Great post Bryan
Sound like you're closing in on the car's sweet spot.
Regarding another extinguisher.......this is what you can end up with when your only extinguisher is depleted:
Rural road, late in the day......without the incredible luck of a couple passer byes, one with extra towels willing to jump in and assist smothering flames, and another with a flat of bottled water who was saturating out towels......I was literally within seconds of have to sit and watch the car burn out.
Get the other extinguisher.