38 coupe
Well-known member
Under specific circumstances an overflow bottle helps on the stock unpressurized cooling system. I added a big ugly half gallon overflow bottle to a stock 40 Ford with good results (I hung it off the batter hold down, not willing to drill any holes in the car). The overflow allows for more coolant in the system. In the case of the 40 Ford since the radiator was now completely full this was about a gallon more coolant. This doesn't make the vehicle run any cooler under most circumstances, but does add a buffer for when the vehicle normally starts running hot. In the case of the 40 Ford, I had 160 thermostats in the car and it ran at that temperature until the outside air got hot. In central Texas the air gets hot a lot. The temperature would creep up when stuck at traffic lights during the summer. The worst was getting stuck in traffic after running down the road. The extra coolant in the system can absorb that much more heat when the engine temperature starts creeping up, buying time. The other thing the overflow bottle did was allowing a more rapid cool down once the vehicle got moving again, since the coolant that was pushed into the overflow bottle was cooling down while the rest of the coolant in the engine & radiator was heating up. When the vehicle got moving again and the coolant in the engine & radiator started to cool down and contract, the coolant from the overflow bottle that was already a bit cooler was pulled back into the radiator, rapidly bringing the cooling system back to normal operating temperature. This was extremely useful in Texas during the summer heat when driving in conditions where you had to stop every mile or two.
