen a single cap like that done, and I can't even think of a simple way to make that work. I suggest if there were a way to do it, Porsche would have updated the twin distributor before now.

.. Gregg

 

You can see the rotor looks the same 12 leads out of the cap what happens inside the cap is a complete mystery. well at least to me it is😁

 

what??

there are several versions out there

I dunno how they work or what they look like inside

 

I just just noticed there are two HT leads into the cap.

 

that setup still use's two ignition coils. I think they went the two cap route because with higher Resistance from the ignition system in our cars you would have more jumping and arching in side the cap. soon I will be distributor less.

 

Exactly ... the only way to fire two banks of 6 plugs that are servicing 6 cylinders ...

.. Gregg

 

I wonder how then the rotor works I suppose two concentric rings inside the cap could work with two pick ups on the rotor. Oh well this will give me something to ponder while I work at my mindless job👍

 

The answer would be clear if someone posted an underside view of one of those 6 + 6 lead caps, as well as the top face of the mating distributor, so we could see the rotors.

Till then, one clue is that one of the HT coil leads appears to be central in the cap, while the second coil lead appears to be offset in the cap. From the little we can see of the rotor, it seems to be constructed in much the same way: a central contact, and an outer ring for the offset contact. That implies that the center contact fires 6 of the plug leads, while the offset one fires the other 6.

.. Gregg