It's hot and it wants to get even hotter. That's because the most basic function of your car's internal combustion engine is to generate heat energy into effective power. But all of this heat cannot be used for motive power. Very generally, about 25 percent is used for power and about 45 percent of the heat escapes in the exhaust and frictional losses, while the remaining 30 percent is transferred into the engine components. The heat that is absorbed into the engine must be discharged, or else the engine will overheat and fail. So to protect the engine against overheating, a cooling system regulates the engine temperature to a proper operating range. However, sufficient airflow, proper water-flow speed, and correct ignition timing are critical to a cool-running engine.
For this month's "How It Works" section, we'll discuss the inner workings of an engine's cooling system and, of course, our discussion will be based on a water-cooled Chevy engine. We'll cover everything from fans to radiators, shrouds, water jackets, and more. When we're done, you'll have plenty of cool ideas to keep your Chevy out of the red.
Because today's high-performance engines produce more power and thus heat, improved cooling systems are drastically needed to maintain a proper operating temperature. Inside your Chevy engine, water jackets surround the engine, and cylinder heads serve to cool it. These water jackets are supplied coolant by upper and lower hoses connected from the radiator to the engine. The water and/or coolant heated in the engine is continually forced by a water pump to circulate through the engine and out to the radiator. The coolant in the radiator, in turn, is cooled by the fan(s) and also by the air traveling through the radiator as the vehicle is moving. Inside the radiator's core are rows (typically two to four) that allow the water to flow in one direction, in and out, while outside air passing through the radiator fins cools the water by temperature transfer.
The thermostat is an important component on any engine, especially a high-performance one. It closes when the water temperature is low (cold) to prevent the water from circulating in the engine and opens when water temperature is high (typically above 180 degrees) to allow circulation through the system. Without a thermostat, the water would simply circulate too quickly and possibly even reduce power partly because more horsepower is required to drive the water pump when the restriction of the thermostat is removed. Thermostats help to provide a fast warm-up and also aid the engine to avoid acid formation in the oil, all the while reducing engine wear. Most new GM engines produced until the late '80s used 195-degree thermostats installed on the outlet side of the engine. GM cars produced since the early '90s typically use 180-degree thermostats (80- to 82-degree C) that are installed on the water inlet side of the water pump from the lower radiator hose. This is because water traveling from the radiator's lower hose is generally at a continuously stable and cooler temperature and does not cause the thermostat's operation to oscillate as frequently.
Pressure-Type Radiator Cap
Pressure-type radiator caps serve a purpose beyond just keeping the water in the radiator. On late-model cars, the pressure-type radiator cap is often found on the coolant reservoir instead of the radiator. If the cooling system were left open to the atmosphere, the temperature of the water would never rise above 212 degrees because it would boil out of the radiator and turn into steam. But because a radiator cap maintains pressure in the system, the boiling point is raised and the system is capped off. So the system pressure and the addition of antifreeze (containing ethylene glycol) raises the boiling point; while in cold temps below 32 degrees, the antifreeze lowers the freezing point (depending on the blend and capacity of the system).
Most of today's pressure-type radiator caps contain relief and vacuum valves. The relief valve allows the pressure (positive) in the cooling system to bleed out if it rises above a preset value, such as 16 lbs. When the engine heats up and the temperature rises, the excessive pressure pushes open the relief valve and blows off through the overflow hose. As the engine cools down, the vacuum valve in the cap serves to permit air into the radiator by opening when the internal pressure falls below atmospheric pressure (negative) after the engine is shut off.
Crossflow vs. Downflow Radiators
Crossflow radiators have radiator rows located horizontally so that water travels across the radiator, and downflow radiators have rows positioned vertically so that water travels downward. A crossflow radiator is typically more efficient than a downflow for the simple reason that the pressure cap is located on the low-pressure side (opposite the outlet or upper hose location). This cap location allows sustained high-rpm operation without forcing fluid past the cap. In addition, under-hood-space considerations typically allow a shorter and wider crossflow radiator to utilize a larger core with added surface area, which provides more effective and efficient cooling.
Pump You Up
The water pump serves to force water flow throughout the cooling system. The speed that the water pump turns (at a given engine speed) is dependent on the size of both the crankshaft and water pump pulley. The smaller the water pump pulley, the faster the impeller inside the water pump turns. If the water pump turns too fast, there are cooling losses because the water travels too quickly through the system. On a typical LS1 engine (or most other engine equals), a water pump consumes about 12 to 15 hp at 6,000 rpm. Most of today's water pumps are overdriven about 10 percent to 40 percent above crankshaft speed. Good water pump design balances the flow rate and pressure with the best impeller size. So, in essence, even your water pump is tuned to perform.
Compared to engine-driven fans, electric fans offer the benefits of reducing engine drag and allow the fan to typically be installed in several locations. Electric fans installed in front of the radiator are called pusher fans, and behind the radiator are puller fans. Pusher fans typically impede airflow by blocking portions of the radiator's core, as opposed to a puller fan, which pulls air through the radiator and is generally about 15 percent more efficient. So if space allows, it is usually better to install the fan on the engine side of the radiator to operate as a puller, where it can draw more airflow.
The shape of the fan's blades will change the fan's performance. A straight-blade fan is generally the most efficient fan possible, but often the noisiest. A quieter, curved-blade fan will move about 10 percent less air than a comparably sized straight-blade fan.
Until the '80s, when manufacturers began installing electric fans on most new cars, mechanical (engine-driven) fans that bolted directly to the front of the water pump were the norm. Engine-driven fans can work well in many applications, but they do cost horsepower.
To minimize the power loss, clutch-operated mechanical fans have been installed for decades. Most fan clutches typically engage at about 170 degrees air temperature (185 degrees engine temperature), run at 60 percent to 80 percent of engine speed, and will generally reduce the engine temperature about 20 degrees.
There are two types offered, and both operate on the fluid drive theory. One's a thermal type, which is typically what auto manufacturers have used, and the other is a low-cost and low-performance style called a non-thermal. The better thermal fan clutches varies the fan speed with temperature of the air behind the radiator. An identifying feature of a thermal fan clutch is the bi-metal thermostatic coil located at the front of the clutch. This coil senses the under-hood heat and activates the clutch accordingly. The operation (when engaged) provides maximum cooling, while the disengaged operation provides fuel savings and noise reduction.
Non-thermal clutches (also called centrifugal) are a low-cost replacement part. Although comparatively inexpensive, non-thermal clutches are always engaged, providing less fuel and power savings than a thermal-style fan clutch.
Just like all other facets of a high-performance car, your cooling system's components must be matched to everything else. The amount of power your engine makes, transmission stall speed, rear axle ratio, engine size, ignition timing, and more are all critical considerations when building, troubleshooting, or maintaining a cooling system. CHP