My logic for wanting to keep the heat from escaping the pipe is that if the heat can't get out then it won't radiate to other things around it. Yes, I plan to use heat shields where there is space for them to be mounted with an air gap so the shield can be effective. Here's some pics showing areas where I think it's too tight for a heat shield to be used.
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About 3/8" gap between header and shifter housing on transaxle. Yes, this is a transverse mounted engine so the transaxle is parallel to the engine near bottom of the V.
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Header passes within about 1/2" from oil pan.
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Starter solenoid within 1/2" of header.
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On the other side of header from the starter, coolant pipes/hoses are 3/4" from header.
As you can see, the headers are already built (so utilizing smaller tubes isn't an option) and the header tubes were sized by professionals at Burns Stainless to maximize exhaust flow. Hopefully these examples show why I'm looking for the best options to keep the heat within the pipes.