Twin Plug distributor question

[TrackRat]

I am trying to understand the function of the twin plug distributor. Do the plugs for a given cylinder fire 15 crankshaft degrees apart?

I came up with that 15 degree figure by this thinking:
360 degrees camshaft rotation divided by 12 plugs equals 30 degrees apart at the cam - for each spark "event" . Divide cam degrees by 2 to get crankshaft degrees.. That equals 15 degrees at the crank... Is this correct? That seems like a huge difference between spark events...

I recall that aircraft engines have their twin magnetos timed to spark each of the two plug in a given cylinder by 2-3 crankshaft degrees apart from each other.

What am I missing?

Doug B in CT

[kenikh]

Each pair of plugs in a cylinder typically fires simulateneously. Typically, twin plug dizzies have two arms on the button allowing this.

[TrackRat]

So Simple....

Thanks!

[Wayne 962]

Sometimes they are offset very slightly to allow for flame fronts to propagate, but most of the time, I have seen them fire at the same time.

-Wayne

[JohnJL]

My 993 fired simultaneously.

[304065]

My Lycoming's plugs fire at the same time. Well, at least they are supposed to!

[TrackRat]

I saw good photos (inside and outside) of an original Marelli cap and rotor posted by Cornpanzer on an ebay ad.

Seen here:
http://cgi.ebay.com/ebaymotors/ws/eB...tem=4590719706

His photos shows the distrib cap with a set of contacts present up high in the cap, and a second set ( every other contact) positioned lower in the cap. The rotor has corresponding tips, both high and low to pick up each set of cap contacts.

It's making a whole lotta sense now....

However it is interesting to note that the rotor tips are not 180 degrees apart, as I would have expected. however the skewing of the rotor blade tips would be just a plug wire routing/firing order issue. Why not position the rotor "Blades" 180 degrees apart? There must be som teutonic/latin engineering reason for this..

[cgarr]

Our Continental O-200 is timed 2 degrees off, top and bottom plugs 24 and 26 thus contributing to the drop in rpm when checking the left and right mags

[TrackRat]

Craig,

You are spending money on some cool old stuff: a J4 cub, and an SE5A... wow... Cool old bird.. Does this have the Hispano Suiza motor?? I used to hang out at airports in my misspent early teens and was tagged as a "RampRat" by the old hands flying around. I have phased out of flying about 10 years ago, but now hang out at race tracks instead.

I'm still half-heartedly looking for a Waco 10 or even better- a Waco Taperwing...

[johnsjmc]

There is a mechanical advance unit in the belt driven outer distributer on my 3.6 (1990) .I suspect it is used to widen the spark event as engine speed increases .

[Steve@Rennsport]

Originally posted by johnsjmc
There is a mechanical advance unit in the belt driven outer distributer on my 3.6 (1990) .I suspect it is used to widen the spark event as engine speed increases .

Nossir, not at all.

Thats for cam position reference so the DME knows which cylinder(s) is knocking when the knock sensors detect detonation,...

All 911's equipped with twin-ignition fire both plugs simultaneously.

[johnsjmc]

How does an advance mechanism allow the DME to sense cam position ? I would think the DME would use a cam position sensor and rotor position(ie which cyl is firing) along with knock sensing and retard as needed. I can also see it being able to retard individually. I can,t see where the advance mechanism comes in.

[mpdevelopment]

The advance mechanism is used to insure that the distributor rotor will be properly lined up with contacts in the cap as the computer advances the spark.

[johnsjmc]

"The advance mechanism insures rotor properly aligned as spark advanced by computer" Hardly makes sense as there is only an advance in one of 2 distributors. Perhaps the mechanism accomodates belt stretch between the master dist and the belt driven slave dist.

[mpdevelopment]

Sorry was thinking of 3.2liter Carrera distributor.

[TrackRat]

There is not any measurable stretch to the toothed Gates type belt running the no. 2 dizzy- they are extremely resistant to any stretching.
I would guess that the no. 2 dizzy was the logical place for the Porsche design engineers to locate mechanical advance mechanism

But if I understand correctly, that "advance mechanism" is really an input for the knock sensor portion of the DME .

So I'm guessing it might work like this:
The DME "hears" the knock, and then the DME "asks" the sensor in the No. 2 dizzy for the specific cylinder number that has just fired (or is about to fire). The DME senses all of this, then electronically retards the spark for that particular cylinder (or cylinders) on the next revolution of the cam. Does the DME back off the timing for that one cylinder or for all cylinders - as a group - once is senses a knock?

I guess the input to the DME is modified by the mecanical advance for some reason...