Having hooked up a 555 to an auto ignition coil in high school in the 70's, sparks were blue and almost like jagged lightning strikes about 10" long. Burnt holes through paper. Since I used another project as a driver (A solid state vibrator for a car radio) the frequency was about 200 Hz. It was also, I think a 6V coil operating at 12 V. It would occasionally arc over from the output to the nearby screw.
Automotive systems do have a ballast resistor that limits the current somewhat. I really would not want to be a guinea pig.
For a science experiment I did make a 3 kVDC supply, but it broke, so I ended up taking the anode off a TV and using that to finish the biology experiment.
Later, professionally, I worked on electron beam power regulated supplies (15 kV @ 1.5 Amps) and X-Ray power supplies ( 100 kV @ 0.1 Amp) and even 13.56 MHz RF transmitters with a power output of 1 KW (The voltages inside that transmitter were up to 3000 VDC. Don't forget the occasion work on a TV.