Some thermostats have a valve built in that plugs off the bypass port when the stat opens. That passage looks awfully big to remain open all the time! Check the bottom of the stat housing to see if you need a blockoff-type thermostat. You could also run a pair of hoses from the bypass ports to a ball valve that could be closed, or partially restricted to regulate the amount of coolant that bypasses the radiator.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!