3106 wrote: Polycarbonate has the WORST chemical resistance of any plastic resin, not what you want for under bonnet! ABS isn't much better. Polypropylene would be better, but Tony Dunn's louvered aluminum pieces are extremely nice and they will likely allow a bit more air flow than solid panels. Fasten them securely with rubber washers and they should never rattle. Polypropylene is not just better it is what ever car manufacture in the world uses for under bonnet,inner wings,under tray,etc so it is the material of choice. Polycarbonate is not the worst of all the plastic composites chemical resistance and ABS is a well used and tried material in the automotive industry too. Many OEM car companies us PC ABS 20% talc for for their front grilles,windscreen trims,bumpers,exterior trim,etc. I have powder coated louvred aluminium engine and radiator valances they are nice yes but not for everyone ;)