Many Model T owners ask this question. Recently there was a lot of discussion on the MTFCA Forum about this subject. Several contributors to the Forum suggested that some scientific tests be run to determine the best system but others suggested that we wouldn't get reliable results because everybody seemed to have their own biases and whoever did the testing might get the results they wanted or expected. I think we could get an unbiased test, but not necessarily an answer that was useful to the hobby. All of the options for a Model T ignition system are quite suitable for the car - but the real question is which system is best for the owner of the car. None of the ignition systems are suitable for all Model T owners.
If the owner of the car is experienced with Model T's, capable and willing to maintain his own car, then the original ignition system is excellent. As a matter of fact, this owner is probably not going to be satisfied with a distributor or the True-Fire system because "it's not like Henry made it." It is nice to have the old historic system and show how such an unusual ignition system (by today's standards) works.
A common mistake made by novice mechanics related to an original ignition system is overlooking the fact that the coils are only part of the Model T's ignition system. The condition of the coil box, the spark plug wires, the wires connecting to the timer, and the timer itself are as important to the proper functioning of the ignition system as the coils themselves. It takes experience and a good understanding of the complete ignition system to properly diagnose and maintain an original ignition system. Unfortunately, many of the experienced Model T'ers are old and some are dying. Their son-in-laws or grandsons may or may not have the knowledge to maintain the ignition systems.
It is not at all uncommon to find Model Ters who "adjust" their coils according to the sound that the coils make. In order to get good results, the original coils must be adjusted using a coil tester which measures the amperage draw of the coil. Only with a tester can one determine if the coil even can be adjusted within the acceptable range or if it is a bad coil and needs to be replaced.
It is gratifying to the owner to be able to maintain an original Ford ignition system and show it off. The pride shown by the experienced Model Ter's in keeping the original system tuned is well deserved. But the fact that they take such pride in keeping the system in good working order does say something about what it takes. Experience and patience.
If the Model T owner is just beginning his/her affair with a Model T, a modern distributor may be a good option to consider. It works well, does not depend on the original coil box to be in good working order and driving the car can be enjoyed. Additionally, all modern mechanics are familiar with the workings of a distributor and it can be serviced almost anywhere. The replaceable parts are available from your local NAPA store.
The downside of a distributor is that it does not look like the original ignition system and makes a change to the appearance of the engine. It does not, however, require any permanent change to be made to the car and can be removed at any time if the owner achieves enough experience and confidence and wishes to return to the original system.
If the owner is more experienced with a Model T, or has an overriding interest in original appearance of the engine compartment, the True-Fire is a good system. The True-Fire ignition module is a reasonably reliable and maintenance-free unit that does not change the appearance of the car.
The weakness of the True-Fire system is that it depends on many of the original components of the Ford ignition system to be in good working order just as the original Ford coils do. The nine electrical contacts in the coil box must all be in good condition, the coil box wood needs to be good, and the commutator wires must be in good working order for the True-Fire to function. There have been many Model T owners who bought the True-Fire unit expecting it to solve their ignition problems only to have the issues get worse. Some of those nine contacts in the coil box only affect one of the four coils in the Ford ignition system but would cause the entire True-Fire system to fail if they were not in good working order. A good understanding of how the True-Fire system works is needed in order to troubleshoot any problems that may arise and most modern mechanics will be lost.
If the issues surrounding the original Ford system are in good order or are properly addressed, the True-Fire system performs very well and does not require much attention. It does not change the appearance of the engine and that is certainly a plus for many owners. I compliment Ed Bittner on his ingenuity in the development of the True-Fire.
So the real question is not which ignition system is best for the Model T, but which ignition system is best for the OWNER of the Model T. The best system for our hobby is the system that keeps your Model T on the road and in use. It doesn't matter if the system is original if the car sits in the barn. Most Model T owners want to get in their car and have it run without trouble to take them on a Sunday afternoon drive.
The great thing about the Model T as a hobby is that parts are available and, in many cases, there is a choice. Many other makes of vintage cars do not have the luxury of any parts being available. Enjoy the options. Don't criticize or ridicule those options you don't like because they may be a great solution to your fellow enthusiasts.