I looked through most of the hundreds of topics on the Aluminium AST Upgrade but can't seem to find an answer to my (hopefully simple) question,

Do the replacement AST's on the market increase the BAR pressure in the cooling system?

My current setup runs at 0.9 bar does the replacement AST run at a higher pressure and therefore put more strain on the piping/cooling system?

I know that there was a factory recall in '94 regarding a cooling problem and the system was dropped from 1.1 bar (I think) to 0.9. Was the problem the splitting of the AST Tanks?

 

Actually the recall reduced pressure from 1.3 to 0.9. Some people think that was too much and settle in the middle at 1.1. I think the M2 AST deletion cap is 1.1, and the Petit Aluminum is too.

 

before you go with an elimination kit make sure you bleed the coolant system or other wise it may cause hot spots. I think keeping the ast is a better choice. Even in japan endurance races the ast is being used, go figure.

 

To answer your question, NO, the AST does not increase the pressure by itself. It is just a cannister. You still put a pressurized cap on it that determines the pressure. Any of the aftermarket ASTs you buy will come with a 0.9 bar (13 psi) cap.

 

All I can contribute is that in the Tri Point AST kit, the documantation says it will run higher pressure. I guess it depends on what cap the one you have has.

 

This is simple: the cap determines the pressure. Here's how a radiator (or AST in the case of the FD) cap works:

The cap has a heavy spring. When you install the cap, it seals against the seat in the neck that the cap installs onto. Installing it compresses the spring a little, so the spring is holding the cooling system closed. There is another seal under the cap that seals against the top of the filler neck. When the pressure in the cooling system overcomes the heavy spring in the cap, fluid comes out between the lower seat and the top of the filler neck. The only place for the fluid to go is out the overflow tube to the overflow cannister. The stiffness of the spring determines the pressure at which the cap will vent. A 0.9 bar cap has a lighter spring than a 1.1 or 1.3 bar cap, so it takes less pressure to overcome the spring.

When the system cools down, you get vacuum in the cooling system. The cap has a second, much softer spring in it that holds a little valve closed in the bottom of the cap. If you look at the bottom of the cap, you'll see a little brass (or some other metal) button in the middle. You can pull the little button out from the bottom of the cap. Under pressure, the pressure seals the button against the rubber on the bottom of the cap. Under vacuum, the little cap pulls away from the bottom of the cap and allows fluid to flow back into the cooling system. Provided the overflow tube doesn't have any leaks in it, it will pull fluid back into the system. If there is a leak in the tube, it will just pull air back into the system (which is not what you want -- you need to pull fluid back).

The cap on the filler neck of the FD in the stock configuration just seals against the top of the filler neck and does not vent any pressure. This cap is much simpler and contains no little valves. You can use a spring cap in the position, but you can't use the simple cap on the AST as it will be like a zero pressure cap -- the fluid will just overflow out of the AST when there is any pressure (not good).

That's how it works. The little radiator cap has a lot going on.

-Max

 

Wow Max!

Always have the best explainations as to how things work! Do you work for www.howstuffworks.com?

 

Thanks guys that's a fantastic amount of information!

One Aluminium AST heading to my engine bay right now!

 

great info, now the question is, what is better for the car, a .9 bar cap or a 1.3 bar cap? (or in between?)

 

It's the old benefit/risk story.

Higher the pressure(1.3 bar in you example), higher the temperature of the coolant will reach before it starts boiling. (Boyle's Law PV/T=constant). Therefore the higher pressure cap will allow the coolant to work at higher temperatures.

However, the higher pressure will tax the coolant system more. It's obvious that a higher pressure will likely stress an old hose to rupture whereas a lower pressure will less likely to do so.

So you decide.

 

What is the general opinion on either upgrading to an aluminum AST or removing it all together?? I have heard a little of both, but I'd like some actually info to back up the decision. Thanx in advance. Laterz.

Zach

 

https://www.rx7club.com/forum/showth...threadid=30651

There isn't a general opinion. Without it, you'll have to make sure to burp the system "really good" to get out any excess air (which the AST is designed to do).

There are pros and cons to upgrading it and removing it. Just depends on what you want to do... However, you should in the least do one or the other and rid the car of the stock plastic AST.

 

Max, thats some great information. I'll most definately be keeping the AST, after upgrading it that is. I still havn't read any real reason the eliminate it. I figure if Mazda put it in their, they did so for a good reason.