The Primary Difference

What's really important when evaluating magneto output? Virtually all magneto service shops will deliver only an amperage number. Unfortunately, this number is PRIMARY amperage and has almost nothing to do with what reaches the spark plug. SECONDARY numbers are what is important and there are several adjustments needed in every mag to get the very best output at the plug where it counts. If secondary voltage and current are not tested, how can a shop know they've done the best possible job for the customer? THEY CAN'T!

Primary current is simply the relationship between a certain magnet spinning within a certain lamination/winding combination. "4.1 amps" scrawled on the side of your mag with a felt pen is no assurance that it makes the best spark possible at the plug. For example, at 2500 test RPM, that 4.1 amps could translate into 54 milliamps of current and 13,000 volts at the plug when properly adjusted. Or, that same 4.1 amp mag may only make 45 milliamps and 10,000 volts if things aren't quite right. The difference is how the mag is adjusted. So...primary current is a pretty lame way to measure magnetos. But it's cheap, quick and easy - that's why most places do it that way.

Fuel Injection Enterprises has invested in high tech equipment that can measure all aspects of magneto output. Every mag that leaves our shop gets a full report that includes primary AND secondary current and voltage along with spark duration. We then calculate and display the actual spark energy generated. This is a real "apples to apples" comparison of the unit before and after service. This is also a great way to compare two different magneto types or models. The watts and millijoules numbers generated on our test reports are the actual amount of electrical "work" being done by the magneto. Interestingly enough, the calculations for millijoules and watts do not even include the magneto primary current at all! Primary current is that insignificant. Note in the test report linked above, the primary current is pretty much the same before and after. However, the output of the mag is significantly better in the "as shipped" condition. This is completely due to adjustment...point gap, edge gap, rotor phasing, cylinder timing - it matters!

Secondary numbers for a mag are very dependent on the model and type of coil (transformer) used with them. Each make and model of transformer coil will have a different winding ratio inside and this effects the overall output in a big way. When adjusting the mag for optimal output, any old coil can be used. However, the actual numbers generated on the secondary side may not be the same as what the customer sees. For this reason, we recommend you send your coil in with your mag for service. If not, at least be sure to tell us what make and model coil you use so we can get real world numbers for you.

Different coils can exhibit different behaviors. Some magneto/coil combinations can result in hard starting at low cranking speeds. Some are not very efficient and don't transform the mag's energy into very good output. A powerful mag matched with a small, inadequate coil can create very long spark duration - so much, that crossfire and serious problems can result. Since we carefully measure the secondary side of the mag and coil, we may be able to recommend a much better combination for you.


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