Think of the engine in your Jeep as a giant gas heater that turns 20-33% of that heat into horsepower. The rest goes out the exhaust or is absorbed into the engine’s cooling system. The age of heat turned into work is an engine’s thermal efficiency. Most of the combustion heat, around 1000 degrees Fahrenheit, goes out the exhaust. Some 20-35% of that heat is absorbed into the coolant through the water jackets in the block and head and some is transferred to the oil. If too much is absorbed by the metal parts, you have problems like melted pistons, scored cylinder walls and burned valves.
Some people get into cooling system trouble due to neglect. Others because of modifications. Neglect is curable by maintenance and you can find solutions to those situations elsewhere. Engine and vehicle performance modifications often require cooling system modifications to keep pace and that’s where this Advisor will focus.
More Horsepower or Load Equals More Heat
Jeep built a cooling system based on the factory power output, usually with enough capacity that you can roam from the frozen woods of Minnesota to the blazing slick rock of Moab in the summer without worry. More power, whether that comes from modifications or an engine swap, changes that complex equation. The thermal efficiency%ages remain largely the same but you are left with more heat to shed using the same cooling system.
Vehicle changes can have the same effect as adding more power. More weight, big tires, changes in gearing, reduced aerodynamics and the blocking of radiator airflow with winches and lights up front will also play a big part. Climatic conditions will have a huge impact.
The Two Things
The two things you need to keep your engine cool are water flow and airflow. Both are variable according to engine speed and vehicle speed. Engine speed controls how fast the water pump and fan are operating. Vehicle speed controls how much more airflow is running across the radiator but engine speed also applies going down the road by supplying water flow at all speeds and fan speed when airflow derived from vehicle speed isn’t enough. The deadly situation is a high load without enough air or water flow.
Life’s Blood: Coolant
Plain water is a decent cooling medium. If it wasn’t for corrosion, freezing at low temperatures and boiling it would be ideal. The cure is anti-freeze/coolant, typically ethylene or propylene glycol. Mixed 50/50 with water, it raises the boiling point of the mixture by approximately 35 degrees and lowers the freezing point by 60 or 70 degrees Fahrenheit. It also contains corrosion inhibitors and water pump lubricants.
Ethylene or propylene glycol alone do not transfer heat well and that’s the important part water plays in the mix and why you never want to reduce the water content below 40%. There are some lifetime waterless coolants, such as non-aqueous propylene glycol, that offer much improved heat transfer and a much higher boiling point than a 50/50 glycol mixture but they cost in the neighborhood of $32/gallon.
A simpler and cheaper way to improve heat transfer is to use a surface tension reducer, such as Royal Purple’s Purple Ice. This additive increases the heat transfer abilities of the coolant mixture as well as carrying some additional anti-corrosion and water pump lubrication additives.
An additional tip is to always use distilled water in your 50/50 mix of coolant. Other sources of water may be highly mineralized and can cause scaling and corrosion, both of which reduce cooling system efficiency over time.
The Thermostat: Not a Throwaway
Misinformation on the role of the thermostat is common. Two common misconceptions are:
One: You need the thermostat to slow water flow in the block and/or the radiator so the water had time to absorb heat.
Two: You don’t need a thermostat at all because you want all the flow you can get.
Two comes closer to being correct than one but let's break them down individually.
Slowing the flow in the block does allow the coolant to absorb more heat but it often results in localized boiling, steam pockets and generally higher block temperatures. Turbulent, high-flow coolant results in a cooler engine because it moves away before it has a chance to boil. Most experts say that coolant flow should be as high as the coolant passages in the block will allow before flow restriction raises the pressure significantly.
For cooling alone, you don’t need a thermostat but there are good reasons to have one. Your engine is most fuel efficient at water temperatures above 180 degrees and you want the engine to warm up as quickly as possible. The fuel system is calibrated to run rich while the coolant temperature is below a certain level. Overfueling for cold running is necessary but it washes down cylinder walls with fuel, dilutes oil, creates carbon deposits, drastically increases emissions and decreases fuel economy. You want to keep that period as short as possible. On top of that, most of ring, piston and bore wear occurs when the coolant temp is below 180 degrees. The moral: on a daily driver, especially a short hopper, don’t use a thermostat with a rating under 180 degrees.
With a good thermostat installed, there should a negligible difference in coolant temps in a stock system with or without it. If removing the thermostat has any significant beneficial effects it’s usually a band-aid. To maximize coolant flow start by making sure the t-stat has the largest coolant opening that will fit in your housing. If a replacement has a smaller opening than the original, take it back. Beware of the cheap, “one-size-fits-all” replacement thermostats that have small coolant openings. This is done so they can fit into a greater range of housings. They may or may not be adequate for a stock engine but they will likely not be adequate for a modified engine. A simple and effective upgrade is opting for a high-flow, performance thermostat. The balanced sleeve designs from Robertshaw not only open more efficiently against water pressure (a problem with some t-stat designs), they flow more water.
The Water Pump: Go With the Flow
Virtually all engines use centrifugal water pumps. Pump efficiency is controlled by a combination of impeller design and pump speed. The average factory water pump is geared by the size of the pulleys to run at a ratio 1.25:1 to 1.30:1 of crankshaft speed. That’s 25 to 30% faster than the crankshaft. For sustained low speed, high load operation, you can run the pump faster for more low speed flow by installing a larger diameter crankshaft pulley and/or a smaller diameter water pump pulley. Most pumps develop their maximum flow at between 4,000 and 6,000 pump rpms, so an engine at 3,000 rpms with a 1.5:1 pulley ratio will be right in that ballpark. The caveat is that at high rpms and high temperatures cavitation can occur (the impeller is spinning so fast that it creates steam pockets in the liquid). Cavitation can lead to erosion inside the pump and the area around it.
Aftermarket high-flow water pumps are often more efficient at every rpm range because they use better impeller and housing designs. A simple water pump change could increase your flow significantly at any pump speed. When evaluating advertised water pump flow data, determine whether the rating is at system pressure and at a temperature approximating normal operation. Flow rates decrease with pressure and heat, so a big number at ambient temp with no system pressure isn’t anything to brag about.
The Radiator: Heat Rejection Specialist
The first division in the radiator world are between vertical and horizontal flow (also called crossflow) radiators. As far as cooling goes, there is nothing to choose between them but the vertical radiator is a lot more sensitive high coolant flow because the cap is on the high pressure side of the pump. Swapping in a high flow pump without also increasing the cap pressure can result an effect similar to boil over. That is the root of the “fast flow is bad” myth that still permeates the performance world. Crossflow radiators don’t have this problem because the cap is on the suction side of pump flow.
Another division is between copper/brass and aluminum radiators. While that “aluminum” heading could include the plastic/aluminum radiators used by the OE, we are talking about the welded aluminum performance radiators. The difference between them starts in their construction. Aluminum radiators can be built more robustly because the material is a little lighter. Aluminum radiators use fewer but much larger tubes that both flow more water and have more surface area to radiate heat. They also tend to allow more airflow than the average copper/brass radiator because the tubes are less densely packed.
An important aspect of any radiator type is the density of the cooling fins between the tubes. These help radiate the heat from the coolant filled tubes. Fin density is measured by fins per inch (FPI). Many older Jeeps used 9-15 FPI. Newer ones are common with 18-20 and sometimes a bit more. An increase in the number of fins around the tubes can add to the cooling efficiency of any radiator type, up to a point. Too many may restrict airflow and that’s counter-productive. In a dirty environment, a high FPI radiator can be a liability because they tend to clog more easily with mud or chaff.
When it comes to sizing a radiator for a swap, there are some general rules to get you in the ballpark. The first job would be to find the dimensions of the radiator in the engine’s original application. These can be found in replacement radiator catalogs or in service manuals. You want to get an area measurement by multiply the length by the width by the thickness.
Say you want to swap the 350 from a ’94 half-ton Chevy into a ’91 Jeep Wrangler. The Chevy core is 34x17.25x1.38, which comes out to about 809 square inches (34x17.25x1.38=809.37). The Wrangler radiator is 18.13x20x1, or 363 square inches, about half the area of the Chevy. Obviously, you can’t put a Chevy-sized radiator in the Jeep but you can substitute core thickness for frontal area to a certain degree.
The max-cool 3-core Jeep radiator offers 725 square inches using our handy-dandy formula. If you substitute a 4-core radiator (18.13x20x2.5) you are at 906 square inches. Is that enough? Most likely, but not in every case. The first couple of cores do most of the work and the airflow through a thick radiator is somewhat reduced, so the number gleaned above may not reflect a totally accurate estimate. Bottom line, the bigger the engine in your swap, the more time and effort you should spend into fitting a bigger radiator.
The Fan: Heavy Breathing
Moving down the road, you have lots of airflow to carry away heat. At low speeds on the trail, all you’ve got is the fan. There are a couple of fairly simple and low cost options to improving low speed airflow. As with artificial respiration, step one is making sure the “airway” is clear. Make sure the radiator isn’t blocked by too much bric-a-brac. The best fan in the word can’t suck air though a winch bumper, six auxiliary lights, an AC condenser, three oil coolers and a blanket of bug corpses. Second, consider speeding up the fan (the water pump will too come for the ride). There is a power cost to this but the trade is usually a good one if you have heating problems.
The two main divisions are between mechanical fans and electrics. Mechanical fans usually win the airflow contest. With either type, a shroud is vital to airflow. With a mechanical, you want the fan about an inch from the radiator core, taking into account any chassis flex conditions. Tip of clearance to the shroud of around one inch (or as required accounting for chassis flex) is ideal. For low speed operation, a fan bolted straight to the water pump, no clutch of any kind, is the bulletproof alternative. The downside comes if you cross water often. A clutch fan will often slip but to ensure no damage, the old standby was to loosen the fan belt for deep water crossings.
Fan clutches come in two varieties, thermally controlled and torque controlled. Both have a viscous coupling but the temperature controlled (identified by a thermal coil on the front) cycles on and off based on the temperature of the air flowing through the radiator. The torque sensitive fan responds only to engine speed, freewheeling above a certain rpm. Both can be effective in low speed work but the temp sensitive unit is preferred for low speed economy and towing.
A thermally actuated electric fan, such as those from Flex-a-Lite, only draw power when coolant temperature warrants it. That can translate into significantly increased fuel economy and a bit more power (typically around 5HP) to put into driving the vehicle. Until recently, electric fans didn’t generally have the airflow capacity to adequately cool a hard-worked trail vehicle. That’s no longer true.
Flex-a-Lite’s Black Magic fan, for example, can pull 3300 cfm and combined with an adequately sized radiator and enough coolant flow, that‘s enough for all but the hardest worked Jeeps, or those with the biggest engine swaps.
For daily drivers, an electric fan can offer up to a couple of miles per gallon. A nice side benefit to an electric for the more aquatic Jeepers is that you can install a cutout switch to disable the fan when fording. A tuning feature of the electric fan is that you can control the cycling temps by choosing different thermal sensors. Even better, the Be Cool adjustable fan control allows to adjust those temps from the driver’s seat to account for changing conditions. Regardless of type, the pitch of the fan blade will dictate airflow. A deep pitch blade will move more air but with a greater cost to power.
Oil and Oil Coolers: Oil! The Other Coolant
The lubrication system absorbs about 5% of engine heat. The lubricating qualities of the oil reduce internal friction and the flow of it carries away heat. Most engines shed lube oil heat is through the oil pan. Very few have oil coolers.
In most cases, oil temp runs at about the same temperature as the coolant, higher during a long, hard run. Before we get into how to shed that heat, bear in mind that you want the oil to be around 180-200 degrees. That’s the temperature at which the hot viscosity is obtained (the “20,” “30,” or “40,” in the SAE grade). When the oil is cool, it’s less viscous (thicker) and it doesn’t flow as well. Good oil flow aids both lubrication and cooling, so running oil with a higher viscosity than indicated by temperature conditions (e.g. running 20W-50 in a cool climate where a 5W-30 is specified) is counter-productive because fluid friction can actually add heat. In general, you run higher viscosities as ambient temperatures increase. A Jeep running in the cool northeast (even summer), for example, is fine with an grade 30 but a Jeep running hard in the hot deserts might need a 40 grade due the higher ambient heat.
The time to consider an oil cooler come after you have addressed other problems. An oil cooler might provide a quick band-aid fix on those really hot days but at other times it may be allowing your oil to run too cool. Oil that stays too cool cannot bake out contaminants, such as fuel or moisture and they can cause serious damage to your engine over the long term.
An oil cooler can aid in overall cooling but how can you find out if you really need one? The best way is to install an oil temperature gauge. A temp sender can be installed into the oil pan or into an oil gallery. Teeing off the oil sender doesn’t usually produce accurate results; the temp sender needs to be immersed in a flow or volume of oil. If oil temp is consistently above coolant temp, say above 230 degrees, that’s a good case for an oil cooler.
The ideal solution is to use an oil thermostat inline with the cooler. These devices open to allow oil flow to the cooler only at a predetermined oil temperature. In general, an engine oil cooler doesn’t need to be huge but the cooler and hoses need to be robust and well protected. Avoid putting the cooler in front of the radiator because you just add to the heat load of the radiator. In extreme conditions, a cooler with an electric fan might be warranted.
Another Radiator: The Exhaust System
A high%age of combustion heat is carried away in the exhaust. A restrictive exhaust system holds more heat in the engine for the cooling system to carry away. Decreasing exhaust backpressure and increasing flow can lower engine temps significantly. Improvements can be as simple as a swap to a low restriction muffler or the installation of a cat-back system. If you want to carry it farther, you can go with headers and free-flow cats. All of them will produce results, assuming you replace the restrictive element with a more free-flowing one. Power and mileage increase, too.
Stuff you Might Not Think Of
An increase in cooling system capacity is a benefit. More coolant equals a larger sink in which to absorb engine heat. A larger radiator will do that. Going from a two core to a three core on a Wrangler adds a half gallon.
On Jeeps with automatic transmissions, the trans cooler can add to the heat load of the cooling system. That load may be enough to tip the balance towards too hot. There are a couple of ways to approach this and both involve installing an auxiliary cooler.
The first opt is to install the cooler in addition to the factory radiator cooler but make sure the hot oil goes to the auxiliary cooler first. The oil that flows to the radiator may be cooler and actually help the radiator shed some heat. This is a good setup for winter climates where the radiator can help warm the ATF.
You also bypass the radiator cooler altogether. This is especially useful with big engine swaps where the small frontal area of Jeeps limits the size of the radiator. Engine swappers using automatics often use a manual trans radiator and a remote transmission oil cooler. The only time this option gets iffy is during cold weather operation, when the trans oil can be overcooled. Trans coolers with thermostatically controlled electric fans, such as Flex-a-Lite’s remote trans oil cooler, are a great addition in this case, because you can mount it anywhere. All auxiliary trans coolers should be mounted out of radiator airflow if possible so as to not add to cooling system load.
Reducing cylinder wall thickness by boring tends to make an engine run hotter. One rule of thumb claims 10 degrees for every 0.10-in of bore removed. On a non-race engine, the tiny power benefit from a to-the-limit overbore may be outdone by the complications that come with it.
Cool intake air not only increases air density but it results in slightly cooler exhaust temperatures. Overall, your Jeep engine will run cooler if you duct in the coolest air possible than if it ingests hot underhood air. The coolest air possible comes from a snorkel kit.
Pump Speed: Determining Pulley Ratios
If your crankshaft pulley is 6 inches and your water pump pulley is 4 inches, you have a 1.5:1 pulley ratio (6/4=2). With a 1.5:1 ratio if the engine is turning 3,000 rpm, the water pump is turning 4,500 (3000 x 1.5= 4500).