Having an incorrect radiator cap on a turbo EJ Subaru is a critical mistake that even seasoned Subaru builders, including us have made. In fact, one of our sharp-eyed readers brought this to our attention when he was looking at one of our articles. The fact that most high-performance radiators for Subarus come with high-pressure caps makes this issue even worse. The problem is that the OEM radiator cap is a round one with no ears and we found that it’s important for this cap to be on the radiator, not the high-pressure cap that came on your performance radiator!
The round earless OEM cap has a unique feature, instead of the typical two-way pressure relief valve that comes on radiators, the OEM cap has a one-way check valve.
The OEM cap with the one-way check valve is pictured here on top, and the performance high-pressure two-way cap that came on our CSF radiator is on the bottom. You can see the difference in the valves here. A two-way cap is used on most cars so as the coolant expands with heat, the valve opens and lets coolant flow into the catch can. When the engine cools down, the valve opens and lets the coolant returns to the radiator. This helps keep some air out of the system and keeps the coolant level more consistent.
Normally a two-way cap is used on the radiator and this is how our car was. However, on a Subaru, you have a circulating expansion tank above the radiator. This is to help ensure that all the air is bled out of the system and is a great OEM feature. We convert most race cars we build to this sort of system for the same reason.