Using a block heater in freezing winter temperatures can improve the fuel economy of your vehicle by 10 percent of more. A block heater warms the coolant, which warms the engine block and lubricants. The engine will then start easily and reach peak operating conditions more quickly. You don’t need to leave the block heater plugged in overnight. It only takes two hours at most to warm the engine. For convenience, consider using an automatic outlet timer to switch the block heater on an hour or two before you need to start your vehicle.Crew Cheif Note On a older cast iron V8 I would suggest a 4 hour + on time News source: canada on line