I’m curious about coolant flow in the engine pictured below.
It’s a closed deck engine (no coolant will flow from deck to head). At the end of the exhaust side manifold (red), is a swirl pot. In my opinion, it is the outlet manifold.
Why would they route OUT water from block (orange line, 3 branch manifold), into the Outlet of the cylinder head ? Why not just make a "Y’’ pipe after the manifold?
You are correct on the flow route…
The small drillings are normal also at deck interface and are a common mod on road engines modded for higher hp which looks like the pictures above tell judging by the tig welding.
The drillings not only allow block to fully fill/vent air at tops of cylinders but also promote vital cooling between the exhaust ports which many older OEM engines lack in.
The block doesn’t care too much about cooling really and it is more-so the head that requires cooling - from memory from F1 Engines, the vital cooling areas are the head(obviously) and the top 20mm of the block. This is why many USA based super high Hp engines can get away with filling the block waterways 80% of the way up with engine grout for bore stability.
A Y pipe is not used as it complicates things and needs more room - the water only really gathers heat going across combustion chamber.
On some older F1 Engines I’ve looked at the cooling was done from the top down - as in into head first, and down into block - but these can be a nightmare to bleed sometimes and are not common.
On an earlier F1 Engine I owned and studied the coolant flows from the block and exits above the plug and vertically into Rocker cover - again exiting as a single pipe to the rad.
You can see more on it here if you like reading,
On My latest F1 engine purchase(V10) the coolant flows up from the block on the clutch end in one large opening, into the head, along the head and out the timing gear end - so there are many ways of doing it. Remember too you can over cool a cylinderhead…the water adds weight too if you have many hoses and manifolds to fill.
Ps…If you haven’t already…change the banjo bolts holding the piston squirters on that engine - they can somethings break off and bad things happen. Use a torque wrench on new ones.
Above is a snippet.