Looking Beyond the Cooling System for Common Overheating Cures
The hardest part about keeping your gauges out of the red on a street rod usually happens after you start changing what the factory did. But wait, isn’t the core of street rodding all about changing the factory design? After V-8 engines get dropped in place of four and six cylinders, space becomes a premium and you're left with figuring out how squeeze the biggest radiator you can fit into a crammed engine compartment. The struggle comes in when there’s no longer a standard guide to use — you become the engineer so to speak. But with the technology involved in today’s performance parts, there shouldn’t be any reason we can’t keep our rods running cool. There may not be a set-in-stone guide on how to keep every street rod cool, but there are some known good rules to go by when it comes to the cooling system.
One of the first things you can examine is: at what point is your engine overheating? Does it happen all of the time or just when you’re in traffic? The radiator must have air moving across it to carry away the heat, so if you’re on the highway and it stays cool, but at a traffic light it starts to overheat, the issue is usually related to airflow. The next questions you can ask are: do you have the proper cooling fan in place? And are you using a fan shroud? Consider the path of least resistance — the same is true with airflow. If you have a shroud in place to channel incoming air, it’s forced to travel through the radiator.
So what happens when you have already replaced the radiator, the water pump, and added a new cooling fan, but the car is still running hot? If we look at the source, a stock engine produces about 42 BTU per horsepower and after performance upgrades even more. When a vehicle is overheating and you have new components in the cooling system, it’s time to start pointing your finger somewhere else. The engine may be creating too much heat due to factors other than the cooling system.
A proper air/fuel mixture will help your engine run cool and to its full potential. Most overheating issues related to air/fuel ratios are the result of a lean mixture, which causes the cylinders to run hotter. On most carbureted systems, fuel pressure typically ranges between 5 and 8psi at all speeds. It’s important to maintain that pressure under all load conditions, because if it drops off at higher rpms, you run the risk of going lean. As a general rule of thumb, most stock engines use roughly 14.1:1 to 13.4:1 air/fuel ratio at idle and slightly richer at high rpm. So having a quality fuel pump and a correctly tuned and jetted carburetor is a must for proper fuel efficiency.
There are a few ways you can check your air/fuel mixture. Many automotive shops use an exhaust gas analyzer, but not everyone has access to tools such as this. As an alternative there are several aftermarket manufacturers who supply air/fuel monitors and wideband tuners that provide readouts from the exhaust. Or another way to gauge your fuel mixture is by examining your spark plugs for abnormal burn characteristics.
Considering the principles of ignition timing, if the spark occurs to soon or too late, the cylinders will run too hot and the engine can suffer from power loss. If the spark happens too soon, the engine fights against cylinder pressure which may cause a pinging problem. When the spark occurs too late it won’t completely burn compression gasses which creates more heat that the cooling system has to dissipate. The optimum ignition spark timing will vary with engine speed, load, and air/fuel mixtures. And the correct ignition timing involves more than just setting the initial timing; the amount and rate of the mechanical and vacuum advance curves are also very important to avoid overheating. This is because cylinder pressure is much higher at wide open throttle than at midrange. The lower the cylinder pressure is, the more time it takes to complete the combustion process.
You should always use the correct thermostat for your engine. Without a thermostat the engine would have no way of regulating the minimum heat range. The bottom line is that the manufacturer designed our engines to run with them installed. Every engine has an optimal heat range to run efficiently, in general 190°F to 210°F. On most carbureted engines, a common thermostat used is a 180°F.
You can check the function of your thermostat a number of ways. One method is by letting the engine come up to operating temperature while feeling the upper radiator hose, once the thermostat starts to open, you should feel the hose start to take pressure and get hot. You can also use an infrared heat gun on the thermostat housing to observe the temperature change as the engine warms up. Some may even remove the thermostat and check its function in a pot of hot water with a cooking thermometer.
If a water pump is leaking it almost always means the pump has failed and needs replaced. Before you install a new pump, always check to make sure the impeller spins freely and has minimal end play. If it’s a high performance engine, high flow water pumps have been proven to help with coolant flow. You should always use the correct size pulleys according to what was on the engine from the factory. It’s not recommended to install under-drive pulley kits on street driven vehicles. Undriving the cooling system (spinning the water pump slower) can lead to poor cooling performance. If you're buying an aftermarket water pump be sure to pay attention to the direction of rotation. Some of the later style aluminum water pumps with a serpentine-belt drive system used a reverse rotation pump, which can lead to serious headaches if overlooked.