I'll stir this pot a little more lol
In my Saab,
Quote:
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The system puts a low voltage over the spark plugs when they are not fired to measure ionization in the cylinders. The ionic current measurement is used to replace the ordinary cam phase sensor, knock sensor and misfire measurement function.
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Saab Direct Ignition - Wikipedia, the free encyclopedia
So yea, some of us have to use the recommended plugs.
But I also believe that wear in the engine, age, modifications, and other factors may be considered.
After changing plugs many vehicles, I believe that indexing works for some cars, but not all. The gain for me was quite small, but the cost was small too - only 1 extra spark plug. Neither gains nor cost are enough to argue.