A fellow on a forum had a question about making his own pressure tank like the one I showed off here. He linked to another welded aluminum box tank similar to the one I chose, and asked if I thought it would work. I only saw one opening for a hose connection. Some of these tanks have only one, some two. Before he pointed out the second hole I hadn’t seen, I explained why the air purge function of the 2.1 wbx cooling system needs two openings in the pressure tank. Turns out he didn’t need the advice, but the explanation might be helpful to some folks anyway, so it’s copied here, read it within that context:
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“With only one connection to the rest of the system, that tank would only act as a place for the pressure cap to control pressure and total coolant volume. You could do that with a p-cap on the end of a short 2” pipe for all that’s worth. It wouldn’t provide any air purge function, which is the only reason to need a tank with some volume.
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The air purge ring on 2.1 cooling systems is there specifically to pull a small stream of coolant from the top corners* of both cyl heads. The purge hoses from the heads to the ring and then the ring itself are small-bore so the coolant flows thru them at high speed. The fast moving liquid entrains any gas bubbles (air, steam, or leaked combustion gases) and carries them along to the pressure tank, where the volume increases abruptly so the flow rate suddenly slows to a relative crawl. There the bubbles have time to separate from the liquid.
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If any gas accumulates at the top of the p-tank, it gets pushed out to the reserve tank whenever the system pressure exceeds the p-cap’s valve setting of ~1 bar, which it does often as a matter of course. The ejected flow enters the reserve tank at bottom and once again, the gases rise out and this time they’re gone for good because the reserve tank is open-vented at top.
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Then after shutdown, the mass of engine coolant contracts, and when the internal system pressure drops to ~1psi below atmospheric the p-cap lets liquid flow back into the system from the bottom of the reserve tank to equalise the system pressure to near atmospheric again.
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In order for that to work, there has to be two flow connections on the p-tank: a way in, and a way out. The water pump constantly pulls some liquid from the bottom of the p-tank, which is what creates a suction in the tank and hence the purge ring.
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*The purge hoses are only directly from the heads on cyls # 2 and 4. On #3 the purge hose draws from the attached t-stat housing, and there’s an operable purge orifice on the t-stat cover to evacuate the radiator return while the t-stat is closed. On #1 the purge hose draws from the coolant distribution manifold, which is itself a tower separator on 2wd’s but there wasn’t room for a tower on Syncros due to the fuel tank filling that whole space, so the Syncro has a purge hose attached to the midpoint of the forward coolant crossover pipe.”
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