So here is the thought process behind a “Mini” Engine Coolant Filler Tank (commonly referred to as the Upper/Turbo Coolant Reservoir)…
The purpose of the OEM Engine Coolant Filler Tank (here forth known as the “ECFT”) is a high point fill. What is a high point fill? Good question! Bear with me as I ramble a bit… On most cars the radiator filler cap is the highest point in the cooling loop. This allows the coolant to be added and/or replaced while allowing air to escape the system at the same time. Without a high point that can be opened, or vented, air can be trapped compromising the effectiveness of the cooling system. A quick thread search will show LOTS of “I just changed my radiator and now my car overheats” threads that are a perfect example of what can happen if air is trapped in the cooling system. The turbocharged Subaru is a bit of an oddball in that the radiator filler neck is not the high point of the cooling system; the turbo charger sits higher than the radiator filler neck. So, to address this situation Subaru uses the ECFT. The ECFT allows the cooling system to be filled and air to escape similar to a traditional system.
For guys running a rotated turbo setup the turbo inlet and/or inlet piping interferes with the ECFT. This means the tank either needs to be relocated or removed. While some have been successful in removing the ECFT, this is not the preferred method as its success is hit or miss at best. The other option is relocating the OEM tank to another location. Firewall, strut towers, and various other areas in the engine bay have been used via fabrication of brackets and other less glamorous methods. While this can work, the relocation requires longer lines that may themselves trap air if the routing is not well thought out, the final location may not be high enough to be as effective as the OEM location and it requires the additional use of fabrication tool and materials, which many of us do not have.
Back to the original point of the post, the thought of manufacturing a “Mini’ ECFT that would utilize the OEM high point location, mounting points, Subaru filler cap, and most importantly, provide clearance for the big rotated turbo setups. All while looking like it belonged there, not as an afterthought.
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