Anybody know?

[Volvostorm]

My engine builder asked me this, how does the 850 ignition advance and retard work?n As the dizzy and rotor arm is bolted to the engine, how does it fire at x amount of degrees?

[Dangerous Dave]

The ecu controls it mate. There is a map which plots the ignition advance depending on load and engine speed

[Porcine_Aviator]

You read all about it did search and came up with this pdf file
https://docs.google.com/viewer?a=v&q...PwyiADVQ&pli=1

[Volvostorm]

I have read that, but what I don't get, is how it manages to fire early?

i.e: on the old vacuum advance distributors, the rotor could move, and was on springs, needed more advance, the rotor arm 'moved' gets to the cylinder contact point earlier.

850 system the rotor arm is fixed, bolted to the inlet cam, to my thinking, its always going to be at the cylinder contact point at the same amount of degrees each cycle.

I know the system works, just trying to work out how!

[Dangerous Dave]

Ah, I see where you're coming from now.

The rotor arm has a wide contact on it, so I guess it covers the programmed advance/retard angles. And the cam rotates slower than the crank so the amount of time 'on contact' the rotor arm has is multiplied.

[nobananas]

As bomb says, the spark isn't actually fired to the plug by the action of the rotor arm passing the spark plug lead contact just distributed to the relevant spark plug. The contact on the end of the rotor arm has a wide blade so that it always next to the relevant plug lead contact irrespective of the advance that the spark is fired by the ecu. The old system with contact breakers used centrifugal weights and vacuum advance system that actually moved the distributor baseplate which in turn moved the breakers around the cam lobes within the distributor effectively advancing or retarding the ignition timing, the rotor arm then distributed the spark to the relevant plug lead by it passing the contact within the dizzy cap.

[Volvostorm]

Thanks for all the answers, it makes sense now