I saw Lynn’s coolant diagram with a restrictor plate in it  …  you guys with evaporator cores and 1” coolant hoses have a 1” restriction, this based on Mazda’s design of 1.5” inlet/outlet on the stock water pump and the stock design includes a thermostat.  With all of that as a background (never had a thermostat), I decided to try a restrictor plate  in my coolant system, using a 0.75” hole in a plate at the water pump outlet into my 1.5” radiator hoses.  I can say that it doesn’t do any harm and may have actually provided about 5% improvement … more testing to follow.

Jeff Whaley