My 1.8 Coolant Reroute

My reroute is based on the coolant route of the 323 1.8 Turbo. Though similar to the 1.6 Turbo, the cooling systems of both engines are not the same.

The cooling diagram of the 323 1.8 Turbo works like this: coolant comes out of the back of the head through a double fitting (blue). The bottom part of this fitting goes to the stock oil cooler (red) and than to the heater. The upper part (green) goes to the throttle body and then back to the metal return pipe from the heater (yellow). The thermostat is at the back of the head. A hose runs from the back of the head to the radiator.

You can see the double fitting in the left picture of a 323 head. The miata has a single plug in that location (right picture). It's the blue fitting in this diagram. The 323 has a heat sensor just next to that fitting (see green connector in the left picture). The miata has that heat sensor in the heater outlet lid. Here's a better picture.

On the miata, the coolant comes out of the back of the head through a single fitting instead of a double fitting. It goes to the oil cooler, then to the throttle body and back to the front of the engine, to a fitting under the thermostat. The heater gets its water from the back of the head, where the 323 has its thermostat. You can remove this cover, put a thermostat in there and put a thermostat cover on it. It's a straight fit. Since you remove the entire thermostat assembly you will need to source water for the turbo from a different location. On my '95 there is a bolt just next to the turbo oil supply (leftside of the orange hose). I took water from that point. Unfortunately that bolt is M12 x 1.25. Since I couldn't find a hose barb with that thread I welded a barb onto the existing bolt and drilled through the bolt. The picture is a bit blurry but it should give you a good idea. For the turbo water return line I welded a barb onto the metal heater return line. In this pic (not mine), there's 2 fittings, I only welded the left one on. The other one is not necessary.

So, to copy the coolant route of the 323, I removed the thermostat housing, closed the hole at the front of the engine (with a 30mm freeze plug) and transfered the thermostat to the back. I used a 1.6 cover because the 1.8 cover points in the wrong direction. It interferes with the coil pack.

The 1.6 has a sensor in the cover, but the 1.8 doesn't so you need to close the hole. I suggest you don't weld it but put a plug in it. I had it welded twice but it kept leaking so I had to buy a new cover and start over. The cover is rather high and comes very close to the firewall. I cut 4 threads of the fitting and put a plug without head in it (how on earth do you call these things?). You know what I mean, a plug that sits flush with the hole. If you point the cover to the left, you can go under the exhaust. Point it to the right and you can go under the intake. I found it too messy to go under the exhaust (with the power steering and airconditioning in the way) and choose to go under the intake.

The hose to the radiator will interfere with the coolant boss at the back of the head so you'll need a spacer to elevate the cover. I used the spacer from a Jackson Racing Cold Air Induction kit. The part number is 051-046 and it costs $15 at Moss Motors. You can order it directly from Moss through this page. I also tapped the spacer to accept the hose barb for the heater. It's very important that you do not put the thermostat in the head. The heater will be after the thermostat that way. Instead, have a lip machined in the spacer to accept the thermostat. The thermostat sits in the machined groove and the thermostat cover sits on top. That way, the heater barb is before the thermostat, not after it. If you put the thermostat in the head, the heater will be after the thermostat and you'll get no heat until it opens.

It is probably easier to route the hoses under the exhaust side. You don't need the spacer to clear the coolant boss that way. You do need to find a way to install the barb for the heater though. You can safely drill and tap the flange on the back of the head (see pic), there's enough material there. However, it might be difficult to do that with the engine in the car.

Now, all you need to do is go to your local radiator parts shop and find tubing bends that allow you to route the flow back to the radiator.

The stock cover bolts to the head with one bolt and one nut. Obviously, you remove the stud and use 2 longer bolts to bolt the spacer and cover to the head. I used a gasket and some silicone sealant on both sides of the spacer. Space is very limited back there and I didn't want to take the risk of a having a leak :)
The weld fitting you see in the picture is for the thermo-sensor that is normally in the heater outlet cover. You cannot put the thermo-sensor directly in the spacer as it is too long and would hit the thermostat (don't ask how I found out) ... Another thing to watch out for is the bolt in the back of the head, just above the coolant boss. It interferes with the thermo-sensor. I removed the bolt and put a plug without head in it. The thread is M12 x 1.5. Notice how close the thermo-sensor comes to the head. Also notice how close the cover comes to the firewall. This is the old leaking (welded) cover btw.

I used 5 90° angles and the original upper radiator hose to do the water rerouting. Since I have an FM2, the intake pipe is to thick to go under it. Instead I went around it. I started at the radiator with a 90° bend to the right, then 90° down, 90° left under the intake pipe. Then 90° towards the back, under the alternator. I then used the stock upper radiator hose pointed to the rear corner of the head. Finally one last 90° bend to the thermostat cover in the back. Fits perfect. I used 10 cm long pieces of 30mm pipe to connect everything. There's a lot of bends, but it looks very good. I had to remove the manifold brace for clearance under the throttle body.

The miata cooling system has one small disadvantage though. You will note that the heater is a bypass around the radiator. The dash heater temperature control mixes heated air with fresh air. It does not affect the coolant flow at all. There is always full flow of coolant through the heater core, even when the heater is set to full cold. On a stock Miata, this is what cools to some extent the back cylinders. If the "genius" Mazda cooling engineers had restricted the flow when the heater is not used, the poor engine would get damaged even faster. The problem is that in hot weather this allows hot coolant to re-enter the engine without being cooled by the radiator. Blocking this flow in the summer will improve cooling. Jason Cuadra tested it by clamping the heater hose with vise grips. Result was 3°C cooler on a hard hot uphill run, which is about the same cooling effect as a small oil cooler (on the coolant temps). I imagine the effect will be greater during low-speed and idle.

One way to stop flow through the heater automatically is with an oil cooler 'stat plumbed in after the heater, such that it closes when the water is hot. In winter it will close only partially; when heater outlet drops below 180F it will partially open - therefore it will regulate heater outlet to be 180F, enough to get a warm heater in winter, yet it will close in the summer.

At the same time, I also made a dual feed fuel rail.

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