Tech Talk #78 – The Big Chill: How to Avoid an Engine Meltdown

DavidTechArticlesBy David Reher, Reher-Morrison Racing Engines

“Most drag race cooling systems are utterly inadequate to dissipate such staggering heat.”

An engine has two fundamental needs: lubrication and cooling. Racers typically devote a great deal of time and money to oiling systems, devising windage trays, baffles, deflector screens, and dry-sump systems to ensure continuous lubrication. In comparison, drag race cooling systems are almost an afterthought – and that’s a grave mistake.

The results are unmistakable when an engine overtaxes its cooling system. Excessive heat does bad things to good engines – head gaskets fail, decks turn blue, valves recede and tulip, pistons seize, and aluminum alloys lose strength. Hot spots in the combustion chambers lead to preignition, which in turn leads to detonation, which initiates a chain of destruction. It’s a blessing that an overheated engine is seldom run to catastrophic failure – usually it loses so much power that the driver realizes that something is wrong and shuts it down.

Why are drag racing engines prone to meltdown? We’ve all watched NASCAR engines run 600 miles at wide-open throttle on a sweltering summer day without problems. At the Bonneville Salt Flats, land speed record racers run flat-out for miles at maximum power. And yet a dedicated drag race car probably wouldn’t last a minute under those conditions.

The answer is in the math. One gallon of gasoline contains roughly 120,000 British Thermal Units (BTU) of heating value, enough energy to raise the temperature of 1,000 pounds of water by 120 degrees. A typical internal combustion engine converts only about 25 percent of the fuel’s energy into useful work (accelerating the car). The rest is turned into waste heat or consumed by mechanical friction. Approximately 30 percent of this waste heat must be dissipated by the engine’s cooling and lubrication systems. So if a drag race engine burns one gallon of gas in the course of staging, burnout, and a quarter-mile run, potentially more than 36,000 BTUs have been dumped into a cooling system with a capacity of only a few gallons of water.

Most drag race cooling systems are utterly inadequate to dissipate such staggering heat. Drag race cars typically use tiny radiators (or sometimes no radiator at all), low-volume electric water pumps, and inefficient fans that simply can’t cope. It’s more accurate to think of these components as “cool down” systems rather than cooling systems, since their chief purpose is to reduce coolant temperature after a run.

A true cooling system would require a massive radiator, a high-volume water pump, and huge fan to balance the input and output of heat. Look at the size of the radiator that’s required to cool a 500-horsepower engine operating continuously in a diesel-powered commercial truck– it’s enormous. Even the cooling systems in passenger cars and light trucks are rarely able to keep up with the heat gain when the engine is run at continuous peak power. That’s why the temperature gauge in my Suburban quickly heads for “H” when I’m towing up a grade.

The cooling system in a typical drag race car is marginal at best, yet we want the engine to be as cold as possible to produce maximum power. The colder the engine, the less likely it is to detonate. That’s why Pro Stock racers use chillers and refrigeration systems to lower the water temperature to below 50 degrees before a run. These systems also have enough pressure to purge air pockets effectively from the system. I realize that such systems are impractical for sportsman racers, but their use illustrates the importance of keeping an engine cool and its cooling system purged of air for maximum performance.

Filling the radiator doesn’t necessarily mean that the coolant is where you want it. It’s vitally important that the water completely fill the passages around the combustion chambers. Air is a much less effective heat conductor than liquid; efficient heat transfer demands that the coolant be in constant contact with the metal. Unfortunately, the water jackets must occupy the spaces between the ports, the head bolts, and the spark plug bosses. These convoluted passages are prone to capture air pockets and bubbles that prevent the coolant from fully contacting the metal surface.

That’s why many raised runner and billet cylinder heads have bleed lines between the ports and valves. These vents allow air to escape when the engine is filled with coolant. Engines with conventional heads can benefit from installing air bleeds at the rear of the intake manifold. In fact, many aftermarket intake manifold castings incorporate bosses that simplify the installation of auxiliary coolant lines and drain cocks.

Cooling systems in dragsters present special problems. Most dragster radiators are mounted horizontally, either well above or below the engine’s coolant connections. In either installation, it takes extra effort to purge air completely from the system.

If the radiator is higher than intake manifold, install a drain cock in the back of the radiator and run an electric water pump until the air is completely purged by cracking open the drain cock. If the radiator is mounted horizontally below the intake manifold, install a drain cock in the rear of the manifold or cylinder heads to bleed the air. Even with these steps, it may still be necessary to jack up the front or the rear of the car to get all of the air out of the system.

A temperature sensor will only give an accurate reading if its probe is in contact with the coolant. If the water jacket isn’t filled completely or contains air pockets, the temperature sensor will give a false reading. The gauge may say that the engine is cool, but it could actually be overheating. It can take time to purge the air from the cooling system, but the result is well worth the effort.

Some racers use a hot engine as a band-aid for another problem. I’ve heard racers say, “The engine stumbles at the starting line, so I have to run it at 180 degrees.” In reality that is a carburetor problem that needs to be fixed rather than covered up with a hot engine.

The objective of any racing engine is to burn as much fuel as possible in the shortest amount of time. The energy that’s released is what propels the car to the finish line. The unavoidable consequence is that more heat is also released into the cooling system. Keeping this heat under control is essential to engine survival.