[Stubby79]

I think you'll find it's a lot to do with the coolant's path...if some of the coolant(what goes through the heater core) can bypass the block, it means some of the coolant going in to the head is already warm, so it can heat up even further on the next pass...don't quote me, I'm just thinking out loud.

The miata flows it all through the block before going through the head...it all has to warm up to get hot.

Besides that out-to-lunch theory, the more likely culprits, in my mind, are the piston oil-squirters on the Miata, robbing heat that would otherwise transfer in to the cylinder walls/block/coolant, the design/materials used in the heater core itself, and the coolant itself...ie your 30 year old heater core is going to have deposits on it that the 15 year old toyota - and it's long-life coolant - aren't necessarily going to have.

Anyway, there are plenty of variables that affect it. It could all boil down to what they designed the cooling systems for...a summer sports car vs a comfortable daily driver. One designed to warm the passengers up ASAP, the other designed to shed excess heat as quickly as possible.

(oh, and the cast-iron block vs aluminum will probably slow down heat transfer from the cylinders, as well as the toyota's cylidners are completely surrounded with coolant/floating, vs passages around them in the iron miata block)