Lets do the maths.
CSA = cross sectional area.
3inches = 76.2mm
CSA = pi x R^2 = 4560 mm^2
For 2 equivalent pipes = CSA / 2
2280 = pi x R ^2
R^2 = 2280 / pi
R = sqrt (2280 / pi)
D = 2 x R
D = 53.87mm = 2.12 inch (2 and 1/8inch)
So you'd want to use 2 x 2.25 inch.
Steel density = 7850 kg/m3
= 7.85 g/cm3
Standard exhaust wall thickness = 1.5mm
So for 3 foot of 3inch vs. 3 foot of 2 x 2.25inch weight..
3inch
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Circumference = pi x D
3inch x 3 foot x 1.5mm thickness
Standardised to cm to make formula easier.
7.62 x pi x 91.44 x 0.15 = cubic volume of the steel in that length of pipe
Mass in grams = 328.24 cm^3 X 7.85
= 2577.5grams = 2.577kgs
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2.25 inch x 2
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Circumference = pi x D
2x (2.25 inch) x 3 foot x 1.5mm thickness
Standardised to cm to make formula easier.
2 x 5.387 x pi x 91.44 x 0.15 = cubic volume of the steel in that length of pipe
Mass in grams = 464.25^3 X 7.85
= 3644.4grams = 3.644kgs
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An extra Kilo..
Talk about doing it the hard way...but I didn't have to leave my computer or bourbon and coke