There's often a whole lot of hot air leakage coming through the firewall after
all these years. From poorly fitted transmission covers to missing brake &
clutch pedal boots, missing wire grommets, and extra holes drilled into the
firewall and never sealed off.
I have seen something at one show that could help the heat a whole lot. It was
a plastic sheeting about 1/4" thick filled with air. Sorta like bubble wrap,
but much more compact bubbles. Aluminum skinned on both sides. I think the
temperature claim was up to 700 degrees F. A sheet of that fastened to the
entire firewall could greatly reduce heat transfer into the passenger
compartment without being particularly in the way or heavy. I wish I'd have
bought a roll of this stuff when I saw it, because I haven't seen it since.
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