I have installed a metal plate with 1/4" dia hole to restrict water flow in the water pump bypass tube.  This is on 292 Merc with recent rebuild.  Though it never boiled over, it always seemed to radiate lots of heat and the not so reliable original 6V gauge showed hot.  Checked with thermo gun and it registered 195 on top of engine.  After installing the restrictor plate thermo gus now shows 175 degrees on top of engine.  What is the down side to this arrangement?  Car has a 160 degree thermosthat.  Thanks for your input and many thanks to the person that suggested this on y-blocks forever!!    

Gary

If you're in a cold climate, the cooler running temp may not be favorable. Otherwise, there aren't really any downsides.

195 is fine and the engine should actually be more efficient at a higher temp. But if it now stays closer to the thermostats rated temperature, you can always swap a in hotter thermostat if it is too cold during the winter.


Lawrenceville, GA

When I put the bypass restrictor in my outfit (3/8 pipe plug with 3/16 hole, stuck in the hose) - I also put in a high flow 180° thermostat. The combination seems to work pretty well to lower the high temp and raise up the low. Narrowing the operating range makes the engine tuning a bit more stable. 

 
 Steve Metzger       Tucson, Arizona

Ditto on Charlie and Steve’s comments.  Using a restrictor in the bypass hose may inhibit the heater putting out any heat prior to the thermostat opening but other than that, no down sides.  And a 160°F thermostat is on the cool side for any engine assuming the engine is actually running at that temperature.

Lorena, Texas (South of Waco)