Throttle body selection

[SkurdalenFox]

I know that essentially one should select the throttle body that will flow enough mass of air at WOT so that it is not the restriction in the intake track. Yet one should not selecting a TB that is so big that one looses too much resolution in part throttle conditions.

However, at part throttle cruise, flowing the same mass of air, what’s more efficient in terms of pumping loss; a larger throttle body opened (for example) 15% or a smaller throttle body opened 20% ?

[coupe]

Not sure about your pumping loss efficiency, but for the same flowrate, the larer TB opened less will spread the flow around more of the blade's edge, so contouring the blade edge can help with tip-in throttle.

But remember: 15% open means 15% of the TPS is open too (vs. 20%).
Unless you're talking larger than 75mm, you will probably not even notice on any of the more common setups.