I'm rebuilding my 91 502 after destroying an exhust valve . as a precaution i picked up a set of stock 98 vintage exhuast manifolds from a freind to rule out the possibility of mine maybe letting water hit the valve because of a leak . upon inspection of his manifold compared to mine the risers are really the only thing that is different . my exit on my stock early risers are an half an inch larger in diameter than the newer . is this comon knowledge than the early carb engines have different risers than EFI manifolds or did merc just change it without reason ? all the pieces are mercury manifolds and risers not after market.