Aftermarket ignition ...do you really need one?

Warning, multi page long incoming post.

One line that I can't take credit for that sums up a lot: "If a match lights the fire just fine, a blowtorch isn't going to do any better." If the timing is optimal, and your current ignition lights off the mixture just fine every time, then you won't see any gain from something hotter.

But there are a couple situations where you can do better.

The "if the timing is optimal" is the first one. A stock distributor would have a good curve for a stock engine; if you have significant mods, you probably need to set the curve for what your engine needs. Sometimes aftermarket distributors or electronic controls make it easier to dial in the timing curve. The right curve can make a huge difference in drivability and economy as well as power. And crank fired ignitions (which will need to be paired with electronic controls for a street engine) can get the timing more accurate.

Related to timing; combustion chamber mods that get the mixture to burn faster will require less timing and make the engine more efficient. This is the deal with dual plug Hemi heads; the stock heads can get in a lot of air and fuel, but it takes longer to burn it than wedge heads. Two spark plugs with a significant distance between them will start two flame fronts and burn the mixture faster. The Gen III Hemis typically only need 20-22 degrees of timing at full throttle.

There are two cases where a stock Mopar distributor (points or electronic) may no longer deliver enough power. One is if you are making considerably more cylinder pressure; more compression or forced induction can increase the pressure to the point a stock ignition no longer generates enough voltage to jump the gap.

The other relates to dwell time. The longer you dwell a coil, the hotter the spark it generates until you hit either the coil's current limit or the ballast resistor doesn't allow any more current. And at high RPM, particularly with more cylinders, you the time to dwell the coil decreases and you get less spark energy.

There are several ways you can improve the ignition if you hit these problems. One is to upgrade the coil. I haven't seen what the larger Accel Super Stock coil (which appears to be discontinued) will do on a dyno, but I've seen their regular size coil lose 5 hp to a 30 year old Bosch stock coil. On a 100 hp four cylinder Porsche 944. It may well have been an upgrade from the stock 1960s era coils, but you can do better.

You can also get a bit of an improvement with more sophisticated dwell control. Points always use a fixed dwell angle, and stock Mopar ignition boxes leave the coil dwelled full time, interrupting it for a fixed time when it receives a pulse from the distributor sensor. A more sophisticated inductive ignition module (such as an HEI) can make better use of the available dwell time. Such ignitions also run cooler and eliminate the ballast resistor - they limit current by limiting dwell time.

Capacitive discharge ignitions get around dwell entirely. They use the coil as a transformer (so some of the coils designed for CDIs won't work with other ignition modules very well) and dump a high voltage pulse from a capacitor through it. So the limiting factor is how quickly the module can recharge the capacitor. They also generate a shorter spark with a faster voltage rise time. This can burn through fouling on spark plugs but can be a disadvantage at idle (which is why you see multispark ones). The faster rise time can also mean that you may need a degree or so less timing to have the flame front start a the same time as with a conventional ignition.

Or you can avoid having dwell fall off by running several coils, letting several dwell at the same time. You'll see this on virtually all automotive engines built after 2000 in the US, except for diesels of course and a few Hondas that held out with distributors for a few years past 2000. The Super Touring Dodges used this approach, and had a very good coil design that I've seen swapped onto other engines to get a hotter spark.