My AST sprang a leak on my 3-92 made R1 while under warranty and it was not a race car and was bone stock at the time. My wifes 11-91 made MX3 AST also broke its AST while under warranty. I am such a nice guy that if you pay shipping, you can have my RX-7's second AST for free!


So much for the theory of breakage on only race cars!

 

I do not like hearsay and gut feelings. I replaced mine with the metal Pettit unit. Nice thing about an AST is over time you will not need to burp your cooling system of air. Air in your system does not cool as well as water/coolant. Mazda knew the 13BREW's heat output would be greater than even the TIIs, therefore logic suggests that they wanted to avoid the problem of air in the system at all times to reduce chances of losing coolant effectiveness. Yes it does add another point of failure and complexity to the cooling system, however IF you look under your hood at least once a week and visually inspect the system, as everyone should with any car/truck, I think such coolant problems can be avoided.

 

Geez Chuck and you were AGAINST the V8 swap?

 

Does anyone know if the last years of the RX-7 in (Japan) kept using the AST? If so I think they "might" have a reason and a chance to test it's effectiveness and thus opted to keep it or ditch it.

 

hugh, thats a decent point. My opinion is to ditch it. No one has reported problems with doing it and the less clutter you have in your engine bay the better. Actually I used the M2 kit and after, I regretted ever doing that since all you really need is a cap from an older rx, some hose, and a few small clamps. I saw the same cap I used on my dads 86 parts car the day I did the removal. Actually if you are really cheap you don't need anything besides the cap. There is enough stock hose already there to just totally bypass the ast. I think you might have to stretch the hose a little though. don't do that.

 

I eliminated it but I am seriously considering replacing it with a OEM one. The original one lasted 7 years.

 

I think Mazda kept the AST, Mazdaspeed has a new Aluminum one for sale now


I personally removed mine, and plugged up the line from the filler housing to the radiator, no problems so far, no overheating, temps stay at a constat 85c/185f when cruising.

I think if you are really worried about bubbles, then tap some AN fittings on the highest part of the radiator, radiator housing, and the throttle coolent and run all of them into an AST type of can and place it at a place higher than the rest of the engine. Something like this....


David

 

David,
Where can I find one of the Mazdaspeed ASTs at?

 

mazda had the AST for a reason. I would go with an aluminum AST. not sure which one is the best. but definately replace it.

1FAST7

 

Japan?

David

 

just polish the aluminum ast's you can buy here... it'll look like the mazda speed minus the name-brand cap.. i did that to mine

 

I have the current Japanese RX7 brochure and it shows a side view of the chassis in it. You can clearly see the same plastic AST that we have in it. as for the laundry list of "bad designs" mazda did have some non-robust designs, but no more that are present in all mass produced vehicles; things that normally get worked out in the subsequent model releases after the weak link is identified. however i'm sure mazda engineers could back up their decision to add an AST to the system with several books full of test data. since they did lower the blowoff cooling system pressure with a recall (i think its at 0.9 bar now, vs. 1.1 bar prior) and cavitation (bubbles) happens at high pressure, you might have a case that the cavitation risk is no longer high enough to need an AST, which would be why those of you who removed it haven't had a problem. But since its still on the J-spec car i think they still have a justified set of circumstances where the AST helps out. So unless you are capable and willing to monitor your cooling system health, I would say don't remove it. Obviously some of the mechanics on this board are familiar enough to recognize when they are having a problem, so the risk is very low for them in trying something experimental like removing the AST.

 

My car runs at 160 - 180 with out it, it is not needed, you wouldn't think that Mazdaspeed a Mazda company would try and sell you a $200 part that you don't need do you? Oh I have some splitfire spark plugs, and a Tornado intake that adds 15 hp to if you want it.

 

Chuck ... you always make me laugh when this topic comes up!

Again why aren't people using the search function? I've been on this board for a long time now and we still get this question??? What's next FMIC vs SMIC?

 

I just had a new highly polished alloy AST designed and made here in the UK. They will also shortly be available in the States. I will post more details when i know myself . Check the pics and let me know what you think..........

 

.....and another......

 

I joined this forum just to reply to the thread on why the air separator tank most likely exists.

The issue no one thinks about is backpressure of coolant through the radiator at high RPM/coolant velocities.

If you eliminate the AST, the radiator cap is now on the high pressure outlet of the water pump. Due to pressure loses from the friction of the radiator lines and coolant going through the radiator, higher pressures can build up at the outlet. At high RPM, the pressure in that line will reduce the effective strength of the spring pressure in the radiator cap. If you have 16 psi, it will open at a much lower pressure.

This is important because it will reduce the boiling point of your coolant, making it more easy to overheat your engine.

By moving the radiator cap to a tank that sees significantly less flow, the cap works at its intended pressure. Removing bubbles from the coolant is just a side benefit.

You're all welcome.

Dan

 

Welcome to the forum Dan.
I think the AST was installed by MAZDA due to the lower height of the radiator relative to the system, not for the reasons you’re describing. My car (and many others) has been running without one for a number of years with an even lower (stock) .9 bar (13 psi) spring cap and the addition of an extended filler neck effectively providing a replacement. No issues, and it’s seen it fair share of high rpm.
And generally the next step up in caps is a 1.3 bar or ~ 18.5 psi. Which I always found to unnecessary on an otherwise sound cooling system.
Maybe I missed it but I’ve never heard of the issue as you’ve described. I think the only time this could happen, if at all, is on a cold engine before the t-stat opens. And no responsible owner would go high rpm before that. Even then the path of least resistance wold be the radiator.

 

Thanks for the welcome. I come from a racing background and systems such as what Mazda designed are used for my exact reasoning (on top of a few other things like air springs), which is what lead to my guess. It's not to say that the other way won't work, Mitsubishi Evos, countless Hondas and I'm sure a million more cars go with a radiator cap on the high pressure side of the system and they work, although we need to keep in mind that these designs are for primarily street (and cost savings) applications.

Given how much heat rotaries produce and how sensitive they can be, I personally feel that a better way to go about it is to use the system as Mazda intended, perhaps with upgraded components that can better handle heat and pressures. This would be more important at high power (aka, high heat) outputs or sustained high RPM usage.

For more reading, I think Steward Components, which manufactures NASCAR level cooling systems, has a great tech section on the matter.

https://www.stewartcomponents.com/in...formation_id=9

P.S. A good friend of mine is working on his FD cooling system and it lead me down a rotary shaped rabbit hole and I'm just trying to offer some information that may be useful and come from a perspective that others hadn't seem to have brought up yet. Thanks again!

 

I don’t have a tech background and didn’t eliminate the AST on a whim But I think MAZDA’S purpose of the AST is in its name...AIR SEPARATION TANK. Not pressure relief tank.

 

I see names dictate the sole purpose of what parts can do. Kindly remove the napkins from your glove box.

Dan

 

No reason to become smarmy... Dan. And I don’t keep napkins in my glovebox and MAZDA didn’t name the glovebox. But they did name the AST.
I gave you my reasonings and experience with the FD. You gave me yours with no specific experience.
Unless I misunderstood, you’re suggesting one small line a few millimeters in diameter from the filler neck to the AST is enough to avoid an over-pressure situation from the water pump that would overwhelm a spring cap if that line and the AST were removed. An over-pressure situation that I haven’t heard anyone experience...even with a lower (13 psi) cap than you suggested.

 

This thread resurrection is almost old enough to legally drink, holy ****! But you couldn't wait one more day?! What do you have against a clean 19 years?? lol

On the topic though, I have been without an AST for about 12 years now with no ill effects. I've never had cooling issues or lost an engine due to coolant seal failure or whatever else cooling related. While I understand there was a purpose that they put it in, I haven't personally experienced any problems with removing it.

 

The extra pressure pushing against the rad cap does make sense from a theoretical standpoint but it doesn't hold true in real-world testing. MANY people have run without AST's for years, under hard and track conditions, with zero problems. Myself included.

If someone has a bone stock car, IMHO just get a good aluminum AST, plug it in, and you're done. When you start modifying the intercooler setup the AST quickly gets in the way without a good home for it, in that case it's time to eliminate it.

The general idea of Mazda's purpose for the AST is to give a high point to burp air out of the system and also have a spot for the bubbles to come out of suspension if their is trapped air in the system. I also think part of the idea was Mazda ran the FD REALLY damn hot stock to try and get fuel economy and emissions where they wanted, the FD will get to 107 deg. C before the fans kick on stock.

Regardless this is an interesting idea, but again not borne out in real-world testing.

Dale

 

I'm just floored by the salt and arrogance required for this dude to make an account solely for the purpose of coming in here to bump a 19 year old thread so he can argue about it.

That said, I *think* what he was thinking of/referring to is a header tank? It acts sort of like an AST, we're adding one in my build.

It functions as the absolute highest point in the engine bay, and coolant lines route to it after cooling the turbo/wg's, the upper port on the rear iron feeds it as well. The pressure cap goes on that, as opposed to an AST, the fill neck, or a radiator.