Ballast Resistor

My name is Geoff. Not hiding from anyone or anything. When I attempted to join this great forum, Geoff was taken....so I used my nickname.
Dave, you were caught out [ not be me, others ] on maxperformanceinc.com providing 'information' that was shown to be incorrect. That is a forum that I visit daily & you have not posted there for months.
Heat conductivity of insulators. The info I provided is readily available on the net or in engineering handbooks. I got it from the Bosch Automotive handbook, 9th edition, 1543 pages. The same book says this, about ign coils: "The windings must have good electrical insulation to prevent electrical discharge & flashovers either to the inside or to the outside. For this purpose, the windings are usually cast in epoxy resin in the ign coil housing."

The metal case of a canister coil is going to feel 'hot'! The metal case, relatively speaking, is a good conductor of heat. The coil is being heated internally during operation & is also absorbing under-hood heat. Of course it will feel hot!
One thing I do agree 100% with D. Ray on is the use/benefit of manifold sourced vac adv.