Caps and Coolant
Speaking of radiator caps, it should be noted that the fill cap needs to be the highest point in your cooling system. This isn't usually a problem with your typical small-block and down-flow radiator or a stock radiator setup, but larger engines (dimensionally) and certain engine placement issues, like a modular engine swap, can sometimes mean the highest point in the system ends up somewhere in the cylinder head or intake manifold versus the radiator cap (you modular and V-6 owners are likely familiar with vent fittings on your intake to help refill an opened cooling system). If the top of the radiator is not the highest point in the system, then you must use a degas tank of some sort and mount it higher than the engine. If you're yanking a modular engine from a wrecked late-model Mustang, do yourself a favor and grab all of the stock cooling hoses and the degas tank from the car and factor them into your build. It will save a lot of headaches down the road trying to figure out hoses, cooling issues, and so forth. The one thing you don't want is trapped air in your system. While trapped air is inevitable in a new build, it is imperative your cap is at the highest point to help extract the trapped air, as the trapped air will always seek the highest point. We've seen people raise one side of a car to move the trapped air out or even add a radiator petcock to a coolant passage in their intake manifold to bleed out the trapped air.
As noted previously, the classic 13 psi cap you'll find as OE on classic Mustangs is not suitable for today's driving environment and should be restricted to “show use“ in our opinion. The higher the rating of your radiator cap, the higher the system pressure, which raises the boiling point of the coolant and increases the ability for the coolant to transfer heat from the engine. Check with your radiator manufacturer and use the highest pressure cap the radiator is rated for. Don't be surprised to hear that performance radiators can often handle a 22-24 psi cap. We highly recommend overflow canisters for vintage Mustangs along with a modern radiator cap. The combination allows for coolant that has expanded into the overflow to be pulled back into the closed cooling system as it cools. This is how all late-model Mustangs are setup.
Water is by far the best “coolant“ you can use in your cooling system. However, water alone does not inhibit corrosion, nor does it have the ability to raise the freezing point of water all by itself. This is why you need some form of cooling system corrosion inhibitor at the least and for cold climates the proper anti-freeze mixture to prevent the system from freezing solid. Believe it or not, a 50/50 mix, as many manufacturers recommend, is actually too high of a ratio for warm climates. If you live in a warmer climate a 70/30 water/coolant mix is better, or like we said, straight water with a conditioner/lubricant/inhibiting agent added to the system is preferred. For those of you who enjoy a good track run, check with the sanctioning body, as many tracks prohibit the use of coolant on the track surface.