This invention provides a system and method for forming a fibrous porous ceramic substrate that employs a screw extruder, and illustratively, a twin screw extruder to form a substrate by directing a homogeneous, wetted and mixed group of substrate components through a screw extruder die under pressure. The components of the mixture can be initially mixed in a substantially dry state by an appropriate mixer to form a homogeneous powder with a high dispersal of materials therein. The powder can be continuously mixed and conveyed to a feeder of the extruder. Along the path of the extruder, fluid can be introduced at a metered rate, along with other additives, such as colloidal silica (glass binder). The extruder's twin, co-rotating shafts include a combination of screw elements for feeding the mixture and shear-inducing mixing elements (kneading blocks) for thoroughly mixing fluid into the dry components. The wetted components pass through alternating sets of transport screws and kneading blocks until the kneaded, wetted mixture finally enter a vacuum section of the extruder where a vacuum is applied to remove excess air pockets and/or bubbles from the mixture. The mixture is thereafter driven through the die head where it exits as a continuous, extruded shape. Such a shape can comprise a honeycomb useful in filtration applications. The extruder can include a cooling system. The fifer in the mixture can be mullite. Alternatively the mixture can form a silicon carbide or other type of porous substrate matrix.