Bob Bundy's 1.6 Coolant Reroute

Written and conceived by Bob Bundy.

I am in the process of rerouting mine on my 90. But I am basically rerouting everything almost identically to the way it came on my 88 323 GTX.

B6t GTX manifold

The hole in the front of the head gets Blocked off with a 30mm freeze plug and the outlet to the rad with the thermostat is placed on the back of the head. I am routing mine to the rad under the intake manifold instead of over the exhaust manifold as others have done.

  The outlet to the Heater comes out the back of the head as well. To do this I welded an area up on the intake manifold and added a hose fitting so it comes out of the head through the intake manifold between cylinders 3 and 4. This is the same as the GTX. A hose goes from here around the back of the engine and back up to the heater connection at the firewall. It needs to be a bypass of the thermostat.

The Green connector on the bottom is the temperature sending unit that is normally in the heater core outlet housing on the 1.6l Miata. The one that is on the radiator outlet housing that I moved to the back of the engine is the fan switch. I think both of these are placed slightly different on the 1.8l versions and the 99+ head is different yet. I re-welded the boss for it to make it easier to get to the connector. It gets real close to the firewall otherwise but it might fit. I am not sure if the M1 1.8l has the same coolant ports hidden by the manifold or not but I am sure the 99+ doesn't.

Some other small hose connections are important. I welded two small hose nipples on the steel hard-line that goes under the exhaust manifold.
One hose connects from the head just before the thermostat at the back to one of the nipples. The Miata head already has a nipple for this hose that is capped off with a rubber cap. This is important to keep a small amount of coolant flowing across the thermal actuator of the thermostat. So it can sense what temperature the rest of the engine is.

The other new nipple is for return coolant from the Turbo. The source coolant for the turbo comes from the side of the block, same as 323 GTX. The Miata block has a plug in this location sort of down by where the turbo oil source location used by FM and the GTX is. Just need a small hose barb that threads in place of the plug. I have one because I have a spare GTX engine.

The small coolant return line from the intake manifold stuff just gets plugged into the nipple on the front of the engine at the water pump inlet.

If you want to see what the diagram will look like you can just look at a diagram for a GTX and imagine rotating the engine 90deg while keeping the hoses attacheed to the rad and the heater core. This is the way it was originaly designed and it seems to make sense, definately works well on my GTX. I might be able to email some pictures. I have a GTX shop manual.

I have everything ready for my to install and I plan to start this weekend.

>Do you mean under the thermostat at the Miata stock location or the 323 stock location?

The small nipple I was referring to is on the back of both heads. It points towards the intake side of the engine and is right on the boss for the Thermostat housing. The GTX plumbs it back into the hard line returning to the water pump from the heater core. On the stock Miata it is still there but it is capped off.

Looking at my 90 Miata head and an 88 gtx head the only difference I can see is that the Miata head has additional machining in the front so the thermostat housing pedestal can be mounted to it. If you take the pedestal off the Miata there is a machined bore that accepts a 30mm freeze plug which the GTX has at that location. The blocks are Identical as far as I can tell.

I have not had the chance to look closely at any of the 1.8ís BP/BPT to see how they might be different or even how they plumb the sandwich cooler but they were used in the same all wheel drive configurations as the GTX so Iím guessing they might be the same or very similar. The 99+ M2 heads might be totally different however.

Well I don't have a 1.8l head to look at but I do know there a little different than my 1.6. The US never got the 90-91 1.8l GTX so I haven't seen that either. We did have a 1.8l Ford Escort GT that had the same engine and head design as the 1.8l Miata and it mounted the engine transversely. So I'm fairly certain there is a way to do it very similar to what I did.  Its going to be a little different though. The way I did it has all routings the same as the 1.6l B6T GTX. My set up seems to work great now.  Before I did it I had overheated on a track day and the temperature would even rise wile driving up mountain roads. I have done two track days since and the temperature stays rock solid. The engine and the heater still come up to operating temperature as it should. I am running an FMII at 15psi and I drive the car as a daily driver as well as autocrosses and open track days so I still have air conditioning hooked up as well.

It looks like there is a freeze plug between 2 and 3 from pictures I have found not sure if you could use that or not. It also looks like the back of the head looks much different as well, might make it really hard to go under the intake manifold like I did. I am not sure what the plumbing looks like for the oil filter sandwich cooler some of the 1.8l's have. It might be possible to tap into that somehow. I am not sure that the 99+ head was used in any car other than the Miata so it may have eliminated some of the options for coolant routing as a cost savings.

I think getting the heater source from the front of the head as others have done will work but its not Ideal. I have heard Richard M's kit might pull coolant for the heater core from the back of the head somehow but I have yet to see pictures of how that's actually done. Pictures I have seen have just swapped things from the front to the back and back to the front.

Something seems really wrong with the way this one is done to me.
The source for the Heater core circuit comes from the upper rad hose after the thermostat. with the thermostat closed before the engine gets hot it will try to suck coolant from the top of the radiator. Since the radiator is being sucked on from the bottom at the same time by the water pump I don't think their will be much if any flow through the heater core or the rest of the engine for that mater until the thermostat opens. And even when the thermostat does open the heater core circuit will serve as a bypass to the radiator in parallel rather than in series as the author seems to indicate.  I bet you don't get your heater to heat your car until the engine gets hot enough to open the thermostat this way. And I bet there is really spiky and unstable temperature fluctuations with the thermostat opening and closing.  Might be fine for a track only car but I think if that were the case then just remove the heater core entirely.