Installing the Lower Cross Brace
Many E36 owners upgrade their chassis with the factory crossbrace that mounts underneath the engine. This crossbrace was installed as stock equipment on all convertibles and the 1995 LTW, a lightweight version of the M3. The convertibles received this bar to stiffen up the chassis since their chassis are naturally less stiff than coupes or sedans because they have no roof.
Installing the crossbrace on a standard E36 coupe or sedan serves a dual purpose: The crossbrace stiffens up the chassis and also protects the engine sump from road hazards.
reference from: http://www.pelicanparts.com/bmw/techarticles/101-Projects-66-X-Brace/101-Projects-66-X-Brace.htm