In automotive racing where standard porting is verboten, an abrasive slurry of a gritty putty consististancy is pumped through at pressure to smooth the transit

In automotive racing where standard porting is verboten, an abrasive slurry of a gritty putty consististancy is pumped through at pressure to smooth the transitions. I think ford even used it for a few performance manifolds. Commonly called extrude honing. https://en.m.wikipedia.org/wiki/Abrasive_flow_machining https://en.m.wikipedia.org/wiki/Abrasive_flow_machining