Storm Knight

iTrader: (1)

OK, here's the scoop.

I have the Pettit style (can) AST, now over a a year and a half old. (I am a firm believer of leaving the AST in, but with more durability (i.e. metal.)) During my morning check, I noted some coolant around the neck of the AST, beading around where the neck meets the rest of the AST. A few drops, mind you, but enough to get my attention. No other leaks were found. All hoses were bone dry. The fluid level was fine, both at the fill cap and the AST. I just replaced the pressure cap this weekend. My question is, has there been a failure of the AST welds to cause a significant leak, or is this the new cap seating itself, or what?

vudoodoodoo

I think it's the cap. I have the same AST and it looks solid.
Sometimes it takes 2 tries to get the cap on just right.

rxrotary2_7

just for ***** and giggles lets hear your reasoning in being a "firm believer" of leaving it in. and please dont say 'because mazda put it there, it must have a purpose'

ronarndt

rxrotary2 7- You should check out the thread I started to get the "opinions" on why the AST was put on the car by Mazda engineers inthe first place. While most people seem to grasp that it some how removes air from the cooling system, I don't yet hear a convincing theory how the air (not steam, not compression gasses, not ethylene glycol vapors or any of the other things suggested) gets into the closed, pressurized cooling system.

rxrotary2_7

that is why i am saying...let me hear why he is a "firm believer"
i am not convinced. what is the link to this thread you speak of?

j9fd3s

iTrader: (3)

the metal ast's fail faster than the plastic ones

mike

rynberg

I don't understand why the "onus" of having to explain why the AST is necessary falls on our side of the argument since we are leaving it in. I would think that those of you who think it ISN'T necessary would want a good explanation since you are removing a part that was engineered into the car.....

j9fd3s: where's your evidence that metal ASTs fail sooner than the stock plastic one?

j9fd3s

iTrader: (3)

ive seen several fail right where the cap goes. it rots out right there

mike

DamonB

Storm Knight, check to make sure that the hose that goes from the neck to the coolant bottle is not leaking a little bit at the AST end. If the coolant were slowly dribbling out there I would expect it to collect around the neck as you describe.

rynberg

How the hell does aluminum "rot" out? I've never heard of anyone else mentioning a failed metal AST.

DamonB

Why all the hate on AST pros/cons? Those who think it isn't needed yank it out. Those who think it serves a purpose keep it but switch to an aluminum one. Whatever floats your boat.

Personally I have yet to hear convincing evidence either way, but I could see the very low and slanted radiator contributing to air trapped in the system. Since I don't know better I leave the AST in.

As for air in the system I cannot see how you can ever get 100% of the air out of the coolant system. At the same time racecars have a swirl pot in their oil systems for the express reason of keeping foaming oil (oil with air in it) from circulating through the system. Basically the oil is pumped into the pot and "swirls" around the walls before being drawn back out. This removes any bubbles and ensures a nice non-turbulent flow.

apexkw

yeah how would a metal ast fail before a plastic one? the reason we switch to metal is so that the plastic doesnt expand and break. so i dont understand how a metal one will break sooner. maybe corrosion? well just let hear your reasons as i am in the market to get a metal ast. maybe you can change my mind. you know mazda speed uses a metal ast one their car...so why would they switch from the stock plastic one to a metal one?

j9fd3s

iTrader: (3)

ive seen several of the metal ones fail where the cap seats, the sympotm is exactly the one storm knight and voodododo complain about.

mike

Leprechaun

The neck on the Petit AST's are a little different and done more cheaply than the ASI ones. They are both on rx7store and both almost same price, but I'd go with the new ASI ones.

I have no idea why anyone would want to remove it. to save 150 bucks? hehe yah, that's a good reason.

spooledUP7

I believe the AST was designed to minimize micro-bubble saturation and reduce flow turbulence caused by the centrifugal agitation of the water pump. Since microscopic air bubbles can saturate the coolant mixture and reform at any point in the cooling system, the AST acts as bypasses, or second option for a percentage of micro bubbles to collect and slows their saturation rate. If you take a look at the stock AST, you will note that the bottom hole is only .09” in diameter, much smaller than the actual nipple, and unequal to the size of the side inlet. This controls the flow rate to a much slower, and drastically less vigorous flow environment.
Mazda was all about efficiency, and I think that the AST is a direct result of this mentality. Yes you can run the car without the AST and still remove the majority of air trapped in the cooling system, but if you want to do it better, and more efficiently, then the AST will fill that task.

As far as gasses as a byproduct of coolant undergoing extreme heat contact, I am not sure about, but if you call 1600-2000 degree turbo housings hot, then maybe there is something to it after all. What temperature does coolant and water mixture bubble under 16lbs of pressure? Anyone know?

spurvo

From the Evans Coolant website:

Efficiency at Higher Temperatures

Engines run more efficiently when they're hot. But up until recently, inefficient cooling has limited how hot you can run a gasoline or diesel engine before serious damage occurs. Conventional coolants containing water boil (vaporize) around 225ºF near sea level. Cooling systems are pressurized to raise the average coolant boiling point, to around 250ºF, but that doesn't solve the real cooling problem, which occurs inside the water jackets of an engine.

Inside the water jacket, heat-stressed metal exceeds the thermal capacity of conventional coolant. The coolant boils, forming a vapor barrier at the metals surface. This vapor barrier acts as an insulator and prevents efficient heat transfer from the metal to the coolant, causing localized overheating and vaporization of the coolant.


Furthermore, this super-heated, vaporized conventional water based coolant doesn't fully condense back into liquid until it circulates back to the radiator. Once it's returned to the radiator, it remains in a gaseous barrier to the heat trying to exit to the air until it's fully recondensed. This means the conventional coolant is constantly losing efficiency as it circulates through the hot engine, and even through the radiator. The problem worsens as the system operates hotter because the radiator cannot condense all the vapor and it passes back into the engine.

So the other numbers I saw on the web for typical pressurized system 50/50 boil over number is around 260 F (Prestone), in line with that above. I find it interesting that what the AST may be doing is what we see above, namely allowing the superheated (i.e.from the turbos, as spooledUP7 mentions) coolant to recondense. At any rate, throwing an aluminum AST into mine, but I'll watch the neck for fluid. Might be leaking from the AST to overflow tank hose, like my system tries to do ALL THE TIME right now? Hmmm...

poss

iTrader: (5)

they use an AST on the 787B, so that's good enough of a reason for me. besides, what can it hurt?

vudoodoodoo

I make sure the cap is on right be I start the car. It's just a bit tougher to get it on than putting the cap on the stock AST. No biggie though.