Subaru Radiator Caps are confusing for a few reasons.
1.) Radiator manufactures don't send or explain the correct caps with their radiators.
2.) Social media is a horrible place for sharing and referencing info.
There are a couple videos on the topic that can be found on YouTube, but in short I'll explain how they work from the factory and how they should work with an aftermarket aluminum radiator.
OEM EJ205 (03-05) EJ255/EJ257 (06-14 WRX 04-21 STI)
Plastic Radiator has a cap. This cap is a 108kPa or 137kPa SINGLE way pressure cap.
Expansion Tank has a cap. This cap is a 88kPa or 108kPa TWO way pressure and recovery cap.
The Plastic radiator needs a cap for 2 reasons. 1.) filling after a radiator swap or flush 2.) save the plastic end tanks from explosion if the Expansion Tank cap fails.
With these two caps in a system, the system pressure is the lowest of the two caps and if it goes over that it will bleed off through the factory lines to the Overflow tank. When pressure reduces the recovery aspect of the Expansion Tank cap kicks in and it allows coolant back into the system. The cap on the Plastic radiator doesn't do anything if the expansion tank cap is functioning.
Now let's talk about increasing pressure, common increase is to 1.3bar or 1.5bar. Increased cooling pressure means higher boiling point and less likely for the coolant to boil and create pockets of steam. This is a good thing (to a point).
In a two cap Subaru system increasing the pressure to 1.3bar becomes confusing and most people do it incorrectly. You can't simply change one of the caps, this won't work. Most aftermarket radiator caps are 2 way caps, which belong only on the expansion tank. Their high pressure is now higher or equal to the 1 way cap on the radiator, causing the system to only pressurize to the lowest pressure cap in the system, not the intended higher pressure from the radiator manufacture.
An incorrectly placed radiator cap can cause a slew of issues on a Subaru, most notably over heating, or excessive overflow. Multiple two way pressure caps also create headaches.
The solution is pretty simple and most videos on YouTube agree. Delete the pressure cap from the radiator.
Aluminum Radiator has a cap. This cap should be solid, no pressure relief or recovery.
Expansion Tank has a cap. This cap can be 1.1bar, 1.3bar, 1.5bar whatever you and your team decide works best. With this being the only pressure cap on the system it now acts as the peak system pressure. It also means the recovery aspect of the system will function as Subaru intended.
How do you get a solid cap you ask?
3 ways to do this, 2 require fabrication, 1 requires a few pieces from me.
1.) Cut off the water neck on the radiator and weld a plate there instead. (This is bad mainly from a filling stand point, bleeding can be a massive headache if you're not using a thermostat less electric water pump)
2.) Cut off the water neck on the radiator and weld a bung or threaded cap (This fixes the biggest issue with option 1, but it does require fabrication and a full coolant drain/refill)
3.) Solid cap designed for Subaru style water necks and a couple silicone heater hose caps and clamps to seal off the no longer needed hard lines. (This fixes the issues with 1 and 2, and is readily available) If you want option 3, click here