Water cooled oil cooler (heat exchanger) Mazda MX-5 Miata

The water cooled oil cooler is the new generation of the air coolers.
We would like to introduce a water cooled oil cooler for racing with Maruha’s original design.
What is the difference between the usual air cooled oil coolers and Maruha’s water cooled oil cooler?

Differences between air cooled oil coolers and water cooled oil coolers

1. The air cooled oil cooler uses air resistance for cooling the oil (it cannot be cooled if you’re not driving)
2. The water cooled oil cooler uses engine coolant for cooling the oil (if the engine is running, it can be cooled)

Most of the oil coolers sold on the market used to be air cooled ones.
An A5〜A4 sized air cooled oil cooler core is built in in front of the radiator.

The features of air cooled and water cooled oil coolers are as follows:

Air cooled oil cooler

1. Merits
• The kit is cheap.
• l The coolant is nothing else but ‘WIND’, that’s why the cooling efficiency is the biggest, when the outside air temperature is low.
• The kit’s composition is quite simple and it is easy to commercialize.
2. Demerits
• If you install it in front of the radiator, the A/C condenser and radiator's cooling efficiency lessens.
• The length of the oil line from the engine to the core becomes long.
• If you’re not driving, you cannot get the cooling effect
• The efficiency is influenced by the outside air temperature very much.
• The cooling efficiency changes accordingly to the running speed, that’s why it’s difficult to stabilize the oil temperature.

Water cooled oil cooler

1. Merits
   • The coolant is radiator water, that’s why we can get a stabilized cooling efficiency.
   • It is not set in front of the radiator, that’s why it doesn’t affect the A/C condenser and the radiator.
   • It is possible to set a very short oil line in between the engine and the core.
   • Even if you’re not driving, it is possible to get the cooling effect just by making the engine run.
   • The efficiency cannot be easily influenced by the outside air temperature. (in winter the oil is heated by the coolant and in summer it is cooled by it)
2. Demerits
   • The kit’s price is high. (especially the water cooling core is expensive)
   • The waterway’s construction is difficult
   • Because it is set inside the engine space, the hose has to have a special shape.

You can see that the original setup of the water cooled oil cooler is difficult. However, it's got lots of merits and for this reason it is being used in these days’ racing engines too.

The details

Let us explain you more about each part.

Air resistance and coolant

The area of Miata’s (Mx-5) front bumper grille’s opening is not too big.
In the back part of the grille the air conditioner condenser and the radiator are set up in a double way.
To that we set the air cooled oil cooler core and we cover it with air resistance. This creates a severe situation.

Air can flow through the core of the air cooled oil cooler, A/C condenser and the radiator.

This construction allows the air to flow through them one after another, but does it flow through completely? The answer is ‘’NO’’.
The core becomes the air’s resistance and so the air that cannot enter into the core, flows by.
Also, it hits the oil cooler of the front row inside the small grille and cannot continue its way, that’s why the reflected air creates a piled-up air space.
We talk about the air that is pressed against the piled-up layer, the air that flows by and the air that cannot enter the grille.

If we think about these facts, we might want to set up the cooler core outside the grille. However, if we don’t use a special aero bumper, the setup might be difficult.

The point which Maruha considered the most is the following: To make the A/C condenser and the radiator function properly, we don’t use an air cooled oil cooler but the water cooled oil cooler.

The coolant and the cooling

Let us explain about the water cooled oil cooler once more.

The water cooled oil cooler’s basis is: water temperature + 10 degrees = oil temperature.
The cooling happens using the water’s temperature, that’s why the oil temperature doesn’t fall under this level.
It is important to verify if this oil temperature is the suitable temperature.

The water cooled oil cooler’s basis is: water temperature + 10 degrees = oil temperature. In other words, the oil temperature should be set to around 90-100 degrees.

On the contrary, in winter it is being heated by the water temperature, that’s why it should be called HEAT EXCHANGER.
It has the function of cooling the engine oil and in other cases it has the function of heating it.
This means that the oil temperature must be constantly controlled. The engine's efficiency can be stabilized by this control. In the cases of racing engines air cooled oil coolers are not being used anymore.

An appropriate water temperature and oil temperature control is necessary to reach stabilized combustion needed for the engine.

The cooling core’s structure

The cooler itself consists of housing and core.

The core is a tube, the outside of the central main waterway is covered with a small honeycomb waterway and in this way it becomes a doubled construction.

Compared to the standard water cooled core its efficiency is 5 times better and we have already obtained the patent.

Water and oil cannot be mixed, but they enlarge the aluminum’s surface and they raise the heat transfer efficiency.

The oil cooler’s efficiency

Below you can see the maker’s official cooler efficiency chart of Maruha's core.

SIDE 1・・・oil flow / minute
SIDE 2・・・water volume / minute

INLET (water temperature at the entrance)
SIDE1・・・oil
SIDE2・・・water

OUTLET (water temperature at the exit)
SIDE1・・・oil
SIDE2・・・water

The general circulating water amount of the 4 cylinders is 40〜60L. In this test we set the oil temperature to 120 degrees and the coolant temperature to 85 degrees.

As you can see, after cooling the oil temperature changes, it is in the range of 98〜113 degrees. When achieving around 20〜7 degrees cooling, the coolant's temperature settles on +1 degree.
The heat transfer efficiency of 40〜60L of water volume is very good and the highest recorded cooling data is 21.5 degrees.
The pressure loss is quite small and it contributes to a certain oil supply.
Furthermore, the radiator water temperature is stable and the volume inside the radiator has reserve of capacity.
The engine’s oil is being cooled, but it isn’t connected to the immediate overcapacity.
If the coolant seems to be overheated, it means that there is a problem with the cooling efficiency.

The core’s location

It is not easy to install a cooler core of this size into the narrow engine space. We have to follow the upper hose’s limitations.

Squeezing it into the lower hose after the cooling might cause that the temperature of the coolant returning to the engine is going to raise, that’s why we installed it into the upper hose.
However, the upper hose’s outside diameter is just a 30mm. It is an ‘S’ letter shaped hose.
We bend the hose coming out of the thermo housing 90degrees and drop it directly below.
At the bottom we turn it into a‘U’ shape and at just at that point we install the water cooling coil.

The bottom stay's shape differs in the case of NA8 and NB8.
Left: for NA8
Right: for NB8
The stabilizer is being fixed by the bush bracket’s bolts.

To the hose we set up an air extracting plug. We use it when extracting air from the radiator’s water. Moreover, the pitch is 1/8PT, that’s why it is possible to install a sensor for the electronic water temperature meter.

Oil block

In the case of the oil block the aluminum billet is treated with alumite.
We make efforts to reach a light design.

We install it to the oil cartridge in ‘’sandwich-style’’.
We set up two plugs and the 1/8 PT’s water temperature and oil temperature sensor’s installation becomes possible.

We use Mazda’s oil cartridge.

The intake manifold’s genuine stay has to be cut in order to avoid interference.

Oil line

The line is AN10 size and it secures sufficient oil volume.
On the market we can find AN8 cooler kits too, though there are some fine differences.

For fitting we use forged type, which is strong and expensive and makes the line solid.
Furthermore, we have to underline the line’s length.

The two In-Out (Collect-Return) lines are extremely short - only 60cm.
In the case of air cooling installed in front of the radiator, the length of the line becomes 2m.
The winding and the long line might affect the oil supply’s pressure drop.

Silicon hose

The silicon hose from SFS comes in the colors RED and BLUE.

We divide the radiator’s upper hose into two and build in the cooler in between.
It is a setup using the narrow space.

In the case of the hose’s part for insertion we used a stainless band.
It is not a type where there is a hole in the band’s center. We only set up a ditch and we use a European style one that can stabilize the torque.

In some of the silicone hoses we set up an air extracting plug.
This part is being plugged up by an 1/8 stainless plug but it is also possible to install an electronic water temperature sensor.

The development’s summary

During these years Maruha has been developing the water cooled oil cooler.
Its efficiency is good and its price reaches that of racing unit items.

For example, this aluminum adapter is the product of accurate machining.
Not only the designing is difficult, but the NC machining costs are quite high too.
It is a product which costs a lot of time and money.

For many years we have been thinking of preparing a high efficiency cooler kit for our customers.
The price might seem high but if you get to know the details better, you might even think, that it is quite reasonable.
There is a possibility of buying only the lower hose.

For NA8

NB8

NEW OIL!!

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