What I am referring to has very little to do with the coolant flow ports. If you have a deck insert that isn't matching the coolant ports, that's a whole different problem in itself related to coolant flow and I would not recommend using one that didn't match. Whats lowering the efficiency of the closed deck to transfer heat is that coolant is no longer in direct contact with that portion of the cylinder wall. The heat now has to go through the insert then transfer to the coolant .That top portion of the cylinder wall is the hottest part of the block that is in contact with coolant. When I was building my motor I looked into 3 company's to do the deck. I was familiar with a closed deck lowering cooling ability from a different motor I had closed . 2 of the companies said that it would lower cooling efficiency but upgrading the cooling system would help offset that. They recommended a larger radiator and lowering the coolant temps which will allow heat to transfer faster because of the greater Delta t. The third company claimed it wouldn't have an affect on cooling. When i asked him to explain why that company couldn't. There are deck inserts that are machined out so that it is not flush with the deck height to allow coolant to contact the top of the cylinder. They do not support the very top of the cylinder wall. That design is a compromise between strength and cooling as the top of the cylinder wall is where the highest pressure is and needs the most support, yet that's where the majority of the heat is rejected from the cylinder sleeve to the coolant.I think you may be confused. An insert will only prohibit heat transfer if the coolant passages in the insert cause any sort of restriction to coolant flow. Maybe there is a specific insert you are saying will do this, but I can assure you that there are N54 inserts that are port matched to the passages on the head to prevent this.
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