The Air/Fuel ratio is actually getting richer as the rpms increase. It starts off at 15:1 air/fuel and fuel increases so that the mixture is 11:1 by redline. The power drop off is due to the small T25 being unable to supple the same amount of boost as rpms increase. The torque peak coincides with max boost which is 22 psi. After that boost decreases down to about 15 psi by redline.