SAVAGEJIM

I do not know if someone has already gone into the fluid mechanics behind the half-tank lean problem, but the half tank lean problem does exist and is all of a result of how pressure from the exhaust pressure line is distributed on the top surface area of the fuel in the tank.

For those of you who do not know, the primary equation in this principles is:
Pressure = Force * Area.

Here is what happens: When the tank is full or near full, the top surface area of the fuel in the tank is twice that of when the tank is half empty. At a given RPM (it does not matter if it's high or low RPM), pressure form the exhaust pressure line feed constant pressure into the tank and this pressure exerts a uniform force on the top surface area of the fuel. The pressure out of the bottom of the tank is therefore relatively high.
In this case, Pressure(to fuel line) = Force(from exhaust line) * large area. See the illustration to the left below.

Now when the tank runs half way down, the top of the fuel surface area is cut in half and at the same RPM as in the above case, the pressure from the exhaust pressure line is the same as when the tank was full. The pressure at the bottom of the tank all of a sudden is cut in half too. Therefore, the velocity of the fuel going into the engine is also reduced.
In this case, Pressure(to fuel line) = Force(from exhaust line) * half the original area. See the illustration to the right below.

Anyways, a fuel tank with a uniform shape from full to empty is why many choose to switch tanks. It should not matter if the new tank in mounted on the centerline or if the tank is mounted where the old tank was (it’s just that the new tank will stick out further to the left of the Savage).

I know the pressure form the exhaust line will vary according to the RPM of the engine, but this is compensated for by the demands of the engine. For example, at idle or low RPM, the engine does not need much fuel-air and its reduced combustions push just enough pressure into the fuel tank to feed it just enough fuel. When the engine increases RPM, the exhaust pressure in the pipe increases too and trerfore pushes more pressure into the fuel tank to therefore push more fuel into the engine.
I know the pressure curve will most likely not coincide with the fuel-air consumption curve, but the engine makers had this in mind and have the right curves when they designed their engines. For example, the fuel consumption demand at higher RPM might be just a slight bit higher where as the air consumption demand would be greatly higher (thus the performance from being leaned at full trigger).

If I am missing something, please let me know.