Neil - I guess you knew that I would be among the first to respond to this new thread. I like the way you have framed the issue. Let me begin by saying that I have been using 2" ID pipes and hoses all the way to the transom for several years. But your discussion about "diminishing returns" is sound and should give reason for pause about going any larger. The engine must always be able to expel the water. This requirement of course is going to vary with each boat's configuration. The greater the "head" against which the engine is working, the more dangerous larger wet sections would be potentially. All this of course needs to be quantified. Just one final thought for now: the manifold exit diameter is 1 1/2" - comments on why the manufacturer would reduce immediately to 1 1/4"?
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