This video is great. To me, spark plugs are always those little doodads that either work good, need cleaning, or tossed; never really think about what goes on inside or how their current form came to be. I sent this link to my dad who is a mechanic of 50+ years and he loved it.
Not to link spam this thread, but this showed up as one of the recommended videos when I clicked the link below:
https://www.youtube.com/watch?v=jdW1t8r8qYc
This guy created a see through head for a Briggs and Stratton; a flat head engine similar to our A4's. Over a series of 4 videos he runs different types of fuel through it to observe the flame kernel. Just multiply by four and you have a good idea of what is turning the prop. Now all I'm thinking about is a way to 3D print a transparent head for my A4 and do videos like these.