Posted for REHBINC@aol.com:

Steve,

This really needs to be adressed on a case by case basis. For simple tensile or compressive loading, where failure is determined by permanent deformation or collapse, adding a small amount of carbon to e-glass to attain a small amount of load capacity probably won't work very well. S-glass would probably be a much better choice, since it has a higher specific strength and a similar elastic modulus. If the goal of the reinforcement is to increase the stifness of the part (i.e.: to resist buckling, change its natural frequency or deflection under load), then adding carbon may make sense. Its specific stiffness is several times that of glass (either e or s). Just be certain that the deflection of the modified part doesn't cause failure of the added carbon. If it does, then you have only succeeded in adding a new stress concentration. Also, in some cases it may be important to consider the effect of the modification on the location of the neutral axis(usually not an issue). An additional layer of carbon will have a much greater impact on this than would glass.

On the surface, it sounds like adding a rib or two of carbon to your firewall would be an appropriate way to address your concerns.However, I don't know the details of your structure or its loading so I cannot give you any direct advice on reinforcing it. Rob