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The question regarding Low-pressure casting and Gravity casting has been asked a lot and it keeps getting ignored when asked so I`ll answer.

In a Wheel Manufacturing perspective:

Gravity casting is a method of manufacturing a cast wheel of an specific metal alloy, typically aluminum, although Magnesium is also used, which involves supplying molten metal to a mould cavity via a feeder through a running system where the molten metal entry point is located above the top of the mould cavity. In this manufacturing process all the metal entering the mould is subject to turbulence. This turbulence can cause severe defects such as oxide inclusions and entrapped gas porosity and in some cases mould erosion and hotspots.

Low-pressure casting in the other hand takes care of the disadvantages of Gravity casting by filling the mould through an in-gate just below the top of the mould cavity from a source under the mould through a method which allows complete filling of the mould. The force of gravity acts against the upward flow of the metal and eliminates the possible turbulence caused by the falling liquid metal. The molten metal is stored in a pressured container and by increasing the pressure metal is then forced into the mould.


Low Pressure Casting Compared to Gravity Casting

In low-pressure casting the method in which the metal is forced into the mould is the main difference/advantage over gravity casting and is absolutely controlled. As explained above it results in a low turbulence or turbulence-free mould filling with little to no corrosion.

The molten metal in the furnace is in a closed container under protected atmosphere. Because of this the metal absorbs less hydrogen and any other impurities as well as the oxide formation being greatly reduced. As opposed to gravity casting, the metal surface is not interrupted constantly since it is been forced from under the metal surface. What you get is a very clean quality metal.

Basically low-pressure casting uses positive pressure to move molten metal into the mold quickly thus resulting in a finished product with an Aluminum that is denser than Gravity cast. The cost involving the production of low-pressure cast are higher than gravity

As per Tirerack.com:

GRAVITY CASTING
Gravity casting is the most basic process of pouring molten aluminum into a mold utilizing the earth's gravity to fill the mold. Gravity casting offers a very reasonable production cost and is a good method for casting designs that are more visually oriented or when reducing weight is not a primary concern. Since the process relies on gravity to fill the mold, the aluminum is not as densely packed in the mold as some other casting processes. Often gravity cast wheels will have a higher weight to achieve the required strength.

LOW PRESSURE CASTING
Low pressure casting uses positive pressure to move the molten aluminum into the mold quicker and achieve a finished product that has improved mechanical properties (more dense) over a gravity cast wheel. Low-pressure casting has a slightly higher production cost over gravity casting. Low pressure is the most common process approved for aluminum wheels sold to the O.E.M. market. Low-pressure cast wheels offer a good value for the aftermarket as well. Some companies offer wheels that are produced under a higher pressure in special casting equipment to create a wheel that is lighter and stronger than a wheel produced in low pressure. Once again in the quest for lighter weight, there is a higher cost associated with the process.

In a Wheel Quality perspective

The advantages of low pressure die casting process are several :
- higher yield achievable
- reduction of machining costs
- excellent control of process parameters
- good metallurgical quality
- leads to an optimal use of the aluminum
- excellent mechanical and technological properties of the castings.

Disadvantages of gravity die-casting:
- High porosity rate
- High absorption of impurities
- Less dense material which results in a heavier wheel to be able to achieve the required load rating
- Very high process variations which leads to more defective units

Gravity casting does get the job done but with a higher rate of defective units as opposed to a stronger and lighter wheel made by low-pressure casting.

Feel free to post any questions you might have.