[QUOTE=BRENT in 10-uh-C;1963865]I have owned 3 dual plug Model-A/B heads, and most of them use(d) a magneto on a side-drive (powered off the outboard side of the timing gear), and a magneto in the cylinder head in lieu of the regular Ford distributor. I had one set-up that used a Early V-8 Distributor that was mounted onto a Model-B distributor in the normal position. Since the early Ford V-8 distributers were basically two 4 cylinder distributers inside of one housing, it worked fairly well. Nissan dual plug distributors (-with leading & trailing firing) have been converted for use on Model-A/B dual plug applications however I don't have any experience with those. As Jim said above, some have used the Stutz distributer, but those things are REAL expensive when you can find them for sale!!
Typically, with the brass-era and A/B engines with dual ignition that I have worked on, the initial timing (BTDC) is set much closer to TDC since you don't need as much lead in the timing to account for flame travel when you have two separate spark plugs lighting the mixture. When only one plug lights the mixture, it typically has only part of that entire mixture that gets lit by the time the piston reaches TDC, so you lose some of the combustion efficiency in that situation. Therefore when all of the fuel & air gets lit, the efficiency is increased which results in a little more RPM increase.
Here again, I'm thinking back to my days in aviation. I believe all the Continental and Lycoming engines had a dual ignition and a four position ignition switch, off, right mag, left mag, and both mags, Prior to take off, we always did a a run up. This involved bringing the engine to 1650 rpm and switching to say, left mag, check rpm (Shouldn't be lower than 1600). Right Mag (Same drop) and then back to both mags. RPM should increase to 1650. So, running on individual mags gave us 50 less rpm. I realize that the purpose of dual mags was not to gain the 50 rpm, but increased reliability and a lessened chance of engine failure due to a faulty ignition. But the dual ignition was a true dual ignition with two separate mags.
Terry