It seems that improving the ability of the air to exit underneath the engine compartment is perhaps the most efficient way to vent underhood heat at high speed. You see attempts by many circle track racers to help this with the aluminum heat shield they build to sit above the exhaust headers at roughly valve-cover height. These shields are designed to deflect heat underneath the engine to keep it away from the induction system and especially the carburetor. With a turbocharged car, it would seem beneficial to use a temperature barrier like High Performance Coatings on the turbo exhaust housing that will keep more temperature in the housing to help spin the turbo and also reduce the underhood air temperature as well. These coatings could also be applied to the headers and the exhaust pipes with the goal of reducing underhood temperature.
More Info
High Performance Coatings; Whitsett, NC; 800/456-4721; hpcoatings.com
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