In spite of others claim of the 20HP or maybe more boost- on modified heads by itself, this is simply impossible with no camshaft or pistons changed. Toyota has casted and machined all their 4 valve heads to pretty close and exact tolerances for Road (and rally) use. Listing of https://knoxxahon.pages10.com/new-step-by-step-map-for-toyota-4age-for-sale-62467008