Undy I have 1 3/16" hole in my thermostat. I am also using Dodge thermostat The outlet is 1/2" larger than the FE one. You where talking about spikes with coolant temp. I am running a shelby 482 motor and the heatup is quick for coolant, 2-3 minutes, the
oil takes about 5-7 minutes. The thermostat is a 180. I had a overheat problem and spike temps, I changed everything. The biggest change was getting rid of overdrive pulleys. I had the March setup. They provided just enough flow to keep from hitting 225 when roadcourse racing. I limit the run to 5 laps. I am now running custom pulleys that are 5 1/2" crank and 5 3/4" waterpump. The temp after racing is in the 190's I still get the temp spike in the 190's until the thermostat opens but I get a drop to the 160's quick and after a couple minutes of driving the coolant is stable a 180-185 with the motor at idle. March makes a nice pulley kit for FE's and on the street they get by as long as you are not in a stand still for a long period of time. The under drive is about 20-25%, and IMO thats too much. ASP built my custom pulleys. I do have the bypass hose on and working. The dodge thermostat does out flow the FE one by alot. If you go to an autozone of pepboys and look at the back where the outlets are you can seen the differents. The only thing is you need to remove 2mm on the edge to fit in the stock location on the manifold. Like said before hole goes on top for air. Hope this helps Rick L.