[Hotfingrs]

On a low horse engine "underdrive" might be ok, but on a high horse engine you would want between 7 and 12% overdrive. what you are looking for is about 20lbs of pressure in the cooling system, and this doesn't take into account the added pressure from the cap. you need to have a pressure gage in the system, warm the engine up to operating temps, then relieve the cap pressure, and start the engine and see what the pressure is...20 lbs is what you are looking for...this pressure in the cooling system will help prevent blown head gaskets in a hipo engine.