Evans states that NPG+ has these benefits (among others):

3. Locally generated vapor immediately condenses into the surrounding liquid.
4. Pockets of vapor do not form to insulate jacket metal from liquid coolant.
5. The coolant pump will not cavitate (unless there is a blockage at the pump inlet) because of the large separation between the coolant temperature and the coolant boiling point.

That's taken directly from their website.

Assuming that you believe the AST in the stock cooling system serves a useful purpose in constantly purging air from the circulating coolant, does NPG+ make the AST unnecessary in a converted system?

Damian's heard that using the AST with NPG+ makes no difference, suggesting that you don't need the AST. Anybody else?

I emailed Evans the question. Their site says that it may take a week to get a reply. That gives you armchair cooling system engineers plenty of time to speculate.

 

If you purged the cooling system properly you will not need an AST.

 

An AST just makes purging the system of air easier, no matter which coolant you use. With an AST one good burping when you first fill the system is good, without it may take a number of burps. Basically, the AST is not really "needed" even with regualr coolant or with Evans. The AST just makes it more of a fool proof system, until the stock plastic one breaks.

 

Trexthe3rd, care to elaborate? Are you saying that you agree with the points from Evans' website?

Tom93R1, it sounds like you simply believe the AST is not necessary for it's intended purpose, but may serve a useful purpose for burping the system.

I believe that it does work for its intended purpose in the stock system-- to remove "air" that may be present in the coolant stream. Even if you have a fully burped system, boiling can introduce more bubbles (of air or whatever it is). For purposes of this thread, I'm more interested in Evans' listed claims.

[BTW, I use the hose going to the back of the throttle body to burp the system. It's worked better for me than other methods, though I have not yet tried the new attachable Lisle funnel I got a few months ago.]

 

I recall seeing ASTs (or AST-looking items) on the Evans site, which would seem to suggest they think ASTs might be useful.

-Max

 

In a healthy cooling system there should be no localized boiling to create any gas, therefor the need for the AST is not present. I have eliminated my AST for over a year now and never had to add any coolant yet since it was purged. (NPG+ 0psi)
As originally stated, the air purging process is somewhat of a pain (It takes quite a few repitition to get rid of all the initial air in the system) and most people tend to neglect the process. The AST is simply a way to compensate for that neglect.
When you eliminate the AST, the filler neck acts like a mini AST, excess gas is expelled to the highest point in the cooling system (the filler neck).
By the way, properly burping the system has nothing to do with which hose you fill the coolant with. It's a process, not a location.

 

In any water based auto coolant system there will be local boiling at hot spots, argue if you like but these are the facts. The vapor generated is water vapor which condenses back into water when it cools; no AST required to remove this vapor as it is water vapor and will almost instantly condense back to water after it moves away from the hot spot.

When you convert to the older rad cap which vents to the overflow tank directly, you will not need an AST but you can run one if you prefer. I see no benefit in running one, an AST adds more potential leak points in connections, hoses, and the tank itself.

 

Ahem, may I point out the fact that Mazda went to extreme in reducing ounces in components. For example, spark plug wires 1/2 the normal length, drilled pedals, lightweight carpeting, smaller rear green house glass, thinner windshield (2/3 of normal thickness compared to my 626 windshield, I measured it). So, for them to leave a worthless piece of crap plastic tank that weighs about 8 ounces (1/2 a pound) tells me there was a compelling reason to leave it.

That our car uses o-ring (which moves around in its groove) could indicate that at cool down, air could leak into the system, however miniscucle. That Mercedes, BMW, Audi, VW, Volvo, Saab, Chrysler, Porsche use AST tells me that these people know something about cooling system that I don't.

You can leave the AST out. For me, these reasons were compelling enough for me to leave it in. I just hope you sleep better at night knowing you've out engineered a bunch of little engineers.

As for me, I'm installing a much bigger AST tank from Canton at www.canton.com (part #80-200).

 

I was just looking at what they are calling expansion tanks. Although they are not marketing them as such, it sure seems like they could serve the same purpose.

 

I take it you are referring to a healthy Evans cooling system. But even Evans does not claim no localized boiling-- they merely claim that where localized boiling does occur, the resulting gas is more quickly and easily condensed back into liquid.

Although you have not stated it, you're also implying that gas does not get into the cooling stream by means other than localized boiling, and therefore the AST is also not needed to remove gas from other sources.

No one in this thread previously stated that the air purging process is a pain, but I doubt that anyone would argue with the point.

Regarding using filler neck as a mini-AST in a converted system, that makes sense to me. I remember Kevin Kelleher posting that theory some time ago.

Hmmm, okay-- but no one claimed that it did.

 

No one I have ever seen has even come to a conclusion on whether or not an AST is needed in a conventional ethylene glycol / H2O system. No one really even seems to understand the role of the AST.

Based on that, you probably are not going to get a good answer on whether an AST is needed for Evans.

I say leave it in. Why eliminate it? I'm going with pomanferrari on this one.

 

Actually, Evans claims that water-based systems are relatively poor at condensing vapor back into liquid, and that is one of their claimed reasons for NPG+'s superiority.

Yep, that's my plan if it looks like Evans' claims are close to true.

 

Have you converted to Evans, or are you planning to? It's not clear whether you advocate keeping the AST for a water-based cooling system, or for an NPG+ system as well.

Try this URL instead:

http://www.cantonracingproducts.com/...ion_tanks.html

 

Actually, I think that a lot of people have come to a conclusion one way or another, at least for a EG/H2O system-- but they're reached that conclusion on whether the car seems to run okay after the AST is removed. I suspect you're right however on getting an answer with respect to Evans.

I've really been hoping that a cooling system engineer would jump in here. But then I don't recall one jumping into the AST/no-AST debate (for a EG/H2O) system before, so I probably should not expect it now.

It should be interesting to see how Evans responds. I really hope that they don't just send some marketing techno-babble.

 

Even if you do get a response from Evans, I would be weary about following it if, for example, they say that you don't need an AST. I really doubt they are familiar enough with the FD cooling system to make an educated decision.

You're right, a lot of people have come to conclusions, but based on what? So far no one has shown conclusive results one way or the other, and also no one has really been able to explain Mazda's reasoning for designing it into the system. I'd really love to hear what the engineers had in mind when they put it in. They obviously had a good reason for it.

 

So what's the proper process for filling and burping at the lower radiator hose?

 

Regarding the utility of the AST in a stock system, I agree with you. I do think it's fair to assume that the AST is intended to help remove gas from the coolant stream, but quantifying any beneficial effect has never been done on this board, the List, or any public forum that I'm aware of.

As for Evans' response, I'm interested in what they have to say, but of course we know that they have an interest in selling more NPG+.

 

One comment about the praises laid on Mazda and all their little engineers who put the AST on the third gen ... if they were so spot on why in the hell do we spend so much time changing up their sub-par designs.

Recognize that many of us on the Forum are capable engineers and/or experienced folks when it comes to this stuff. We study many forms of motor and auto applications and actually try things to prove our own theories. Some may share these proven theories as fact but that's mostly because we don't have time or space to write up our findings in a non-biased report format.

If you look at various cars you will not find many ASTs. Mazda put an AST on the third gen because the top of the rad is below the top of the motor & they did not feel confident that inerts would discharge through the rad cap pressure vent because of the low volume high relative velocity as compared to most cars radiator end tank where the pressure cap is usually located. Is that enough explaination?

 

That is irrelevant. Just because they under-engineered some other parts of the FD doesn't mean that the AST is pointless. This is an *extremely* weak argument.

Yes, that's one thing I really like about the RX7 community. There are a lot of educated people who you can have good conversations with. I am also an engineer.

This is probably the best explanation I've seen so far about the role of the AST. But you still haven't proven, or implied, whether it is needed or not.

 

Let me try to finish quickly.

Before getting started ... I was not trying to say that Mazda screwed up everything on the FD, simply trying to establish that it was "weak" to use the argument that since Mazda put it on the car that there was a valid reason for it or that it was an optimum design. Purely a personal opinion.

Is the AST needed? No. I believe I know why Mazda put it on but I've yet to derive a technical justification for it to be there. Because of this, it's tough for me to explain why you might choose not to run one if you haven't come to the same conclusion as I have.

Should you run it? Your choice.

Either way you should consider being very careful when filling and air bleeding as the AST will not do it for you. My technique is to fill the system slowly, use the upper block nipples to bleed air while filling, and constantly pump the main upper rad hose until no more bubbles come out of the fill neck (remember, AST has been removed). By pumping I mean squeezing it to push any trapped air up to the fill neck on the block. If I have time I'll let it sit overnight then continue filling. On first start I'll leave the cap off and once the motor is warmed up a bit I'll hold it at 2-3k rpm and top off continuously, then let it cool and top off both the motor and the overflow. Then just drive normally and top off each morning when it's cold until I find it full. That's it.

 

Since I started this thread, I'm refining the original question to remove the AST issue:

Compared with EG/H2O based coolant, is Evans NPG+ better at removing vapor from the coolant stream?

We already know what Evans would say (see my original post for their claims). So how could we test their claims? I've got some NPG+, some Sierra, and orange and green EG sitting in the garage. If anyone can propose a relatively simple test that I can perform at home, I'm willing to give it a shot.

 

I am not sure a simple test can be devised.

However, one can set up a flask as a closed system with an appropriate pressure cap(attach with a rubber hose and AST or some sort of a container which can accomodate a pressure cap). A temp probe is also needed.

Fill the system completely with coolant.

Heat the flask at one specific point until boiling occurs. Record the temperature of the fluid.

Observe and record how much time it takes for the vapor to be absorbed as it rises through the fluid.

Repeat for each coolant.

Does this sound reasonable?

 

Sorry - double post

 

Ahem (clearing my throat ...) I've looked at a lot of cars. If that is your explanation for AST (top of radiator below top of motor), please explain why there is an AST from a lowly VW Bug, all Beemers including M3, M5, all Volvo, Saab, Mercedes Benz, some Toyotas, Chrysler? Your explanation makes an AST to be a special case. It ain't so.

The thing I noticed about the AST in VW and German cars in general is that these cars run hot. In the VWs, they run routinely at 235-240 in traffic. May be it's a good idea to eliminate air bubbles and therefore hot spots for these hot running cars?

As to another member question of whehter I have converted to NPG? I think I will and the AST will remain in place once I finish installing a reman.

 

I started to write this response assuming propylene glycol boiled at a higher temp than ethylene glycol... when I looked up the info, I found out I was wrong. Propylene glycol (Evans and Sierra) boils at 369F, ethylene glycol at 385F.

Wow... what I was going to say doesn't really make sense now. Why not run straight ethylene glycol?