So my buddys got a 91 ls and hes getting a vtec head put on by a shop and I saw the list of things they include in the swap and the GSR oil pump was there but I didnt see nothing about a water pump and when I searched a buncha people used a water pump but nobody really said why. I know you need the vtec oil pump but do you also need the water pump? Also how safe would it be to use the stock pistons?

 

they use the vtec water pumps (22t) because the LS water pump was only ment to flow to 7k rpm. at over 7k, the LS water pump runs out of effeciency and simply starts to cavitate the water. the vtec water pumps are meant to flow over 8k, so they're a better choice if you plan on running the engine any higher rpm. the stock pistons work fine, a little low on the compression side, but work fine. swap some p72's in there for about 10.2:1ish c/r depending what head or some pr3's in for about 11.3:1ish depending what head. if you want the motor to last a bit, i'd recommend balance the crank, shot peened rods (stock ls rods), arp rod bolts and head studs. with these things done (and the vtec oil pump/water pump) the motor will safely rev to 8k without a problem and last a while.