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Today at Atelier Kaz - Private NSX Enthusiast, ex-Honda R&D engineer with F1, Indy/CART background

OEM Header Tank 02

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A while ago, I noticed that my blog regarding the OEM header tank here (http://www.nsxcb.co.uk/entry.php?132...nt-Header-Tank) was referenced at NSX Prime with some questions on the tank design.

So, I started writing multiple small topics over the last several days and just started compiling all of them into one piece.

It will take several days if I start talking about the cooling system on our NSX in detail so I'll just touch on the basic.

As each topics were written at different timing, it may sound dis-oriented but hope you will get the idea on the design background.


The main purpose of the expansion tank is the separation of the air from the coolant.
It is also mentioned on the English workshop manual in page 5-15.





With ordinary production cars, the radiator, the engine and the tank (reservoir) are located closely together.
The (radiator) cap is attached to the filler neck of the radiator and it is the highest point of the cooling system.

With this layout, the tank indeed acts as the reservoir.
There will be a tube running from the radiator filler neck to the tank and the other end of the tube is submerged in the coolant inside the reservoir.
There is another short tube at the reservoir allowing the internal tank pressure to be at the atom pressure so that it can act as overflow tube as well as allowing the air to enter the reservoir to prevent negative pressure when the coolant is returned to the radiator during the cool down process.

As the engine warms up, the coolant expands and the radiator cap controls the system pressure.
When the system hits the pressure threshold, the expanded coolant will be transferred to the reservoir.

Any air that may have been trapped or generated in the cooling system will be captured at the filler neck since it's the highest point of the system and the hot coolant normally enters the radiator from the upper hose.

Then, the captured air at the filler neck will be eventually pushed towards the reservoir to eliminate the air from the cooling system.
When the system cools down, the coolant also contracts and reduces the volume. So, the negative pressure valve at the radiator cap will open allowing the coolant to be returned back to the radiator without any air entering the system.

Every time when you start the engine, drive the car and then park it, it's the repeat of the above process so you need to keep the coolant level above the MIN line of the reservoir.
With this layout, the entire cooling passage is fairly short so the air can be separated without extra process.
Because it's fairly short passage and simple design, the system 'may' have wider margin against the flow friction/drag causing cavitation at the WP.



On our NSX, it's a different story.
The engine is at the back and the radiator is at the front. Therefore, the entire cooling system passage is quite long and needs careful consideration for removal of air for the cooling efficiency and prevention of the cavitation especially at the WP.

I have never measured the reduction ratio of WP pulley against the TB drive pulley but if you have ever worked in this area, you can imagine how fast the WP could be spinning while the engine is running.


From WP point of view, the coolant flow returning from the radiaor could be a friction/drag factor if the flow was not smooth enough and could cause cavitation.
This is probably one of the reason why NSX is using expensive WP compared to the cheap one on ordinary cars that tend to use impeller manufactured from just pressed metal sheet.

I saw several cheap WP impeller smashed due to cavitation because the owner left the coolant service for ages.

You don't need to have visible air inside the system to generate bubbles.

While in the hot bath tab, please try moving your hand side way rapidly in the water to fight against the wall of the water.

If you managed to move your hand fast enough with ideal gap between each fingers, you should be able to generate lots of air bubbles. That's the cavitation and since your hand was already submerged in the water, there was no visible air under the water surface. You can see the same thing happening at the screw of the leisure boat, etc under certain conditions.
Under certain applications, the design of screw becomes top secret.
The force of continued cavitation is enormous. It's like shaving off piece of metal from the impeller surface by each bubbles.

Eventually, you will end up with WP spinning with missing impeller resulting in not enough coolant flow and causing over heating situation.

Bubble can be generated when the coolant goes through that narrow channel around the cyl bore or passing through the small holes/pipes.
Also, if the engine was stopped immediately after driven hard, it can cause heat spot. You will be surprised how some of the journalists stops the engine after the hard test driving session.
You feel like you can really hear the coolant boiling inside the engine......


So, it is important to separate the air from the coolant for the efficiency point of view.
Air can't hold much energy compared to the water.



Updated 24-11-2014 at 12:35 PM by Kaz-kzukNA1 (not great writing in English.....)

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Cooling

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