The Ultimate Miata Coolant Reroute
by Dave Coleman
The lameness of Miata cooling systems is well known. They can reasonably handle the 12-13 hp a naturally-aspirated Miata makes, but when asked to deal with both the increased heat load of turbo boost and the reduced airflow caused by the intercooler, the cooling system starts to cry.
Our Frankenmiata makes almost double the stock power output, and can endurance race in 100+ degree weather with the coolant temp staying under 200 degrees. In more civilized weather, it often stays below 180. There's some magic in our cooling system that I'm going to reveal now for the first time. Of course, this is the first time because I've been too lazy to do it before, not because it's some big secret. Still, it sounds more impactful when I use that “first time” line, doesn't it?
When most people suffer through their first sighting of our car's front end, they're usually so distracted by the chaos of creativity they fail to notice the absence of a radiator. When we first started building this car, LeMons races were like demolition derbys where everyone just happened to be driving the same direction.
Our previous car, the pimptastic Honda CR-XXX, died an untimely death when its bedazzled dollar sign grille emblem punctured the radiator after the 45th time we rear-ended someone. Determined not to repeat this fate with our new car, we hid its radiator deep in the middle of the car. The odd radiator placement is just a small piece of the cooling puzzle. The real magic starts on the inside.
One of the biggest reasons for Miata cooling fragility is a hidden quirk of the engine's front-drive heritage. In the 323, where the engine was transverse mounted, cool water from the radiator entered the water pump, was pushed in through the front of the block, and exited the rear of the cylinder head. This front-to-back cooling flow did a reasonable job of cooling all four cylinders evenly.
When the engine was turned north-south and stuffed in the Miata, the rear-mounted thermostat housing/coolant exit was repurposed to feed just the heater. The thermostat and upper radiator hose were relocated to the front of the head. As a result, most cooling flow goes in through the front and out through the front, leaving water flow in the back of the engine relatively stagnant.
This same design quirk applied to 1.6 and 1.8 Miatas until the 1.8 with variable valve timing in 2000 finally introduced a head gasket that only allowed coolant to pass from the block to the head around the #4 cylinder. This finally forced coolant to flow front to back across all four cylinders, up to the head, and then back to front across all four combustion chambers. 1.8 Miatas have different bore spacing from our 1.6, though, so that head gasket won't solve our problem.
Instead we did the fairly-common Miata coolant re-route, where you return to the old, front-drive coolant flow strategy. Several companies sell kits to make this a reasonably-simple thing to do without re-engineering your entire car. With no money and lots of engineers, we did it ourselves.
I stole this picture from some guy on MiataTurbo.net, and I'm pretty sure he stole it from someone else, so I have no idea what the original purpose of those arrows was, but here's what the back of a Miata head looks like. The first thing you do with a coolant re-route is remove this heater hose fitting (red arrow) and replace it with the thermostat housing and thermostat.
Unfortunately, that green sensor is the coolant temp sensor for the ECU, and it has to be on the hot side of the thermostat. Those fancy kits usually include an aluminum spacer, with a tapped hole for this sensor, that goes between the head and the thermostat. We did without such luxuries and instead removed the superfluous hose nipple (green arrow), and tapped the resulting hole for the coolant temp sensor. This is not the kind of thing you want to do with the engine installed in the car, but it was an easy solution for us.
As you can see here, the temperature sensor is very close to the thermostat.
So close, in fact, that we had to remove that praying mantis looking doodleframus from the thermostat on the left, leaving us with something like the one on the right.