The cooling system as installed in a Subaru is incredibly self contained, especially when compared to a VW wasserboxer engine. The wasserboxer is notorious for being an air cooled engine with cooling jackets attached, and as such, almost all the coolant plumbing is external. The Subaru engines are just the opposite. From the engine in a Subaru you have just a radiator feed and return, and a heater feed and return. The header tank in a Subaru is also built into the radiator, and the expansion bottle is attached directly to the radiator, so the whole coolant system could not be any neater.
Some internet resources make out that Subaru powered VW’s are difficult to bleed the air from. In reality, it depends entirely on the design of the circuit. It is no surprise that those same sites feature coolant circuit designs which do nothing at all to encourage the air to self bleed from the system as it is filled. There are plenty of good sites out there too, but be aware that they’re not all good.
A well designed circuit can be as easy to fill as any other car. Yes, a radiator installation with both hoses at the bottom, and the top higher than the header tank, such as a T25 is not going fill immediately, but there is an incredibly simple trick to doing this - no extra bleed points, or jacking one end of the bus into the air required.
The following pages show the system design and components we recommend. This circuit is the result of a lot of research and talking to others who have done conversions. A lot of the trouble people have had with other circuits off the internet comes down to a lack of thought about what is actually happening in the circuit by whoever designed them.
Coolant Circuit Design
Reversed Coolant Manifolds
Header Tank Installation
Coolant System Material Choice