Why does AST goto cold side of radiator
 

I just realized the AST bypass the radiator and goes to the cold side of the radiator

why is that ?

https://cimg9.ibsrv.net/gimg/www.rx7...42ec2257e9.png

the cold side is easier to flow back into, don't overthink it, the AST's job is merely to filter out air in the system.

the reason I'm asking is because we are installing a hks vmount kit, and it reverses the flow of the radiator
so now the side of radiator with ast port is the hot side

will that be an issue ?

There is pressure at the thermostat outlet, and suction at the lower pipe of the water pump. That is why water flows though the radiator, due to the difference in pressure. If you leave the AST's top hose connected to the thermostat outlet but move the AST's top hose so it also connects to the thermostat outlet, there won't be any flow through the AST.

as long as air still rises in coolant it will work fine.

it is literally an Air Separator Tank

There is a bit more to how it functions.

Consider in the stock plastic (awful quality) AST tank that the outlet hose going to the cold side of the radiator has a very specifically "put there by Mazda" orifice in it- very small, about 2 millimeters in dia or even smaller.
The outlet hose might look normal sized but the AST tank outlet port is restricted really small with this orifice inside the tank nipple.

Mazda did this to keep the coolant in the AST tank LONGER, so bubbles and air could have time to rise to the top and be bled off out of the system continuously.

All the aftermarket AST tanks are manufactured wrong- they don't have the tiny restrictor in the outlet port and don't hold the coolant long enough for the champagne bubbles/air/whatever to bleed off properly as the factory AST does. The coolant just gushes into the aftermarket AST and straight out again, which is useless and defeats the point of having an AST tank in the first place. You need to make and insert a restrictor pill into the aftermarket AST outlet line if you go with an aftermarket. Or there's little to no point in having it.

Not all. Some had it. And I suspect w/o the restriction they still work...but agree it wouldn't be as efficient...

http://i1126.photobucket.com/albums/...ps580c0382.jpg

http://i1126.photobucket.com/albums/...ps1ef2158d.jpg

The other reason (besides residence time) the restriction is there is that the OE flow path is short-circuiting coolant from the hot side of the engine past the radiator to the engine cool side. So minimizing that very slightly improves coolant flow through the radiator.

who made that? It looks nearly identical to the one I made for my car haha

The real question is why your FD is still using that POS that is not needed?
Many of us have removed it before time began and our cooling systems run without any problems.

YMMV. I'd keep it.

Yeah, whatever. I removed mine several years ago because that aftermarket AST didn't fit well with my choice of SMIC and I really didn't want to put a stock AST back on. If you're not prepared to remove it properly then I'd recommend keeping it. And until I bought a lisle funnel it was easier to get the air out of the system w/the AST every spring when I changed coolant.