Originally Posted by Pete R
The ID of the stacks should be approximately the venturi size of the carb (34mm),or just slightly bigger.
If you made them 2-1/4 ID,the wave would be returning from the carb entry,and not from the end of the end of the stack.

Here's what the 2-1/4 inch airhorn looks like. Notice the air bleeds . 1-1/2-1-5/8 inch diameter stack will have a large flare end to fit the airhorn...I have no experience with this but wonder if the air flow will be disturbed in a bad way. I suppose actual testing might be necessary ...

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79 T140D, 96 900M Ducati ....On a bike you can out run the demons..