Mark Houlahan
Tech Editor, Mustang Monthly
June 14, 2012

The last thing to consider when it comes to radiators is whether to retain the original down-flow radiator style with tanks on the top and bottom, or to update to a more modern crossflow radiator. If there's room for a proper crossflow core design we recommend the crossflow design for the reasons mentioned previously, plus they position the radiator cap on the low-pressure side of the system. A high-flow pump can create high system pressures, which will force water/coolant past the radiator cap at high rpm. This is terribly common on down-flow radiators, which is why we also recommend the highest cap rating you can find if you're going to keep your standard down-flow radiator due to space constraints (the stock 13-psi cap doesn't cut it with today's engine rpm capabilities).

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, 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. 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 to have 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 end 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 and Fords is not suitable for today's driving environment, and should be restricted to "show use" in our opinion. The higher the cap rating, 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- to 24-psi cap.

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. Certain racing sanctions will expect you to run straight water to reduce cleanup times should your cooling system be compromised in some way on track.

Cooling Fan Choices

Having the biggest radiator you can fit in your engine bay and a high-flow water pump won't mean much without adequate airflow. In this regard, you have two avenues; electric cooling fans and beltdriven fans. Each category has its good and bad points. For most, the traditional beltdriven fan is what we're used to. The fan bolts to the water pump's hub and is driven by a pulley and belt combination. We've seen fans installed backwards, we've seen the wrong pulley ratio used (salvage yard pulley swap for example), and we've seen belt routings that don't put enough belt surface on the pulley, causing slippage. Of course, you can have too small of a fan, or a fan with not enough blades to cool as well. When using a beltdriven fan, we recommend the "A/C" style fan with deep curved blades to really pull the air through the radiator core. If you're using a beltdriven fan, then use a shroud, no excuses. Airflow will take the path of least resistance, and that can, and often will, mean around the radiator instead of through it. Ensure the fan blade pitch is half into the shroud and half out by changing or adding a fan spacer as required. This will also ensure the airflow is drawn through the radiator instead of around it.

Electric cooling fans are pretty much the norm on new cars these days. Just about everything but fullsize trucks have an electric fan or pair of fans. The Mustang has used an electric fan setup with multiple speeds since 1994 (that's nearly two decades now), so electric fans certainly aren't just for race cars or for added towing insurance anymore. Electric fans allow for more room in the engine bay and offer better control over your cooling system, as they are an "on demand' cooling device, only running when the thermostatic switch in the coolant stream deems the fan necessary (low speed driving, long periods of idling, and so on). In some instances, like modular V-8 swaps, they are mandatory since there's no way to mount a beltdriven fan to the engine's beltdrive system. There are essentially only two ways to mount an electric fan, either as a pusher or a puller; meaning on the engine side of the radiator pulling air through, or on the grille side of the radiator pushing air through. An electric fan is going to be more efficient as a puller, but if you must use a pusher type, ensure it has adequate core coverage and moves enough air for your application.

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