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Off-topic but fendamonky, that engine bay is looking pretty damn cool. I don’t follow the build threads like I probably should. But look forward to seeing that when it’s complete.
Thanks I don't have a build thread up (yet). I was thinking of putting something together once it's done. I'd rather have a conclusion before I start any hype
. . . . 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. . . . . Dale
Simple solution and I have had no issues thus far.
Yeah that's the nicer/newer version of what most (including myself) have gotten in the past, which is the FC part that is plastic and exactly the same functionally. I like that billet piece, and I'll probably get one at some point, but it's tough bringing myself to spend $75 on something so small, especially when I already have something that works lol. Although I would like it for the simple fact that it's billet instead of plastic in contact with hot coolant.
EDIT: Boy, that was a lot of words for saying nothing at all that's important. Sorry guys.
Actually you brought up a good point and the reason I got the Banzai piece. I heard of and saw pictures of the plastic piece or the ‘O’ ring failing after some years. I had the FC piece on for maybe 4 or 5 years with no issue but went ahead and got it for peace of mind. Probably as close to bulletproof as you can get with the viton ring.
My car is mostly stock and for reliability I replaced the OEM plastic AST with a nice one from Tri-Point Engineering 7 or 8 years ago, so I have no reason to eliminate the AST. But I see that the current replacement for the thermostat housing is a single casting that doesn't have a separate filler neck for the cap. I'm curious, what does one do if you want to eliminate the AST but have the newer, one-piece version of the thermostat housing?.
Last edited by Retserof; Feb 16, 2021 at 09:51 PM.
I have seen people drill/tap that housing for an overflow tank nipple. You can also source a good thermostat housing from an earlier car that has the bolt-on radiator cap neck, they typically aren't hard to find used.
My car is mostly stock and for reliability I replaced the OEM plastic AST with a nice one from Tri-Point Engineering 7 or 8 years ago, so I have no reason to eliminate the AST. But I see that the current replacement for the thermostat housing is a single casting that doesn't have a separate filler neck for the cap. I'm curious, what does one do if you want to eliminate the AST but have the newer, one-piece version of the thermostat housing?.
Just to add to what Dale has already confirmed. There is 'just' about space to drill and tap the top to take a 1/8 NPT fitting. I did mine this way to remove the AST:
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.
I never intended to be smarmy, my response was the way it was because your response came off as smarmy. If that wasn't your intent, my apologies. The internet doesn't have the best reputation for getting intention across. Still, I wouldn't throw out the concept I brought up just because it doesn't align with longstanding views.
Originally Posted by Sgtblue
You gave me yours with no specific experience.
The experience I've seen is with turbocharged piston engine cars "blowing their head gaskets" at the track. What would happen is the cooling system would become over pressurized and drivers/builders would blame it poor head gasket sealing or head stud clamp load. What took a while to learn was the cooling system would heat up, burp fluid (before it got outside of normal operating temps), lose pressure, then have thermal runaway (not measurable in the coolant as the lose of coolant would cause localized hot spots), warping the head/block and cause a head gasket to blow. After that, is when the temps would actually spike.
After having this process happen a handful of times, we started to monitor coolant pressures and could see this issue occurring. Moving away from the standard cooling system design to running a tank, similar to what Mazda uses on the FD, was a big part of the solution to our problem. We would then learn, after making good connections with OEs and proper race cooling system manufacturers, that this is a known issue for all engines, which is why you will never see a high end, race cooling system with a radiator cap on the higher pressure size of the radiator.
The long and short of it is that it struck me as odd that people would be removing a design that is regularly used in the motorsport world and felt compelled to offer some insight to save people from the headaches and cost I dealt with.
Originally Posted by Sgtblue
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.
The small millimeter diameter line is exactly why it avoids an over-pressure situation. This tank is intended only to see the pressure caused by the heating of the coolant. The small hole reduces flow and damps pressure created by water pump but not the pressure caused by the heating of fluid.
Originally Posted by Sgtblue
An over-pressure situation that I haven’t heard anyone experience...even with a lower (13 psi) cap than you suggested.
Maybe someone has experienced this and not known. Without multiple coolant pressure sensors, it can be tough to diagnose. Someone would think that a seal would let loose internally, causing pressure from the combustion chamber to enter the cooling system. There are 5,060 results for "overheating fd rx7" on this site and the only thought for someone posting is to try and diagnose their issue. I'm just saying it's plausible that this issue could be happening and shouldn't be dismissed without a little more research. Hell, I'd be even willing to help. I have the sensors and datalogging equipment.
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 appreciate that you acknowledge the concept. As I don't have data, you may be 100% correct that people can run without an AST without issue, but if they did have that issue, no one would know to say otherwise.
If someone overheated and noticed missing coolant, you'd do all the normal things. Check the radiator cap seal, cheap line integrity, check radiator integrity, pressurize the system to ensure it's not leaking.
Unless the concept of thinking the water pump could be flowing more coolant than the radiator can efficiently allow to pass through it, causing excess pressure in the upper coolant hose, which then caused the radiator cap to burp a little fluid, which then caused the cooling system to no longer be pressurized optimally, which then caused the coolant to boil and/or not come in contact with all important coolant surfaces, which then caused it to fully overheat... it's quite the conclusion I wouldn't guess anyone to come to without data telling them it was the issue. I hope that makes sense.
As mentioned above, I am glad to test the concept out to see. Maybe we learn something cool that can be beneficial to the community.
I never intended to be smarmy, my response was the way it was because your response came off as smarmy. If that wasn't your intent, my apologies. The internet doesn't have the best reputation for getting intention across. Still, I wouldn't throw out the concept I brought up just because it doesn't align with longstanding views.
The experience I've seen is with turbocharged piston engine cars "blowing their head gaskets" at the track. What would happen is the cooling system would become over pressurized and drivers/builders would blame it poor head gasket sealing or head stud clamp load. What took a while to learn was the cooling system would heat up, burp fluid (before it got outside of normal operating temps), lose pressure, then have thermal runaway (not measurable in the coolant as the lose of coolant would cause localized hot spots), warping the head/block and cause a head gasket to blow. After that, is when the temps would actually spike.
After having this process happen a handful of times, we started to monitor coolant pressures and could see this issue occurring. Moving away from the standard cooling system design to running a tank, similar to what Mazda uses on the FD, was a big part of the solution to our problem. We would then learn, after making good connections with OEs and proper race cooling system manufacturers, that this is a known issue for all engines, which is why you will never see a high end, race cooling system with a radiator cap on the higher pressure size of the radiator.
The long and short of it is that it struck me as odd that people would be removing a design that is regularly used in the motorsport world and felt compelled to offer some insight to save people from the headaches and cost I dealt with.
The small millimeter diameter line is exactly why it avoids an over-pressure situation. This tank is intended only to see the pressure caused by the heating of the coolant. The small hole reduces flow and damps pressure created by water pump but not the pressure caused by the heating of fluid.
Maybe someone has experienced this and not known. Without multiple coolant pressure sensors, it can be tough to diagnose. Someone would think that a seal would let loose internally, causing pressure from the combustion chamber to enter the cooling system. There are 5,060 results for "overheating fd rx7" on this site and the only thought for someone posting is to try and diagnose their issue. I'm just saying it's plausible that this issue could be happening and shouldn't be dismissed without a little more research. Hell, I'd be even willing to help. I have the sensors and datalogging equipment.
Dan
Interesting explanation, and with actual experience to back it up. However, as I said in the other thread...
While you are technically correct, in practice, this is, as Sgtblue says, a non-issue IN MOST APPLICATIONS. I "could" see in an application where the water pump was very efficient and the hoses, radiator were too restrictive, plus there was no restriction exiting the block, etc., that this would be correct. The WP is placed where it is, pressurizing the coolant in the engine, to raise the boiling point a few degrees and prevent localized boiling as high flow creates localized turbulence. Usually, however, a restrictor or thermostat at the block exit creates the pressure rise, and since the pressure-relief radiator cap is after that restriction, it sees little if any pressure rise due to the WP.
So, in practice, if the system is designed properly, this is usually a non-issue. In racecars, often stuff that works great for street applications is inadequate for race applications in highly tuned vehicles. So there is a lot of truth in both of your (Sgtblue & defjux23) posts.
Interesting explanation, and with actual experience to back it up. However, as I said in the other thread...
While you are technically correct, in practice, this is, as Sgtblue says, a non-issue IN MOST APPLICATIONS. I "could" see in an application where the water pump was very efficient and the hoses, radiator were too restrictive, plus there was no restriction exiting the block, etc., that this would be correct. The WP is placed where it is, pressurizing the coolant in the engine, to raise the boiling point a few degrees and prevent localized boiling as high flow creates localized turbulence. Usually, however, a restrictor or thermostat at the block exit creates the pressure rise, and since the pressure-relief radiator cap is after that restriction, it sees little if any pressure rise due to the WP.
So, in practice, if the system is designed properly, this is usually a non-issue. In racecars, often stuff that works great for street applications is inadequate for race applications in highly tuned vehicles. So there is a lot of truth in both of your (Sgtblue & defjux23) posts.
Agreed, in most applications, it probably doesn't matter. As mentioned in my second post, Honda and Mitsubishi have been designing systems like this for years with very high output engines and obviously, they get away with it.
On further reflection, Mazda was seemingly designing the car to run really hot coolant temps from the jump (perhaps for performance, perhaps emissions as someone else mentioned). Undersized radiator, high coolant pressure cap. These ideas are straight from motorsports where teams are trying to maximize packaging for best weight and aerodynamic benefit. F1 runs 50 psig cooling system so the engines can run at higher temps without boiling coolant. Higher temps mean the radiators can be smaller.
If you go with a larger radiator and adjust fans to turn on at lower temps, the need for a separate coolant tank is less important. That being said, the AST likely gives you more headroom.
...If you go with a larger radiator and adjust fans to turn on at lower temps, the need for a separate coolant tank is less important. That being said, the AST likely gives you more headroom.
Dan
No doubt. It serves 2 purposes - air bubble minimization and pressure relief optimization.
Thanks for the insights.
I don't have a build thread up (yet). I was thinking of putting something together once it's done. I'd rather have a conclusion before I start any hype 
