Deputydog,
I have the pretuned pipe and header.The manual calls for a 10x6.It is not for a specific engine.They list about a dozen motors and only one prop.They are pretuned for a certain rpm which a 10x6 gives you.Using a higher pitch will work as long as it has the same load as a 10x6
Do you have one of their pretuned tuned pipes?
Did you read the manual?
The macs muffler is not tuned for anything.