The majority of the cooling system designs published online for Subaru powered VW conversions are not very well designed. Some are not too bad, others are absolutely awful, with the latter giving away more signs that whoever came up with them had a limited understanding of what they were doing. In general it is not as bad now as it was in the mid 2000’s. We first published what we consider to be the optimum circuit design in 2006 or 2007, and quite a few diagrams which others have published since then bear more than a passing resemblance to it. There are still some pretty bad ones out there though. The worst we have seen, which used to be very prominent, really did look like it had had no more thought put into it than ‘what can I join all these pipe ends to so that when I pour coolant in, none of it pours on the floor’. It had certainly had no thought as to how the coolant gets to where it needs to go during filling, or to using the coolant flow to aid separating trapped air from the coolant during filling.
If you want to get an idea of whether a coolant system schematic or plumbing set is any good, check it against the following, and add up its score. The header tank is the tank which is pressurised when the engine is at running temperature if the system has more than one tank:
- Does the coolant flow out of the header tank go to a radiator pipe? (Yes =3, No = 0)
- Excluding any small diameter vent pipes, does the header tank have only one pipe? (Yes =1, No = 0)
- Does the inlet pipe to the header tank (or the only one pipe to the header if there is only one excluding small diameter vent pipes) join the pipe which it is connected to away from its nearest local high spot? (Yes =2, No = 0)
- Does the inlet pipe to the header tank (or the only one pipe to the header if there is only one excluding small diameter vent pipes) join the pipe which it is connected to at the side or bottom, rather than the top? (Yes =1, No = 0)
- Does the cooling circuit add additional breather points which were not there in the Subaru or VW? (Yes =1, No = 0)
- Does the cooling circuit send multiple pipes in directions in the vehicle which are nothing to do with the locations of the key cooling system hardware? (Yes =1, No = 0)
- When constructed as whoever designed it intended, does the cooling circuit feature multiple hose joints which are to do with hose shape rather than hose function, because the hose shapes needed are constructed from multiple generic off the shelf hose bends? (Yes =1, No = 0)
Add up the total score from the above questions for the system you’re looking at. The higher the score, the less thought and cooling system knowledge went into its design. 0 = a well designed system, 10 = a pretty dreadfully designed system.