It’s more the combination of the St bottom end with the Bt top end and the thicker gasket that gives a budget but more capable motor.

The st bottom end has fatter rod bolts and a bigger gudgeon pin with a bit more meat around the piston crowns. All the week points of the Blacktop.

The bt top end has milder cams that suit a mid power turbo with plenty of breathing capability for the job in hand with stiffer inlet valve springs from the St.

The st head breaths better but this is not the limiting factor at the aimed power level. Then you have the higher compression, more duration and lift from the inlet cam than is needed and the hassle of properly managing the vvt. You get none of this with the bt head.

This is not a 500hp capable engine, it is a diy budget motor.