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Off Road Cooling Systems


All engines make power through heat and heat is as much a part of the equation as fuel, spark and air. Maintaining proper engine temperature means more complete combustion, which in turn means greater fuel economy, more power and lower emissions.

Too much heat however, just like too much of anything in life, can cause major problems for your engine. Too much heat can cause detonation, power loss, and in extreme cases, failure of engine components. Naturally the more power your engine produces, the more heat you will have to deal with. Off road engines also have their own, special concerns. You don't want to have an overheating engine when you far out in the wilderness. The fact that most off road driving is done at a blazing Mach 0.0002, means that airflow issues are of particular concern.

The cooling system of your off road vehicle (or any vehicle for that matter) is relatively simple. However enough variables are involved to cause problems in extreme off-road conditions, or when modifications to the vehicle have been made. (There was a somewhat technical discussion on the thermodynamics of engine cooling a while back, which some of you may find interesting.) In the most simple sense, the cooling system utilizes a liquid to transfer heat from the engine, to the outside air, via the radiator. The coolent is pumped through the block and heads to extract heat from these critical components. The coolent is then passed through a large intercooler (the radiator), where it's heat is transfered to the outside air.

Well, that all sounds simple enough. But making the system work on your off road rig is a bit more important than keeping the 'ole grocery getter cool. Here are some things you can do to ensure that your off road vehicle brings you home safe, every time.

Measure Cooling Performance
Airflow (Fans)
Shrouds and Ducting
Water Pumps

Measuring Cooling Performance

One of the first things you should do if you have a cooling system problem is invest in a quality water temperature gauge with the appropriate sending unit. In fact any engine should have a good quality gauge. Never trust your expensive motor to an idiot light. It's a good idea to test the sending unit with the gauge to make certain that it properly calibrated. A faulty gauge reading is as bad or worse than no gauge at all. You can test the gauge by comparing it's readings for a pot of boiling water with that of an accurate cooking thermometer. Coolant temperature readings can also be checked on the car with a device known as a pyrometer.


Before you begin replacing expensive cooling system items, make sure that the rest of the engine is working properly. Make sure that your overheating problems aren't a result of an out of tune engine. An engine running too lean or with the bad timing will run hot, and all the cooling system modification in the world won't help. Check the simple things first, before spending lots of money. Many times overheating problems are caused by inexpensive items.

One of the first things you should check is the thermostat. A thermostst is nothing more than a heat sensitive valve which controls coolent flow according to temperature. To check the termostat, place it into a pot of water, along with an accurate thermometer. Heat the water, and note the temperature at which the valve opens. Turn off the heat and make sure that the valve closes again. Changing the thermostat heat range will alter coolent temperature in the system, but beware: cooler is not always better.Choose your thermostat based upon the requirements of the engine, Most modern engines run better warm; high compression (>10:1) engines may need to be a little cooler, but they don't need the extra heat for better ignition becasue they have the extra pressure.

If you put in a thermostat that is too cold then it will be wide open at all time. The water will run through the radiator with no time to cool and just go back through the engine again and get hotter, until eventually it overheats. A properly operating thermostat will show a variable engine temperature and you can watch the temperature rise and fall. Another advantage of a high temp thermostat is greater cooling capacity. The object of cooling the engine is to get rid of excess BTU's. The greater the difference between the hot and cold temperature, the greater the cooling effect ( don't all our truck's cool better on a cold day than a hot one?). For example: since a 192 thermostat is 12 degrees hotter than a 180, I will lose as many BTU's on an 82 degree day as a Bronco w/ a 180 will on a 70 degree day, and as many as you will on a 50 degree day.

Be certain to check the coolent itself. Old coolent, coolent that's full of rust and crud or an improper coolent mixture can cause overheating. Flush and change coolent on a regular basis. the proper ratio of antifreeze to water will vary depending on season and location, but if you don't know than mix in at least a 50/50 ratio.

If you found crud or rust in the cooling system, there is a good chance that the radiator is clogged and you should have it checked. The passages in the radiator are very small and it doesn't take much to plug them up. If the corrosion was severe, you may have to have the radiator routed out by a professional radiator shop. This is generally around $50.

Check the belts and hoses. An overheating problem could be caused by something as simple as a collapsed return hose or a slipping water pump belt. The return (bottom) hose can actually have enough suction applied to it so as to pinch the hose shut. If this is the case, replace the hose with a good quality, reinforced hose and make sure it's not bent or kinked at odd angles.


Since the radiator is arguably the most critical piece of the cooling equation, special attention should be paid to selection and modifications for off road applications. Custom radiators offer many advantages over O.E.M units when high demands are placed on the cooling system.

The two primary materials used in radiator construction are aluminum and the traditional copper/brass. Aluminum's primary advantage is one of weight. Aluminum is not as "tough" as copper/brass, and is not recommended for off road use. Most importantly, they are not as efficient heat exchangers as copper/brass. Some claim that they are as much as 1/3 more efficient than an equivelent aluminum radiator. They are also much more expensive.

There are two common primary core designs in modern radiators: the serpentine fin and the flat fin. Flat fin radiators have a regular pattern of straight, horicontal fins connecting the coolent tubes. Serpentine fins can be identified by their long rows of accordion like folded fins.

If your engine truly needs additional cooling, the only way to do it is by increasing the size of the radiator. The size of the radiator is directly proportional to the amount of heat it can dissipate. But how you define bigger is important too. Given the same amount of cooling surface, a bigger, thinner radiator will cool better than a smaller, thicker one. This is because the most effective part of the radiator is the front surface (where the air is coolest), and the bigger radiator will have more front surface. In a thicker radiator, the back fins receive air that has already been heated.

Radiator caps are also important. The cooling system is pressurized, which effectively raises the boiling point of the coolent. Radiator caps carry a pressure rating. This rating is the point at which the cap begins to act to bleed off excess pressure, before the pressure destroys other components of the cooling system. The higher the rating, the higher the pressure, and the higher the boiling point of the coolent. Once you are sure that you have selected a radiator that will do the job, you must ensure that the air flow through the radiator is sufficient to cool your engine.


Even the best radiator in the world won't keep your engine in the green if there isn't sufficient air flowing over it to remove the heat. Most off road driving is done at very low speeds, and may involve a lot of ideling. Therefore, we can not rely on the vehicles motion to force air through the radiator, like many cars do. We must provide our own source of airflow. This is accomplished by the fan.

There are two kinds of fans: electric and engine-driven. Engine driven fans are realiable and efficient. The only drawback may be space, but this is usually not a concern. They do drain a bit of horse power, but this is generally not harmful at off-road speeds. (Remember, we're not building a race car here.) The blade size, number and pitch can all be tuned for optimum airflow. For example, a fan with large, high-pitch blades will move a great deal more air than one with small, flat blades. Do not get a flex fan. Flex fans are for race cars, the object of a flex fan is to push less air when the engine goes fast. This is bad for off roading.

Electric fans do not draw any engine power, and can fit into very small spaces and be mounted in front of or behind the radiator. They can also be programmed to keep running after the engine shuts off, to prevent boil over. They are very effective at low engine speeds. If choosing a location to mount an electric fan, be aware that it is as much as 10% more efficient when mounted behind the radiator. Pulling the air through the radiator is less restrictive to airflow than pushing, since the fan itself is an obstruction. If you really want some airflow, you can mount an electric in front, and either an electric or engine mounted fan behind.

Shrouds and Ducting

Many people overlook the importance of the fan shrouds and inlet ducts. For off road use, a fan shroud is mandatory due to the very low speeds involved. The shroud serves to maximize airflow through the radiator, while minimizing aerodynamic drag.

At low speeds this can best be accomplished be an efficient fan shroud behind the radiator to help the fan do it's job more effectively. Early fan shrouds were quite primitive, but with using wind tunnels and computer programs, modern shrouds are quite effective at improving low speed airflow. The idea is for the shroud to pull air through the entire core section, not just the area in front of the fan. Fan tip clearance should be at least 1/2 inch, and 3/4 inch is better. The fan shroud should be positioned so that one half of the fan blades project into the shroud for maximum airflow.

It is also important to pay attention to the radiator and shroud mounts. You want to avoid the hot air that just passed through the radiator from passing back over the top of the radiator and shroud and back into the radiator. Air will flow through the path of least resistence, especially towards a low pressure area like the one in front of the radiator. Make sure that the area around the sides, bottom and top of the radiator direct air into the radiator. A small airdam below the radiator may be very effective.

Once you've got the air flowing through the radiator, you must make certain that it has an escape route. If there isn't sufficient airflow out of the engine compartment, the radiator will not be as effective. This is not usually a problem at off road speeds.

Water Pumps

The water pump is the final variable in the cooling equation. Unlike radiator air flow however, higher coolant flow rates are not always desirable. Most expert agree that you don't want to force coolant through the radiator too fast, or the radiator simply can't remove the heat fast enough. Both impeller design and pulley size determine coolant flow rates. The pulley controls the speed of the pump, so changing the pulley size will either slow down or speed up the pump. Be aware though that overspeeding the pump can cause cavitation, which aerates the coolant and reduces its heat dissipating properties.

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