The pressure regulation in a car is done by the radiator cap, and those typically go to 25lbs max, with 15-17 being the norm. However, maybe it's regulated to that due to the hoses, radiator, pump, or something other than the block sealing (which is the only thing we are dealing with in a jet boat without a water pump). I don't know. Cylinder pressures are 10X+ what the radiator cap would regulate it to, so maybe it isn't an issue. It would be good to know.