AST Dissected
 

Today I cut apart a spare Air Separation Tank that I've had lying around for a while. It's not from my car, so I don't know how many miles it's seen.

I didn't take a 'before' photo, but here's what it looks like all pieced together:
https://www.rx7club.com/attachment.p...hmentid=117006



I was surprised to see that there isn't any baffling or trickery inside:
https://www.rx7club.com/attachment.p...hmentid=117008



The inlet (from the hot side of the radiator) is restricted:
https://www.rx7club.com/attachment.p...hmentid=117007

https://www.rx7club.com/attachment.p...hmentid=117009

https://www.rx7club.com/attachment.p...hmentid=117010
-s-

The middle outlet goes to the fill neck, bypassing the radiator:

https://www.rx7club.com/attachment.p...hmentid=117012


The top outlet goes to the overflow tank. It is on the bypass-side of the pressure cap, and the diameter is restricted also:
https://www.rx7club.com/attachment.p...hmentid=117013



-s-

Removed mine 1500 miles ago, on advice of Big Islands Seven ( luv ya Bro) to see no adverse effects and 1 less piece of clutter and part to break.

Interesting, I too was under the impression the stock unit had baffles. I've also found that to be a point of discussion in a couple threads where it was suggested the after market aluminum ASTs weren't as effective because of the lack of baffling. Guess this debunks that myth.

>The inlet (from the hot side of the radiator) is restricted:

That is the ast outlet at the bottom with .086" dia restrictor, going to the lower tank. by deleting this restrictor, aftermarket pettit and other asts look good, but bypass the rad with the hottest coolant that needs to be cooled.

This is very interesting. Yesterday I was going through my Hyper Rev Vol.91. Most of the 350-450ps 7s are running the stock plastic AST. Why don't the tuners in Japan change to the aftermarket ones? There are plenty of aftermarket ASTs available. RE, Sard, Mazdaspeed etc. Was just wondering.

Thanks for the correction Kevin, it makes sense that the outlet would be restricted, not the inlet.

For those with aftermarket ASTs, it would be a good idea to add a restrictor in the outlet hose, something similar to the 'wastegate boost pills.'


-s-

I took my ast apart when I got my seven, but I split it at the seam between the top and bottom sections, which is where it was leaking. Other than the shape of the reservoirs and the restrictive orifice, both features perhaps facilitating the air separation function, I could see no reason that the aftermarket unit would not provide the same funcitonality. Isnt this devices sole purpose to take the air bubbles out of the cooling system, thus preventing hot spots where air pockets may form in your engine. I dont believe that the ast serves an integral fucnction for the actual cooling of the liquid passing through the system. Perhaps the tendency for the ast to develop leaks is compounded by the restrictive orifice size (0.086") creating excessive pressure buildup within the chambers. The part I replaced it with was a cylindrical aluminum piece with top and bottom inlet and outlet and no restrictive orifice. I have had no adverse affects with the upgraded part and best of all - no leaks.
Any idea what those yellowish deposits are in your ast. Kind of looks almost like sulfur.
chuck

so an aluminum one prevents from cracking but it doesnt help in any way?
might as well remove the ast in the first place and save some money

Very nice pics.Now,If we can get this AST version...


https://www.rx7club.com/attachment.p...chmentid=72017

The ast degrades normal cooling, since it bypasses the rad. It removes air that has leaked into the coolant, but it only can do this efficiently when the t-stat is wide open, and the rpms are very low so the coolant flow through the t-stat is slow, and air can rise to the fop of the filler neck, and get pushed into the ast. Most of the air accumulated in the ast will get pushed to the exp tank when the engine is shut down and cooled.

Note many race cars (mazda 4-rotor) have a large ast that takes all of the flow from the rad, and it can purge air (or combustion gas leaks) continuously.

you must have been really bored to do that

Yes, I had some time to kill while waiting for the car to cool enough to remove the radiator cap for a coolant change. Cutting the AST took about 5 minutes (using an old hacksaw), and taking the photos was about that quick also. I still had to wait before I could change the coolant.



EFS.O, I've seen that photo before, and was thinking about making something just like that. I don't have connections to a foundry to make one, and it would take some time because I haven't cast anything since metal shop in junior high.

The reason I cut this one open was to see what's so special about the stock one, and the answer is 'not much'. I've come to the conclusion that the only trick to it is the restrictor on the outlet line so that you're not letting too much fluid bypass the radiator. There's no good reason to make a new one using a different shape, or find a coolant expansion tank from a different car at a junkyard.



On a side note, the throttle-body coolant line is the highest point in the cooling system right now, so I wonder what effect that has on the functionality of the AST. You could see this when refilling the coolant with the TB line open to prevent air bubbles. The AST fills and overflows before coolant comes out of the throttle body line.

-s-

It seems that the expansion tanks for most other cars are larger than ours:
http://www.scienceofspeed.com/produc...nk/default.asp
http://www.cantonracingproducts.com/...ion_tanks.html

-s-

Tanks in your links like the top Canton, and some oem high mounted replacements, are dual purpose .... they combine air separation and expansion functions. Nsx oem example:

http://www.nsxsc.com/cnmnsx/DSC02224.jpg

The offer an added advantage, air cushion helps maintain high system pressure with typical slight temperature changes in coolant. Most oems are like this now, with no extra vented overflow tanks.

I cut my plastic AST apart and epoxyed the brass restrictor piece into my pettit tanks bottom tube.