when you buy the Greddy radiator cap 9 (1.2 bar) does it go on the thermostat housing or on the AST tank???

 

The AST. The thermostat housing is just a filler neck.

 

what if you have one of those Aluminum AST's with the cap with the vent ontop. Do you remove it or leave it on?.....I have my Greddy cap on my Thermostat housing. Should i put one on the Aluminum AST?

 

filler neck

 

Caps with no spring in them do not go on the thermostat housing. There is no overflow from that neck therefore no reason to have a pressure cap. The overflow comes off the AST, that is where the pressure cap goes. If you have a pressure cap on the filler neck it will do nothing, the system will be pressurized to whatever in on the AST.

 

i have one on my FD on the T-STAT housing

 

it appears no one really knows for sure...
I have an aluminum AST with a cap that has a pressure release valve on it.
It is in my opinion that both caps would need to be rated at the same pressure or the system will only operate at the pressure of the smallest rated pressure cap, being they are both part of the same system.
It shouldnt matter where in the system these caps are located ( housing or AST ) they are both in the same cooling loop.
As an example: If you had 2 pop-off valves on the same intake pipe and had one with a spring rated for 12psi and the other pop-off valve with a spring rated at 17psi, the 17psi pop-off valve would never open because the 12psi would always relieve the pressure first. I feel the same theory applies to the 2 different rated coolant system caps.

 

No, I do know for sure. Only the cap on the AST matters. If you look at how a pressure cap works you will see 2 seals. The seal on a spring pushes up into the cap when pressure builds. When that seal is pushed far enough pressurized coolant escapes around it and goes into the hose to the overflow. The thermostat housing has no hose and no inner shoulder for that seal on a spring to seal against. Therefore it does not matter what pressure cap you put on the t-stat housing. Sheesh, it is a very simple system. Just look at it and think for a second. If you had a cap that releases pressure at 5psi on the t-stat housing where is the pressure going to go? It is going to get stopped by the outer seal on the cap and pressure will continue to build. No matter what this is the case unless you eliminate the AST and put a neck with overflow hose connection on the t-stat housing.

 

I must be looking at this all wrong...
Rotary Performance says it goes on the AST and RX7- store says it goes on the T-stat housing...

 

Go with what RP says, hands down.