Originally Posted by eocoolj
This may be a stupid or simple question, but I've been wondering about the differences lately between the 1st and 2nd gen (LT) small block chevys. In reading about the LT engines, it says that the primary difference between the 1st and 2nd gen engines is the reverse flow cooling featured on the LTs.
My question is why cant you just make a water pump that pumps in the opposite direction and mount it to a 1st gen small block? What makes it so that the coolant has to flow in one direction?
The reverse flow has to do with where the coolant enters the engine. Conventional design puts the pump output into the block where it circulates up to the heads and returns to the radiator. Reverse flow is where the coolant first enters the head then flows downward to the block and then returns to the radiator. The direction of pump rotation is not an issue with either choice. With centrifugal pumps, like those used on engines, coolant flow is in the same direction regardless of pump rotation. The impeller is configured for efficient pumping in the intended direction of rotation so it does way better when turning properly, but never-the-less either direction will pump. To reverse a conventional pump either the inlet would have to be connected to the engine and the outlets to the radiator, not easy to do; or the pump's output has to be intercepted before it goes into the block and rerouted. The latter system has been done many times on race engines where the hole going into the block is plugged. The horns of the pump are tapped for fittings and the coolant plumbed into the heads. On the Chevy the forward soft plugs are removed and the casting tapped for return fittings which then hose the coolant back to the radiator. The process of blocking off the flow into the front of the block has also been used for conventional flow on race engines where the supply hose puts the coolant into the block at one or more of the soft plug locations along the side of the block. Another is to feed a stream directly into the head between the paired exhaust valves for which sustained high RPM cooling is always a problem for engines with this configuration of valves while sending most of the flow into the block whether in the usual front location or into the side.
The reason for reversing conventional wisdom is that the coldest coolant is put into the heads first. These are the hottest part of the engine with their combustion chambers. Reversing the flow lowers the combustion chamber temps more than is seen with conventional flow which preheats the coolant in the block before it goes to the heads. The temperature reduction of the combustion chamber of the reverse flow process allows the engine to run at higher compression ratios and with leaner mixtures before the detonation limit is hit. This provides higher power outputs with better gas mileage than possible with conventional coolant flow.
Pontiac tried this with their first V8 from 1955 to 1959, it proved to have a lot of in-service problems mostly due to the era where water without any overflow recovery cooling systems were used and people were more familiar with earlier flat head technology that didn't require so much cooling system care. So in 1959 they went back to a conventional flow system.
The second generation of the LT-1 and 4 in the 1990s brought the system back with some changes; certainly better coolant and more sophisticated cooling systems which people became more acquainted with in the 1970s and 80s along with a vapor vent from the upper rear of the head made the major difference compared to the Pontiac experience of 35 years earlier.