Heat wrap does reduce the life of the mainfold because it gets hotter as you said. They tend to crack.
And the number are the primaries, secondaries and collector numbers. So 4-2-1 would be 4 pipes into 2 pipes and finally to one pipe. 4-1 would be 4 primaries collecting straight into 1 pipe. If you google each you will see pictures. Speaks a thousand words
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