ronarndt

To upgrade the AST to an aftermarket replacement or to remove the AST. There is no clear winner in the opinion poll. However, why is the AST there in the first place? (I know- to remove air). But why would the Mazda engineers, who were trying to remove every last gram of unnecessary weight (drilled brake and clutch pedals, no body sound deadening material, etc.), put the AST on the car in the first place? It was not needed on the 1st gen or 2nd gen, so what about the 3rd gen was introducing enough air into the cooling system to warrant a new piece of equipment? Keep in mind the cooling system on the rotary engine is nothing special. It is a sealed, pressurized system with a syphoning overflow tank, just like on grampa's Chevy station wagon. Air will not just seep into the cooling system against the pressure, so it has to come from a source of higher pressure, like the pressure side of the turbos or from engine combustion chamber compression. Or...during the cool down phase when it back syphons from the overflow tank through a leaking hose connection. Also keep in mind many people have removed the AST with no ill effects (mine is removed 60,000 miles and two years ago)

OK, you armchair prognosticators- this is your chance. Any theories? Or better yet, does anyone have access to the Mazda design committee notes on the 3rd gen during 1990 and 1991?

pnoy1duh

iTrader: (8)

conspiracy theory says: extra part to break, part cost about $15 to produce, mazda charges ridiculous amount to replace, more money for mazda
j/k

13brv3

Here's a SWAG (scientific wild *** guess). The air would come from boiling of the coolant at the hot spots of the cooling system. Ideally, it shouldn't ever happen, but it probably does anyway under the right conditions. I still don't understand why they use that low pressure cap, unless it's due to all those stupid spring clamps.

Cheers,

ronarndt

Yeah-like $125. Could not believe the dealer price.

Boiling coolant would produce either water vapor (steam) or ethylene glycol vopor, both of which would condense back to their liquid form as soon as they got away from the hot spot.

Trexthe3rd

After a lot of reading, studying and experimenting, here is the answer.
1. The AST wasn't needed in the FB or FC because they did not run as hot as the FD.
2. The 13B coolant paths did not change going from the FC to FD (ie. the same flow capacity but now a hotter running engine, just look at the ratio of cooling related failiurs in the FD vs FC)
3. The AST was put there because the Mazda engineers knew that it would be the cheapest way to expand the cooling capacity to allow for the additional heat generated (instead of reworking the entire 13B block).
4. Is the AST necessary? It depends on a combination of your mods and driving style. Example 1, if you use the car just as a normal daily driver then no AST will not have an adverse effect. Example 2, If you do track events (on stock system with higher boost), you definitely will need the AST. Example 3, single turbo running rich a/f mixture with occasional track time, no AST is fine. It all come down to how hard (hot) you push the engine. The cooling system design did not provide much margine for conditions other than stock.

I hope this answers you question.

The DJ

I've said this alot of times:
The AST is there for capture of air bubbles in the cooling system. This is due to the extreme angle the stock radiator sits at. When adding a FMIC (front mount intercooler for those who don't know), the stock rad is pushed back farther into the engine bay and is sat upwright.....thus eliminating the need due to minimal air bubbles forming.
The people who usually eliminate it have a FMIC themselves so really you can go either way........I hope this helped........

j9fd3s

iTrader: (3)

i remember seeing a mazda bulliten years ago dealing with engine failure.
the 79-85's fail because of plugged radiators
the 86-91's fail due to air pockets in the system.
if you look the fc's did have problems with air pockets in the system, they went thru several revisions of the cooling sysem over the life of the car.

mike

ronarndt

OK, the AST removes air from the cooling system. Already knew that, stated in the original post. Air bubbles forming in the coolant. How? Air will not just mysteriously appear. Assuming the system is properly purged of air after the initial fill of the system, the remaining small amount of air will collect in the highest part of the system, either the AST or the top of the coolant filling opening. After a few heat and cool cycles, the air will go past the radiator pressure cap and be purged into the coolant overflow tank. Again, assume there may be a small amount of air left in dead space pockets in the block that do not circulate well with the water pump. This air will mostly stay in place. So........... where does the additional air come from that warrants the AST in the 3rd gen? There are some additional coolant passages in the throttle body and around the turbos. Any evidence that these may leak slightly to allow the pressurized air from the turbos to get into the system?
Japanese forum members- any chance of getting design notes or speaking to one of the design team engineers at Mazda? In addition to learning the reason for installing the AST, I would like to know why they used such a ****-poor design- I mean, gluing the top on the thing- that is bush league. They must have known that it would be one of the first things on the car to fail.

KevinK2

fd has 221F fan trip (emissions spec related?), and 240+F before gauge moves. right combo of low glycol% and low pressure and high boost could result in sluggish condesation of vapor from nucleat boiling.

vapor pressures, coolant psi:

temp __ water __ 50/50

212F __ 0.0 ___ -4.0
230F __ 6.1 ___ 1.1
250F __ 15.1 ___ 8.0

1.3 cap = 19 psi (orig cap).

also, with flatter rad, more hoses, could be more difficult to get air out, so ast would help collect and purge trapped air.

I keep system healthy, and just use the 'mini' ast that results from tossing ast and plugging ports. I described it in cirian's site a few years ago. coolant always 100% full.

note stock or replacement ast's not to be confused with hard core race units that take the full flow of the coolant, not just a very small %.

ronarndt

Some interesting thoughts, not sure anyone has really nailed down the real reason for the AST. The whole poorly designed AST system seems like a last minute fix to meet a production deadline by the engine design team. If it were a problem with the total volume of the cooling system being inadequate, a larger radiator would be the obvious solution. Was there not enough time to get the bigger rad in place? Was the increased cost a factor? Was there more air being introduced somehow that the conventional syphon tube-to-reservoir tank used in the 1st and 2nd gen cars and most other cars would not keep up with it? Someone mentioned the angle of the rad- I don't see much difference between the angle in my FC and my FD, so if that were a problem it should be in both gens. It seems that the only source of air going against the pressure of the coolant system getting in would be the pressure from the turbos. But how- leak around throttle body? Leak around turbo in the coolant passage? If this were the case, it would explain the AST being a bolt-on, last minute fix to meet production deadlines. To re-work the turbos or the throttle body would be a major delay and would have stopped the introduction of the car on time. Any thoughts on this?

pomanferrari

iTrader: (1)

You ask where the air is coming from?

The combustion chamber!!! The o-rings are not labyrinth type and will not keep air out. Furhter, due to repeated heat cycling, air will enter the combustion chamber.

Ever wonder why the Eurpeans all run AST on their cooling system? Must be something to air pockets in the system right?

Anyway, I installed an expansion tank from a Volvo S70 (20psi cap) instead of the OEM AST so I can see coolant level by just opening the hood. The tank is 2 or 3 times the capacity of the OEM AST and works great!