Cooling system woes are as old as hot rodding itself. Any time you build more horsepower, the byproduct is more heat. In recent years, the problem has gotten more acute. Street cars with 700, 800, or even more horsepower are becoming commonplace and these giant numbers necessitate a cooling system just as powerful.
But you don't need horsepower like this to over-burden a cooling system on a classic car. Many of our favorites from the '50s and '60s had cooling systems that were barely marginal from the factory. If you've increased the horsepower of your factory powerplant by 50-percent or more—something that's pretty easy to do with a head and cam swap—you could be at the limit of the factory cooling system's capabilities when it was new. That radiator in your car is probably not exactly as functional today as it was 40 years ago, either.
Then there is the proliferation of aftermarket air conditioning systems for classic automobiles. These are great for keeping passengers comfortable, but was your factory cooling system designed to accommodate the extra heat they throw off? Probably not. For every degree the air conditioning system is taking from the cockpit, it is moving it right to the condenser sitting in front of the radiator. Something else to note: R-134A A/C systems are not as efficient as the old R-12 systems were, therefore they work harder and produce more heat to cool the cockpit the same as R-12. This makes more heat at the condenser and it's blowing right into the radiator.
When we put a 383 stroker in Project Homewrecker, our '72 Corvette project, we had the radiator professionally cleaned by a radiator shop. Everything was hunky-dory until the car and its owner moved to the west coast of Florida from the northeast. This really put the old system to the test. What we noticed at first was in the summer, when temps could be in the mid-90s with 80- or 90-percent humidity, the Stingray would run hotter on the highway than it did in town by about 20 degrees. Not optimal, but we were still in the safe range. Still, this was a sure sign the cooling system was not efficient enough and causing us concern.
The real trouble came after we installed the great C3-specific A/C kit from Vintage Air. While the unit did a remarkable job of keeping the passengers comfortable, the factory radiator and clutch fan was overmatched. At 70-75 mph in the summer on the highway, the orange needle was swinging right near the 250-degree red zone on the temp gauge. Once you got off the road and onto city streets the temp would come down—again, a sure sign the system was overwhelmed—but once parked and shut off, the radiator would puke coolant all over the ground, something it never did before. (This car was never equipped with an overflow/expansion tank.)
Corvettes of this vintage have always run hotter than other Bow Ties because styling took precedent over function. They were the first factory bottom breathers. They take in air from under the bumper, not through the grilles. The chin spoiler was not only a functional aerodynamic piece, but it was also tasked with directing air to the radiator. There's very little underhood area on these early C3s, which exacerbates the problem. Big-block cars were known to overheat in traffic when new.