What is a magneto?

[Ace-Garageguy]

More mag stuff regarding "period look" cars from another of my posts.....

More

One more small point. There are cars out there now, doing the period look, that have hollowed-out mags with capacitive-discharge ( like MSD) guts. Mags do have limitations, in that they have a lot of precision mechanical parts to wear, they have points that also wear and need adjustment, and ultimately, they won't make as hot a spark at high rpm as current technology. All that said, they can make an excellent and fully functional ignition system on a daily driver.

And a further caveat. Mags work great for dragsters, top-fuel and otherwise, because the high maintenance factor isn't an issue. The bearings and points etc. just aren't expected to last very long in a professional race car, and they get replaced frequently. Electronics can have their own reliability issues, and there is probably a very good reason (or reasons) why pro drag cars still use mags, but I won't get into that, 'cause I just don't know.

Edited July 3, 2012 by Ace-Garageguy

[Harry P.]

My head hurts...

[Ace-Garageguy]

A further clarification of the actual function of a magneto:

A rotating permanent magnet inside a magneto induces a current in the PRIMARY winding, creating a larger magnetic field (an electromagnet) adjacent to the secondary winding. When the points open (they are connected to the primary winding as a timed-switching-device, and they perform exactly the same function as in the distributor workings described above, namely to tell the magneto the position of the crankshaft and cam) the current stops flowing in the primary winding. The magnetic field collapses, and the moving lines of magnetic flux of the collapsing field induce a much higher voltage in the SECONDARY winding, which is more-or-less analogous to the COIL in a battery system (with a distributor). The hot "spark" output from the secondary winding is directed to the correct plug by a rotor-and-cap and secondary wires, just like in a distributor ignition.

Because the magneto is capable of producing its own primary current AS LONG AS IT IS TURNING it doesn't require a battery or generator or alternator to function, and this, again, is why you always see old mag-equipped race cars either push-started, or started with an external device, like old Indy cars. Because the battery and something to keep it charged (generator or alternator) aren't required to keep a mag-equipped engine RUNNING, there is no onboard battery to run a starter either. SO, the engine in a mag-equipped car THAT HAS NO STARTER has to be made to turn some other way, so the mag can make a spark, by pushing the car in gear, or by an external starter that spins the engine.

But one more time, you CAN have a street-driven car that has a battery to start it (which spins the starter motor), a generator OR alternator to keep the battery charged and run lights and accessories, and a magneto, that does absolutely nothing but fire the spark plugs.

All clear now?

Edited July 3, 2012 by Ace-Garageguy

[Aaronw]

In closing, a mag won't blow out lights as posted above, because it's constructed so that it's spark discharge is directed to a SPECIFIC spark plug as the magnetic field collapses, and not into the car's general electrical system.

This perception may be due to how early generators worked. My Dad owned several Ford Model As in high school and college (late 50s - early 60s). He borrowed a friends older Pontiac and blew out the lights on the highway. He said as he got onto the highway it seemed like the lights kept getting brighter and brighter, until they went out. Turns out the generator on those old cars had two speeds, one for streets, one for highway but his friend forgot to tell him about that and it wasn't something that needed to be done on the Model A.

[Ace-Garageguy]

Electro-magnetic, point-type voltage regulators (on both generator and alternater equipped cars, '40s through '70s) were occasionally prone to sticking in the full-charge position, sometimes when the points burned. This could result in lights being fried and batteries boiled, but this is the only direct experience I've had with the over-charge problem. It was also a fairly well known practice to run with lights on when doing extended highway driving, the theory being that the extra current draw from the lights would keep a faulty regulator from boiling the electrolyte out of the battery. Burned out lights were cheper to replace than batteries.