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The Novak Guide to 

Cooling for Jeeps®

Jeep engine water coolant temperatureWhether for conversion or stock Jeep powerplants, cooling is a key issue. We've been advising customers on cooling since our company was founded in 1967, and this article is the concise culmination of the knowledge that we and many of our customers have earned on the topic.

The radiator is only one part of the complex cooling equation. You should keep in mind that there are many variables involved with the process of cooling an engine that it’s nearly impractible to use hard and fast rules, but if you apply the following principles, you will be able to make the right decisions to ensure that your cooling system keeps up with your Jeep® vehicle. Overheating should not be a problem if some basic principles are observed.

Engine Condition and Tuning

454_big_blockAn often overlooked area is the condition of the engine itself. Mechanically it must be sound and the water jackets of the block and head(s) must be free of contaminants and build-up that could act as an insulator, preventing proper heat transfer to the cooling media. A good flush of the cooling system and engine with the right detergents is a good way to eliminate this possibility. You should also be confident that engine compression, timing - both ignition and camshaft - are optimal.

The carburetion and injection systems must be sound and tuned. An engine running too rich will generate significantly more heat. Running fuel injection systems “open-loop”, i.e.; without O2 sensors, Vehicle Speed Sensors, etc. can cause this situation. Induction tract air leaks are also a source of cooling problems in that they can cause the engine to run on too rich of a mixture.

The Radiator Cap and Cooling System Pressure

The cap of the radiator acts as a filling location, but is also a safety valve for the cooling system. The cap should be located at the absolute highest point in the cooling system to allow for the release of air and for proper filling. The location of the cap and configuration of the radiator has some import. A cross-flow radiator is less likely to push the cap open prematurely by the pressure of the flowing fluid instead of the excess pressure of the cooling system itself. This brings us to our next topic... Pressure.

One very effective way of raising the boiling point of a fluid is to place it under pressure. Vehicle cooling systems’ operational temperatures vary. “Running hot” in one system’s application may be normal temperature to another. In other words, an open (or non-pressurized) cooling system will, of course, boil at 212 degrees Fahrenheit at sea level. This same system will be normal at 195-220 degrees F. with a 15-16 pressure cap and 50:50 mix of glycol coolant and water. A higher pressure in the cooling system offers a higher boiling point. In a sealed cooling system, the pressure the hot coolant creates itself performs this pressurization function automatically. Higher coolant pressures also transfer heat from the cylinder heads more efficiently. But cooling systems can only withstand so much pressure, and a relief valve built into the radiator cap provides this function.

As mentioned earlier, the location of the cap is important. It should always be located at the highest point of the cooling system; this includes the engine itself. This allows for any air in the system to rise to the cap area where it can be released, preferably to an overflow reservoir. The cap area should also see the lowest velocities within the system, further allowing the coolant to de-aerate near the cap. This is also the part of the system where the pressure is lowest, which is crucial. You only want excess heat pressure to open the cap, not the flow of the coolant. This is why thermostat housing style caps are not recommended.

Naturally, you will want to use a radiator cap with the higher pressure ratings that the radiator is designed to accept. As a general rule, standard systems operate at 15-21 PSI, performance radiators operate at 22-24 PSI, and professional racing radiators will go as high as 29-31 PSI. The coolant will typically only build to 16-18 PSI, due to expansion up to 200 °F. However, if the engine does overheat due to external factors, the pressure inside the cooling system could reach as high as 28 PSI. Once the radiator cap has opened and vented coolant, it is a slippery slope. The engine will not cool down until the engine has been turned off! We do recommended that for Jeep applications, moderate pressures (22-24) be used. Higher pressure systems become increasingly difficult to service the further away from civilization you travel.

Cool water

Water has some of the most unique physical properties of the all compounds. One of these is its ability to carry heat. As such, pure water is the most effective media to carry the damaging heat away from the engine. The reason we add an ethylene glycol mix to the water is to:

  • Eliminate the possibility of freezing of the coolant (engine and radiator breakage being the risk)
  • Effectively raise the boiling point of the coolant past water’s natural limit of 212 degrees F.

Like other alcohols, ethylene glycol extends the boiling and freezing points of the water to which it is added. However, when mixing the two, remember that more is not better. Any ethylene glycol concentration above 70% begins to raise the freezing point of the cooling media. What’s more, ethylene glycol is significantly less effective at transferring heat than pure water. Mixtures of higher than 50% concentration are only recommended in the more severely cold climates. Follow the coolant manufacturers guidelines as you make mixture decisions based on this principle.

The Novak aluminum RadLock and other radiators require the use of an aluminum-safe coolant. GM’s Dexcool and other equivalents are recommended and widely available.

With the inception of aluminum radiators and now more aluminum engines, silicates and some organic acids have been introduced into the coolant mixtures. As a rule, don’t mix coolant types and brands together in your system, or the life of the media will diminish quicker. And don’t rely on coolant colors alone to indicate compatibility, as different brands and different national and corporate standards do exist.

We should mention coolant disposability. Most Jeep owners have the class and responsibility in the disposal of coolant media, but some may not. Most locales have governmental and environmental information resources to assist you in finding a facility to dispose your coolant. Many parts stores and garages will also accept coolant drained from your vehicle. The thought of having that sort of foul substance percolating into you or your food’s water supply should be deterrent enough.

Water Pumps, Thermostats and Flow

One of the myths being perpetuated in the aftermarket is that of high-flow water pumps - either conventional or electric - being able to cool all engines better. Faster is seldom better in terms of coolant flow and heat transfer. The coolant needs enough time in the engine to pull the heat out of the block and heads. Likewise, coolant needs enough time in the radiator to shed its accumulated heat before returning to the engine. As such, fooling with coolant flow speeds without a very solid basis for doing so can be counterproductive.

There is a balance to acheive, especially with Jeeps performing hybrid functions.

If your vehicle overheats while at cruising speeds (and consistently higher RPM’s) than you may:

  • be pumping coolant too fast
  • have a radiator that does not dissipate heat quickly enough (more below)

If you overheat while in traffic or crawling in the low RPM’s, you might:

  • not be flowing coolant fast enough
  • have a radiator that does not dissipate heat quickly enough (more below)
  • have a fan our shroud situation that does not pull enough air at low speeds (more below)

On the topic of coolant flow velocity, it is popular in some horsepower tweaking and race circles to use underdrive pulleys. It is only in very rare circumstances that one should consider these for any vehicle, and we can think of no situation in a Jeep where such a water pump underdrive system would be indicated.

ThermostatThis leads us to our next subject and precaution; the thermostat. Heat is a miserable thing, and too much will destroy your motor. However, too many people arrive at the incorrect conclusion that a lot less heat will benefit the engine. Your motor is designed to run optimally within a certain thermal window. This window is even narrower for fuel injected engines. Running a 160 degree thermostat is not a good thing. Many do so either because they think they are doing their engine a favor, or they are trying to increase the cooling efficacy of an inadequate cooling system. The latter is nearly always an excercise in futility! If your cooling system can’t hack it, adding a cooler thermostat will benefit nothing. Of equal concern is when people run the engine without a thermostat, presuming that the increased flow will increase cooling capacity. This is another commonly couterproductive philosophy. The thermostat acts as a necessary restrictor in the system to regulate the flow rate of coolant. This is contraindicated, per the above discussion.

To restate somewhat, many mistakingly think that all there is to decreasing temperature of the engine is to decrease the temperature rating of the thermostat. If the cooling system cannot keep up with the heat the engine is generating, a cooler thermostat will help precious little. The second misconception that if some engine cooling is good, more must be better. Again, most engines run optimally at the recommended factory temperatures. Significant departures from these recommendations are most often counterproductive. The temperature is critical to the efficiency of the burn of the fuel and the tolerances of the motor itself. 180-195 degrees F is common on conventional engines. Modern motors see higher temperatures (205-210F) , mostly due to fuel and emissions efficacy.

Flow through the thermostat is important. Coolant should flow well enough to avoid the build-up of hot spots in the engine. Some thermostats feature ports that allow coolant to flow slightly even with the thermostat closed. While this delays the engine warming up in cold climates, it does promote better engine cooling, generally.

Heat Exchanger Materials 

Gen III radiator
The materials used to build the radiator are an important factor. Copper/bronze cores are the most common, but they are slowly being supplanted by aluminum units, especially where more rigorous cooling requirements exist. Heat transfers through different materials at different rates. Aluminum has some of the most impressive transfer capabilities of the common metals. Generally, an aluminum radiator with its high level of thermal conductivity will cool better with less core area.

Plastic units, for the discussion of Jeep conversions, are mostly worthless. They usually don't have the strength or cooling capacities required. Some radiators employ plastic tanks with a metallic core, and prove difficult with which to work.

Fans and Airflow 

The relationship of the core to the fan is quite important also. The core should be parallel to, and not more than one inch away from the fan and should be centered on the fan. The core cannot be mounted to the back of the grill as there must be space for air to spread out so it can flow through the core.

While on the subject of fans, we have finally seen electric fans become strong enough to effectively cool an engine conversion. These are typically within the 2200 - 3300 CFM range. With some good fan choices on the aftermarket, some of the cooling difficulties once experienced with mechanical fans, are now lessened. Additionally, many aftermarket electrics come with a built-in shroud and housing, further facilitating the process. Many units also carry a temperature sensor and relay to trigger the fan based on radiator heat; a convenience to the setup process. As for mechanical fans, both flex and clutch style fans can be less than effective.

It is important to note that by the time air has passed through three sets of radiator tubes it has just about reached the temperature of the coolant and additional thickness does little to improve cooling. This varies with ambient temperature and speed of airfiow. It simply means that core area is much more important than core thickness. Another important factor related to airflow is mounting or carrying items on the front of the vehicle. Most Jeeps have a rather limited grille opening area. License plates, oil coolers, driving lights, toolboxes, winches, etc. will usually cause enough turbulence to disrupt airflow at some speed or other and this could result in overheating. Also, the mounting of an automatic transmission cooler in front of the radiator will measurably diminish engine cooling capacity as well. By the same token, a transmission cooler mounted behind the radiator will be subject to the heat flowing through the fins of the radiator, sometimes making it a poor location for transmission cooling purposes as well.


This shows a conversion where the shroud was taken seriously. It was fabricated from sheet aluminum and gave outstanding airflow

There may be situations that will result in less than ideal radiator to fan locations and these can be solved by shrouding the fan. Actually, a fan shroud is a good idea under any condition. This causes an air accelerating vacuum which improves airflow particularly at low vehicle speeds. The best installations will have the rear edge of the shroud centered over the middle of the leading edge of the fan blades.

While on the subject of shrouds, some ask about pusher fans vs. the more conventional pullers. Pushers are measurably less effective due to the shrouding differences and the usual lack of a shroud with such scenarios. If you find yourself having to resort to a pusher type fan to even get the motor to fit, you may take it as an indication that the motor is mislocated or too long for the vehicle being converted.

Differences in Engines

Some engines (and some automatic transmissions for that matter) create more heat when what they should be creating is more power. Different engine brands and designs of similar displacements burn hotter than others.

In our long experience of cooling engines, we have discovered interesting and significant differences in the classic V8's, including (from hottest to coolest burning) AMC, Dodge, Ford (302 & 351) & Chevrolet. Some engines are notorious for needing larger radiators and/or oil coolers than a similarly displaced and power producing engine of another design and brand. Heat is (as per Carnot) a byproduct of power, but also a product of internal friction and other inefficiencies.

Engine Oil Cooling

engine_oil_coolerEngine oil cooling is underrated and under-utilized.

No longer considered just a source of additional cooling, engine oil cooling may be one of the most effective ways of drawing heat out of the engine and prolonging the quality and efficacy of the engine oil itself.

Some choose a radiator with an integrated oil cooler (that can be used for the engine oil or the auto transmission fluid). Some choose a dedicated unit that uses either passive or active (usually an electric fan) cooling. A combination of both is feasible and will also add to the volume of oil for your engine.

Gauges & Sensors 

Another overlooked item is the accuracy of the temperature gauge. Some gages may be considerably out of calibration. Either electrical or mechanical gages can be off by 20 to 30 degrees. One should always cross-check the temperature with a known good gauge. However, it has been our experience that the good mechanical gauge is more accurate and often preferable, though the routing of its pressure line is arguably more difficult than stringing a wire into the cab of the vehicle. One misconception should be debunked here, and that is that the temperature of the coolant media is the same as the metals adjacent to it. In fact, some spots of the engine see dramatically higher temperatures. This is why coolant flow rate is crucial. Should the coolant flow too slowly past these hot spots, it will boil and then lose is cooling capabilites, leading to a potential and dangerous chain reaction within the engine.

Again, a coolant overflow recovery system should be used on all pressurized cooling systems to make sure they stay full of coolant at all temperatures.