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You can use whatever coil your heart desires. Different coils are set up for different primary circuit VOLTAGE which is what the resistor is affecting. When energized, your electrical system feeds power to the primary windings of your coil. That sets up a magnetic field centered around the core of the coil. The coil is grounded through the points on the distributor ("normal" system). When the points open the field "collapses" and a high energy pulse is created in the secondary winding of the coil. If all works properly and the stars are in alignment the pulse travels down the coil lead to the distributor, the rotor sends it to the appropriate plug lead and good things occur in the combusiton chamber.
The design of the various coils are varied by potting (oil vs epoxy) wire diameter and number of windings in both the primary and secondary. Also, they vary by resistance (here we are again). The main reason for lowering voltage through the primary circuit is to protect the points from frying (also lowers heat build up to a small extent but requires larger wire for the same current capacity). The condenser is utilized to "damp" the "bounce-back" of current in the primary circuit which would tend to cause spark flutter and further fry the points (lots of little arcs instead of the two big ones when the points separate and join again.
So-called "pointless" (breakerless) sytems do much the same thing except a "trigger" is incorporated somewhere other than the points. Many breakerless systems require STILL LOWER voltage to protect the often delicate 'tronics inside the boxes.
MSD is nice because it can set your redline with user installable selecotr plugs and the "6" has a soft reduction wherein it randomly fails to "fire" a cylinder to keep the revs at the desired point (not so harsh as cutting out entirely, especially in the middle of a corner!).
I happen to like the Mallory Unilite but that's just me and it takes twice as much ballast as the stock system.
Oh, Bosch makes two "red" coils. One is a stock unit for the older VW's and (I'm sure) other European cars (along with the various black ones), and the other is the high performance unit (really a very bright red, requires external ballast, i.e. resistor). The Bosch "blue," if you can find a real one, is internally ballasted and will work quite well in the Volvo. It is also considered a high performance unit.
In short, consult the instructions that come with any ignition system you get (or find instructions!) to determine how much ballasting is required.
Isn't this fun?
Mike!
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