you need to have the thermostat to keep the coolant in contact with your bores and other engine internals long enough for effective heat transfer. Your temp gauge will not be giving you an accurate reading of the temp of your engine. The coolant will run cooler, but the temp of bores and such will be higher. If you dont want to fit the thermostat, at least cut one up and just put the backing plate in to give you some restriction to flow.