We’ve all heard the usual advice about beefing up our rod’s cooling system. Bigger radiator, more air, etc. But before you lay out any big bucks, read on. When I built my rod it was prone to overheating, and in the course of taming it I discovered these cooling secrets. But first let’s define overheating in the context of this article. Overheating: the coolant temperature rises relentlessly, and steam and boiling water spurt from the overflow line. It is not simply a “hot” temperature. You may not like it that your rod runs at 230F, but as long as it can stabilize at some temperature, it is not overheating as used herein. That said, the secrets in this article can also lower your temperature if you so desire. Now the propensity to overheat is a function of variables, some of which are fixed in the engine design, and some of which we can control. Four variables we can control are 1) coolant composition, 2) coolant system pressure, 3) coolant flow, and 4) radiator air flow, and we will explore how to manipulate these to eliminate overheating.
But before we start, check your timing. I’ve seen many a motor running hot simply because the timing was not advanced enough, sending still-burning gasses around the exhaust valves and head passages. A small block Chevy should be running at least 15 degrees before TDC at idle. Mine idles at 35 degrees BTDC (initial of 20 plus vacuum of 15). And use full manifold vacuum; ported vacuum or no vacuum advance is not for street cars.
On to secret #1: did you know that water is a better coolant than antifreeze? And the clichéd 50/50 mix is not the optimum mix for many of us? You actually need only enough antifreeze in your water to ensure the coolant doesn’t freeze at the coldest temperature your car will encounter while it is sitting and the engine has cooled to the ambient temperature. Now when I lived in Texas and my rod sat in the garage, I didn’t need any antifreeze; it never got down to 32F in there. So I ran straight water. But not just any old water. I use distilled water. See, tap water has minerals that will eventually deposit on the inside of the radiator and the engine, reducing the efficiency of the cooling system. And I use Red Line Water Wetter, an additive which lowers surface tension, thereby improving heat transfer. Even if you need some antifreeze for your climate, you can include this additive to improve your cooling efficiency.
Secret #2: Do you know the pressure rating of your radiator cap? Who cares, you say? And why do we need any pressure in the cooling system, anyway? A little physics: the boiling point of any liquid increases with pressure. So the higher the pressure, the less the chance of coolant boiling into steam in the hottest places in the engine, namely around the combustion chambers. Now steam is a lousy coolant and is impossible to pump, and once you get a little of it, the hot spots can get hotter and make more steam and you have runaway overheating, which is serious business (engine damage) and you better get parked pronto. Luckily, caps are available in many pressure ratings, from 7lbs to nosebleed, and you might want to experiment with higher pressure caps if you are overheating. But be careful: use the lowest pressure cap that will do the job, because higher pressure puts more strain on hoses, gaskets, and the radiator itself. Be alert for leaks if you increase your pressure. I use 7lbs in winter and 13lbs in summer. And get an overflow tank if you don’t already have one. With the system full, as the coolant and any air in the system heats and expands, some coolant will go into the tank. Then once the engine is off and cooling down, this coolant will be sucked back into the system. Without a tank, you will lose some of your coolant and the system won’t be as effective as it would be if it were completely full. And make sure there is always some coolant in the tank; if the tank is ever dry, then the system will be sucking some air. You need a bigger tank if it continues to run dry. And note that coolant overflow increases with increasing temperature, so the tank must be sized for the highest temp you will encounter. When in doubt, go big.
Secret #3: Coolant flow rate: an old rodders tale says if coolant flows too quickly through the radiator it will not have time to cool “properly.” But if you keep the coolant in the radiator longer, it also stays in the engine longer, which increases coolant temperature. Coolant in the engine can literally boil away from the hot spots if not forced through the cooling system at a high enough flow rate. This is one area where more is always better. High-flow water pumps are an easy install and can add to your peace of mind. And try removing the thermostat to see if it is restricting flow.
Secret #4: radiator air flow. Let’s talk fans. Conventional mechanical fans are fine if they are doing the job, but one or two electric fans can help in nail-biting airflow situations, like sitting in summer traffic jams. I don’t have a thermostat, so to modulate my coolant temperature the two pusher electric fans on my radiator are wired to a Jegs fan controller. This nifty little gadget controls the speed of the fans. The fans come on at half-speed, and ramp up in sync with coolant temp. This keeps my engine at a stable temp regardless of air temp or engine load. I also have an electric water pump and I wired it so the pump runs along with the fans for a minute after key-off to cool things down, avoiding heat soak and possible boilover. Electric fans can also be controlled by simple thermostatic switches and/or manually with a dash switch.
And that’s it. I’m assuming your thermostat (if you run one; trust me, it’s not mentioned in the Ten Commandments) is working, and your radiator is clean inside; if you have any doubt, pull it and have the local radiator shop clean it. Then optimize your coolant as described and make sure the system stays full with a properly sized overflow tank. If you’re still overheating then try a higher-pressure radiator cap and/or add electric fans and a high-flow water pump. And check your timing!
I’ll be delighted to hear from you; contact me at dr.kerrysmith@gmail.com with any comments or questions. I don’t have all the answers, but I will pretend I do.
But before we start, check your timing. I’ve seen many a motor running hot simply because the timing was not advanced enough, sending still-burning gasses around the exhaust valves and head passages. A small block Chevy should be running at least 15 degrees before TDC at idle. Mine idles at 35 degrees BTDC (initial of 20 plus vacuum of 15). And use full manifold vacuum; ported vacuum or no vacuum advance is not for street cars.
On to secret #1: did you know that water is a better coolant than antifreeze? And the clichéd 50/50 mix is not the optimum mix for many of us? You actually need only enough antifreeze in your water to ensure the coolant doesn’t freeze at the coldest temperature your car will encounter while it is sitting and the engine has cooled to the ambient temperature. Now when I lived in Texas and my rod sat in the garage, I didn’t need any antifreeze; it never got down to 32F in there. So I ran straight water. But not just any old water. I use distilled water. See, tap water has minerals that will eventually deposit on the inside of the radiator and the engine, reducing the efficiency of the cooling system. And I use Red Line Water Wetter, an additive which lowers surface tension, thereby improving heat transfer. Even if you need some antifreeze for your climate, you can include this additive to improve your cooling efficiency.
Secret #2: Do you know the pressure rating of your radiator cap? Who cares, you say? And why do we need any pressure in the cooling system, anyway? A little physics: the boiling point of any liquid increases with pressure. So the higher the pressure, the less the chance of coolant boiling into steam in the hottest places in the engine, namely around the combustion chambers. Now steam is a lousy coolant and is impossible to pump, and once you get a little of it, the hot spots can get hotter and make more steam and you have runaway overheating, which is serious business (engine damage) and you better get parked pronto. Luckily, caps are available in many pressure ratings, from 7lbs to nosebleed, and you might want to experiment with higher pressure caps if you are overheating. But be careful: use the lowest pressure cap that will do the job, because higher pressure puts more strain on hoses, gaskets, and the radiator itself. Be alert for leaks if you increase your pressure. I use 7lbs in winter and 13lbs in summer. And get an overflow tank if you don’t already have one. With the system full, as the coolant and any air in the system heats and expands, some coolant will go into the tank. Then once the engine is off and cooling down, this coolant will be sucked back into the system. Without a tank, you will lose some of your coolant and the system won’t be as effective as it would be if it were completely full. And make sure there is always some coolant in the tank; if the tank is ever dry, then the system will be sucking some air. You need a bigger tank if it continues to run dry. And note that coolant overflow increases with increasing temperature, so the tank must be sized for the highest temp you will encounter. When in doubt, go big.
Secret #3: Coolant flow rate: an old rodders tale says if coolant flows too quickly through the radiator it will not have time to cool “properly.” But if you keep the coolant in the radiator longer, it also stays in the engine longer, which increases coolant temperature. Coolant in the engine can literally boil away from the hot spots if not forced through the cooling system at a high enough flow rate. This is one area where more is always better. High-flow water pumps are an easy install and can add to your peace of mind. And try removing the thermostat to see if it is restricting flow.
Secret #4: radiator air flow. Let’s talk fans. Conventional mechanical fans are fine if they are doing the job, but one or two electric fans can help in nail-biting airflow situations, like sitting in summer traffic jams. I don’t have a thermostat, so to modulate my coolant temperature the two pusher electric fans on my radiator are wired to a Jegs fan controller. This nifty little gadget controls the speed of the fans. The fans come on at half-speed, and ramp up in sync with coolant temp. This keeps my engine at a stable temp regardless of air temp or engine load. I also have an electric water pump and I wired it so the pump runs along with the fans for a minute after key-off to cool things down, avoiding heat soak and possible boilover. Electric fans can also be controlled by simple thermostatic switches and/or manually with a dash switch.
And that’s it. I’m assuming your thermostat (if you run one; trust me, it’s not mentioned in the Ten Commandments) is working, and your radiator is clean inside; if you have any doubt, pull it and have the local radiator shop clean it. Then optimize your coolant as described and make sure the system stays full with a properly sized overflow tank. If you’re still overheating then try a higher-pressure radiator cap and/or add electric fans and a high-flow water pump. And check your timing!
I’ll be delighted to hear from you; contact me at dr.kerrysmith@gmail.com with any comments or questions. I don’t have all the answers, but I will pretend I do.
Last edited: