IMHO perhaps round tube for the plenum with a round butterfly is a valid
possibility.
Maybe cheaper, easier, lighter, more efficient, and better looking.

Terry Adams
Thorp T-18C

Cheaper than $2.75 a foot? Not easier as a round butterfly is harder to make
than a square butterfly. Also harder to get a nice bell mouth shape where
the intake tubes join the plenum. No doubt about it. It looks better to the average 
onlooker.

Paul Lamar
 
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