I looked at the "Heatsheets" with a view to installing them on my SC. The side that the cat is on gets hotter than hell, and anything would help. But since I put on a test pipe and put the cat into storage, there is far less heat to worry about.
All the Heatsheets are is pieces of thin metal (SS?) bolted to some of the valve cover studs, using metric threaded nut couplers to space out the sheet an inch or so. You could make all this yourself for a few bucks.
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Doug
'81 SC Coupe (aka: "Blue Bomber")
Canada West Region PCA
members.home.net/zielke/911SC.htm