Let's review the basics first. On the Gen I engine, the water pump bolts to the front of the block. It draws coolant from the radiator and pushes it into both sides of the block, where it circulates around the cylinder bores and up through a passage at the back of the block into the cylinder heads. Coolant then flows to the front of the heads and out through the crossover in the intake manifold where the thermostat is located. If the coolant temperature is high enough, the thermostat will allow coolant out of the engine into the upper radiator hose and back to the radiator.
This system worked fine for almost 50 years, but it could be more efficient. The engine is not cooled evenly with this flow pattern because the bores of cylinders 1 and 2 will always be cooler than the bores of cylinders 7 and 8. But the reverse is true for the combustion chamber temperatures because numbers 1 and 2 are the last to be cooled.
In the LT1, however, coolant is pumped into the cylinder heads first. Though the water pump still bolts to the block per the original SBC design, the coolant makes a sharp, 90-degree turn just inside the block and is routed up into the cylinder heads first. It then flows, like a parallel electrical circuit, evenly down the coolant jackets of all the cylinders in the engine block, then returns to the water pump through the lower pair of ports at the pump-to-block mating surface. A two-way thermostat (also unique to the LT1) is located in the water-pump housing. If the coolant temperature is hot enough, it is routed out to the radiator. If not, coolant flows through a bypass circuit and back into the engine.
The combustion chambers are the hottest part of any engine, and they are cooled first in a reverse-flow system. And because of the flow pattern, they are closer to uniform cooling from cylinder to cylinder. This provides a more stable and efficient environment for combustion to occur. More efficient combustion means lower emission levels. Plus, cooling the heads first allows the engine to safely run a higher compression level than would otherwise be possible, and we all know more compression means more power.
The reverse-flow design can also contribute to longer engine life. Since temperatures in the combustion chambers can reach 1,400 degrees, the result is a substantial difference in temperature between the top and bottom of the cylinder bores, and therefore the bores are wider near the top than they are at the bottom due to thermal expansion. The piston rings have to expand and contract slightly as they seal the bores from top to bottom, causing wear on the ring faces and ring lands. And since the temperature at the bottom of the bores can be below the dew point, water vapor, a by-product of combustion, can condense on the bottom of the bores and drip into the crankcase, eventually contaminating the motor oil and contributing to sludge buildup over time. However, in a reverse-flow system, the bore temperatures are more even from top to bottom, reducing the effects of those two problems.