Scott Ingram's Coolant Reroute
Written and conceived by Scott Ingram and Ray Ayala
The factory cooling system is poor at best. The 5-speed radiator is a single row unit and it just cannot handle much more heat - just about any performance modification will make the system overheat. Cooling system upgrades such as a larger radiator or larger CFM fans only provide minimal benefits because of the design factory cooling path.
The real culprit to overheating when performance parts are installed is the factory coolant routing. The original cooling path for the 'B' engine as used in FWD cars is front to rear but in the Miata the "engineers" rerouted it to a bottom to top path system. It doesn't really work as designed for two reasons 1) the hot coolant from the back of the head is mixed with the "cool" coolant just coming from the radiator in a manifold (just behind the AC compressor) making the incoming water hot again 2) that hot incoming water then goes to the water pump where most of it circulates right back to the radiator through the front thermostat housing. This makes the water at the back on the engine (#4 cylinder) super hot and doesn't circulate near as fast as it should (also contributing to the lean #4 cylinder lean condition).
The best fix is to reroute the water from the back of the head to the radiator input and eliminate that "mixing" manifold. It is a lot of work but the true fix. Below is writeup from Scott about his heater core reroute.
My car was originally equipped with the coolant reroute kit made by Richard M to pull the coolant from the rear of the head. This worked but I would still overheat occasionally. In talking with Randy S at R-Speeds open house 8/11/02 he suggested that I try re-routing the coolant from the heater core to the radiator instead and not back to the block so it won't mix with the cool coolant.
First, remove the hose exiting the heater core. Be care as you can easily damage the copper fitting (cutting it off is generally best)
Run a new length of heater core hose. This will go to the front of the engine.
Plug the old return line that goes to the "mixing manifold".
Here is the test adapter to accept the new hose. I used 1 1/4 copper pipe and soldered a 3/4" 90 degree elbow that the 5/8" heater core hose fits onto.
The final production piece is only 3".
Completed reroute. The coolant now runs from the back of the head to the heater core, then to the radiator as designed by the original engineers. Note, the thermostat is now in the back of the head (there is machining there to accept it) and the front thermostat housing port is plugged with a freeze plug (that is how the factory does it for FWD too)
The results were that I drove the car for 25 miles with the air on in the city and highway and never overheated as it used to do. I found that the max temp displayed on the LINK was 106c in stop and go traffic with the AC on and this cooled down to 96c running down the interstate with many WOT runs (The air temp that day was 88-90°F). With the AC off the temps never got hotter than 86°C.
provided a nice graphic for the route: