2.75" vs 3" pipe on supercharged vehicles.

Sh0velMan

Oh and your figure for 2.5" dual is a little off.


2.5" Dual = 9.817 sq inches
2.75" single = 5.939 sq inches
3" Single = 7.068 sq inches
3" Dual = 14.136 sq inches
3.5" Single = 9.621 sq inches
4" Single = 12.566 sq inches
4.5" Single = 15.904 sq inches

Edit: re-read your post, I see where our numbers are different, you're subtracting wall thickness. Pipe diameters should be ID rather than OD. So subtracting thicknesses should be unnecessary.

Edit 2: And the reason they over-estimate pipe size is because most of the flow is happening at the outside perimeter of the pipe, so a general rule of thumb is to add 10-15% (with the percentage being smaller the larger the pipe diameter) to the pipe size to allow for turbulence losses at the center of the pipe. Sounds silly, but it's been worked out many years ago by folks a lot smarter than us hahaha.

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Alstann

Erm, not to dispute, but according to my fluid mechanics knowledge, the main flow of a fluid through a closed pipe is in the center. The velocity on the edges of the pipe is actually approaching zero. It's actually a large reason why polishing an intake manifold adds little to no gain, even a loss, depending on the application.

A straight increase in pipe diameter does not net a linear change in exhaust velocity, if I'm correct.

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Sh0velMan

You're right, I got my facts confused.

The boundary layer @ the perimeter of the pipe has little flow but is (obviously) where the pressure is, whereas the center flow is high speed and lower pressure (naturally).

The same 10-15% loss applies, I just had my facts twisted.

And yeah, spot on the polishing bit. Actually get better flow if you texture it so the boundary layer can latch on easier.

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