Building a powerful engine is only part of building a dependable off-road vehicle. That engine also needs a cooling system capable of removing the heat it produces—not only while cruising down the road, but while crawling at low speed, climbing under heavy throttle, racing for hours, or sitting behind another vehicle in dust and mud.
Cooling problems often start before the engine ever runs. A radiator gets selected because it fits the available space. Fans are chosen because the advertised CFM number looks good. The thermostat gets removed because someone believes it will increase coolant flow. Plumbing gets routed wherever it is easiest to package.
Then the vehicle hits the trail and the temperature starts climbing.
At Wide Open Design, we approach engine cooling as a complete system. Radiator core area, engine horsepower, fan performance, shroud coverage, thermostat selection, coolant circulation, system pressure, plumbing, and air removal all need to work together.
This guide explains how to choose the right cooling system for your rig, how we size the radiator packages used at Wide Open Design, and how to diagnose a cooling problem when the temperature does not stay under control.
Bonus Wide Open Design Downloads:
Download the Trail Ready Extended Cooling-System Inspection Checklist
Download the Trail Ready Radiator Recommendation Guide
Why Off-Road Vehicles Are Hard to Cool
An off-road engine can produce a tremendous amount of heat without receiving much natural airflow.
A street vehicle usually has air being pushed through the grille and radiator as the vehicle moves. A rock crawler may be working hard at only two or three miles per hour. A rock bouncer may go from idling to wide-open throttle almost instantly. An endurance race vehicle may stay under load for hours.
Common off-road cooling challenges include:
- Slow vehicle speed with high engine load
- Long hill climbs
- Repeated wide-open-throttle pulls
- High ambient temperatures
- Mud, dust, and debris blocking the radiator
- Restricted airflow around a tube chassis
- Hot engine or exhaust air recirculating through the radiator
- Rear-mounted cooling systems with long coolant runs
- High-horsepower engines installed in tight chassis layouts
- Racing conditions that cause heat to build throughout the entire vehicle
This is why radiator selection cannot be based only on whether a radiator physically fits between two chassis tubes.
The radiator must have enough usable core area for the horsepower being produced, and the fan package must be capable of pulling air through that core when the vehicle is not moving fast enough to create natural airflow.
Start With Horsepower and Usable Core Area
Wide Open Design uses a practical starting point of approximately:
0.67 square inch of usable radiator core area per horsepower
This is based on the actual cooling area of the core—not the radiator’s total outside dimensions.
For example, tanks, mounting areas, and other parts of the radiator do not contribute the same cooling area as the exposed core. That is why a radiator described as 27.5 inches wide does not necessarily have a full 27.5 inches of active core width.
The basic sizing calculation is:
Usable core area ÷ 0.67 = approximate horsepower capacity
This gives us a practical starting point for matching one of our radiator packages to an engine.
It is not an absolute temperature guarantee. The final result will still be affected by fan quality, shroud coverage, engine load, ambient temperature, coolant circulation, radiator placement, installation quality, and how the vehicle is used.
WOD Radiator Recommendation Guide
| WOD Cooling Package | Usable Core Area | Approximate Recommended Output | Best Application |
|---|---|---|---|
| 27.5" × 15.5" radiator with dual 12" fans | 22.5" × 15.5" = 348.75 sq. in. | Up to approximately 525 HP | Compact LS/LT swaps, trail rigs, crawlers, and tight chassis installations |
| 31" × 15.5" radiator with dual 14" fans | 26.5" × 15.5" = 410.75 sq. in. | Up to approximately 625 HP | Most LS-powered trail buggies and general high-performance off-road builds |
| 38" × 17" radiator with dual 16" fans | 34" × 17" = 578 sq. in. | Up to approximately 875 HP | High-horsepower builds, rock bouncing, Ultra4, racing, and severe-duty use |
These horsepower numbers are rounded recommendations. They are meant to help you choose the right starting point without pretending that every vehicle creates and manages heat in exactly the same way.
WOD 27.5-Inch Radiator Package: Up to Approximately 525 HP
The WOD 27.5" × 15.5" Dual Pass Radiator Combo is our most compact complete cooling package.
Its usable core measures approximately:
22.5" × 15.5" = 348.75 square inches
Using the WOD sizing guideline, this puts it in the range of approximately 525 horsepower.
This package is a strong choice for:
- Lower-horsepower LS and LT swaps
- Trail rigs
- Rock crawlers
- Custom tube chassis
- Compact engine compartments
- Builds where radiator width is limited
- Vehicles operating within the recommended horsepower range
The package uses a dual-pass aluminum radiator, -16 AN inlet and outlet fittings, a clamp-mounted full shroud, and two 12-inch SPAL puller fans rated at 1,870 CFM each.
The smaller outside dimensions do not mean this is a low-performance radiator. When the engine is within the recommended horsepower range and the system is installed correctly, this package works well in tight off-road applications.
The mistake is trying to use the compact package on an engine that exceeds its intended cooling capacity.
Packaging should determine which suitable radiator fits the chassis. Packaging should not be used as an excuse to install less radiator than the engine requires.
WOD 31-Inch Radiator Package: Up to Approximately 625 HP
The WOD 31" × 15.5" Dual Pass Radiator Combo is our normal recommendation for many LS-powered trail buggies and custom off-road vehicles.
Its usable core measures approximately:
26.5" × 15.5" = 410.75 square inches
Using the WOD sizing guideline, this package is recommended for engines producing up to approximately 625 horsepower.
This is the middle of the WOD cooling lineup and fits a wide range of builds:
- Naturally aspirated LS-powered trail buggies
- Higher-output LS and LT swaps
- Rock crawlers
- Recreational rock bouncers
- Custom Jeeps
- Tube chassis vehicles
- High-performance trail rigs
- Builds requiring more cooling capacity than the compact package
The 31-inch package uses a dual-pass aluminum radiator, -16 AN fittings, a clamp-style aluminum shroud, and two 14-inch SPAL puller fans rated at 1,864 CFM each.
For many off-road builds, this is the best balance of radiator size, horsepower capacity, airflow, and chassis packaging.
When choosing between the 27.5-inch and 31-inch packages, horsepower should be the primary decision. The 27.5-inch package works well when the engine is within its range and the chassis is tight. The 31-inch package adds cooling margin for higher-output engines and more demanding use.
WOD 38-Inch Race Radiator: Up to Approximately 875 HP
The WOD 38" × 17" Race Radiator with Dual 16" SPAL Fans is the severe-duty package in our cooling lineup.
Its usable core measures approximately:
34" × 17" = 578 square inches
Using the WOD sizing guideline, this package is recommended for engines producing up to approximately 875 horsepower.
This is the radiator we look toward for:
- High-horsepower engines
- Big-block applications
- Forced-induction combinations within the recommended output
- Competitive rock bouncing
- Ultra4 racing
- Long hill climbs
- Repeated heavy-throttle use
- Severe-duty off-road vehicles
- Endurance racing
The package includes a TIG-welded aluminum radiator, -16 AN fittings, a full shroud, and two 16-inch SPAL puller fans producing a combined rated airflow of 3,836 CFM. Wide Open Design has used this style of radiator package in demanding off-road applications for more than 15 years.
The 38-inch package should not be selected only because it is the largest radiator available. It should be selected when the engine output and intended use require that much core area.
At the same time, high-horsepower and racing applications are not the place to save a few inches of radiator width.
Endurance Racing: Move Up One Radiator Size
Horsepower is the starting point, but endurance racing adds another layer.
During a short trail ride or hill climb, the cooling system may have time to recover between hard pulls. During endurance racing, heat continues building in the engine, coolant, chassis, surrounding air, and other vehicle systems.
For endurance racing, WOD recommends determining the radiator size required by horsepower and then moving to the next available size whenever packaging allows.
Examples:
- A 500-horsepower recreational trail rig may fit within the 525-horsepower compact package.
- A 500-horsepower endurance race vehicle should generally move to the 625-horsepower package.
- An engine near the 625-horsepower limit should move to the 38-inch package for endurance use.
- A racing engine approaching the 875-horsepower rating requires an application-specific review rather than assuming the radiator will have enough reserve capacity.
This additional margin helps account for heat soak, sustained RPM, repeated loading, restricted airflow, high ambient temperatures, and the fact that a race vehicle may not receive a cooling break for hours.
Do Slow-Speed Crawlers Always Need the Largest Radiator?
Not necessarily.
Slow-speed rock crawling reduces natural airflow, but that is why a properly designed electric fan and shroud package is important.
When the radiator is correctly sized for the engine’s horsepower and paired with strong puller fans and a full-coverage shroud, the fans can provide the airflow needed during low-speed operation.
A slow vehicle does not automatically require the 38-inch radiator. A 450-horsepower crawler can still use the compact package when the installation, fan package, and airflow path are correct.
The radiator should be sized for engine output and intended duty. The fan system must then handle the lack of vehicle-speed airflow.
Radiator Size Is Only Half the System
A large radiator with weak fans can still overheat. A strong fan attached to the wrong shroud can still overheat. A properly sized radiator and fan package can still overheat if air is trapped in the coolant system or the water pump is rotating the wrong direction.
Engine cooling depends on two types of flow:
- Coolant flow through the engine and radiator
- Airflow through the radiator core
If either side of that equation is wrong, heat will remain in the engine.
Not All Electric Fans Are Created Equal
One of the biggest mistakes in cooling-system selection is assuming every fan with a similar advertised CFM rating will perform the same.
They do not.
Advertised airflow is only part of the picture. Fan performance is also affected by:
- Motor strength
- Blade design
- Fan depth
- Static pressure capability
- Radiator core restriction
- Shroud design
- Electrical supply
- The way the fan was tested
- How much of the radiator core the fan can actually pull air through
Wide Open Design uses SPAL puller fans because of the results we have seen in actual cooling systems. Our 12-inch and 14-inch SPAL product listings specifically note that radiator restriction, shroud design, blade efficiency, and motor strength all affect real-world cooling performance.
Do not select a fan using the advertised CFM number alone. A cheaper fan may claim a large airflow number in an unrestricted test but fail to maintain that airflow when mounted behind a thick radiator core.
Use Puller Fans
WOD prefers puller fans mounted behind the radiator. A puller fan draws air through the radiator core and into the lower-pressure area behind it. This arrangement works well with a properly designed shroud and allows the fan to pull air across a larger portion of the core.
A pusher fan mounted in front of the radiator may be necessary in unusual packaging situations, but it is not our normal recommendation. All three WOD radiator packages use puller fans.
The Shroud Must Cover the Core
A fan only pulls strong airflow through the section directly in front of its blades unless a shroud helps distribute that suction across the radiator. A proper shroud should cover as much of the usable radiator core as possible.
Without full coverage, large areas of the radiator may receive very little fan-driven airflow at low vehicle speed. The radiator may have enough total core area on paper, but only part of it is doing meaningful work.
This is especially important for crawlers, buggies, and race vehicles that cannot rely on steady road-speed airflow. WOD radiator packages use clamp-style or mounted shrouds that avoid pushing mounting hardware through the radiator core.
Fan Control Settings
On the off-road systems we build, Wide Open Design commonly operates both radiator fans together rather than staging one fan at a time.
Our common control settings are:
- Fans on at 180°F
- Fans off at 175°F
The exact wiring, relay, and fuse requirements should follow the recommendations for the specific fan being installed. The important point is that the fan circuit must be built for the actual electrical load. Both fans need to reach full operating speed when commanded on.
Front-Mounted Versus Rear-Mounted Radiators
WOD does not automatically recommend front mounting or rear mounting for every vehicle. Radiator location is determined by the chassis and the space required by the other components.
A front-mounted radiator may compete for room with:
- Steering components
- Suspension components
- Winch mounts
- Engine accessories
- Chassis structure
- Bodywork
- Air intake components
In some vehicles, a rear-mounted radiator is the only practical way to install enough radiator core area for the engine’s horsepower. Rear mounting does not automatically change our radiator horsepower recommendation. We still begin with usable core area and engine output.
A rear-mounted cooling system does, however, create additional plumbing and air-bleeding challenges.
Does a Rear-Mounted Radiator Have to Sit Above the Engine?
No. The radiator itself does not have to be higher than the engine.
The important requirement is that the fill point or expansion tank is located at the highest point in the cooling system. Air naturally moves toward high points, so the system needs a place where that air can collect and be removed.
A radiator mounted below part of the engine can work correctly when the system is properly designed, filled, and bled.
Bleeding Air From a Rear-Mounted System
Long coolant runs create more opportunities for air pockets. Depending on the chassis and hose routing, it may be necessary to add an air bleed at a high point in the system.
When laying out a rear-mounted system:
- Identify every high point in the coolant path.
- Avoid unnecessary rises and drops in the plumbing.
- Place the fill point or expansion tank at the highest location.
- Add a bleed where air could otherwise become trapped.
- Verify that the thermostat opens and coolant circulates before loading the engine.
- Recheck the coolant level after the first complete heat cycle.
A cooling system can have the correct radiator, fans, and thermostat and still overheat because an air pocket prevents proper circulation.
Using Chassis Tubes for Long Coolant Runs
On purpose-built WOD chassis, we commonly use chassis tubes for long coolant runs. This creates a clean route between the engine and a rear-mounted radiator without running long sections of flexible hose through the vehicle.
The chassis must be designed for this purpose. Tube routing, connections, sealing, filling, bleeding, and service access all need to be considered when the chassis is built. For the systems we build, -16 AN plumbing is our normal choice.
Although WOD offers both -16 and -20 thermostat housings for different customer applications, we use -16 AN plumbing on the vehicles built in our shop.
The -16 AN WOD LS Thermostat Housing converts the factory-style LS outlet to clean AN plumbing while retaining the thermostat. It is designed for GM LS-based engines and the 1997–2008 Corvette-style thermostat and seal.
LS Steam-Port Routing
Steam-port routing is an important part of an LS cooling system. On the LS systems built at WOD, we tie the steam port back into the water pump. We do this by drilling and tapping a 1/8" NPT port into the water pump to allow circulation.
This provides a path for trapped air and vapor to move out of the upper areas of the engine rather than collecting in the cylinder heads.
This modification must be completed carefully. The location must be selected correctly, the pump must be protected from metal debris during machining, and the fitting must be properly sealed.
Improperly routed or blocked steam ports can contribute to trapped air and unstable engine temperature.
Thermostat Selection
Wide Open Design uses a thermostat in its cooling systems. Removing the thermostat is not our normal solution for an overheating engine. If the engine is getting hot, the cause should be diagnosed rather than assuming thermostat removal will fix it.
Our general recommendation is:
| Thermostat | Recommended Use |
| 160° or 167° | Off-road use, trail rigs, crawlers, racing, and high-load performance builds |
| 187° | Mixed-use vehicles that see both street and off-road operation |
| 195° | OEM-style street vehicles designed and calibrated around factory operating temperatures |
WOD currently offers both a 160° LS thermostat and a 187° LS thermostat. Both are compatible with WOD’s LS thermostat housings. For the dedicated off-road and race vehicles we build, we generally prefer the lower-temperature thermostat.
Coolant Recommendation
Wide Open Design uses a 50/50 mixture of water and antifreeze in the engine-cooling systems we build. We use this mixture for:
- Trail rigs
- Recreational off-road vehicles
- Rock crawlers
- Rock bouncers
- Race vehicles
- Vehicles exposed to freezing temperatures
The cooling system should be filled completely and bled thoroughly. Do not assume the system is full because the radiator or expansion tank appears full before the thermostat opens.
After the engine reaches operating temperature and cools completely, recheck the coolant level.
Radiator Cap and Recovery System
Every system should also have a properly routed coolant-recovery system rather than only allowing coolant to discharge onto the ground. The WOD 16 PSI Vented Radiator Cap is the normal cap offered with our radiator packages.
One mistake we see is using a cap with too much pressure. More pressure is not automatically better. The radiator, hoses, fittings, expansion tank, and recovery system all need to be designed for the pressure being used.
Use the correct cap rather than installing the highest-pressure cap available in an attempt to hide a cooling problem.
What Temperature Is Actually Overheating?
Not every temperature above 200°F means the engine is overheating.
A useful general guideline is:
| Coolant Temperature | WOD Guidance |
| Below 190°F | The engine may still be warming up, depending on thermostat selection |
| 190–210°F | Normal operating range |
| 210–220°F | Getting warm; watch whether the temperature stabilizes or continues climbing |
| 220–230°F | The cooling system needs attention |
| Above 230°F | Reduce the load, stop when safe, and diagnose the problem |
The temperature trend is as important as the number. An engine that climbs to 215°F during a hard pull and then holds steady may be operating within the system’s stable range.
An engine that reaches 215°F and continues climbing through 220°F, 225°F, and 230°F is not under control. Watch for:
- The temperature at idle
- The temperature at 2,000 RPM
- The temperature during a climb
- Whether the fans cause the temperature to drop
- Whether the temperature recovers after the load is removed
- Whether the temperature climbs every time the engine is loaded
- Whether the problem is worse at low speed or high speed
These patterns help determine whether the problem is related to airflow, coolant circulation, radiator capacity, or engine load.
Engine Cooling Troubleshooting Guide
A cooling problem should be diagnosed as a system. Do not immediately replace the radiator because the gauge is showing a high temperature. Do not remove the thermostat without testing it. Do not assume a fan is good only because it is making noise.
Start with the basics, confirm coolant circulation, and then measure the radiator’s temperature drop.
The Five Most Common Cooling Problems
1. Air Trapped in the Cooling System
Air pockets can prevent coolant from circulating through part of the engine or radiator.
This is especially common with:
- Rear-mounted radiators
- Long coolant runs
- Systems with multiple high points
- LS engines with incorrect steam-port routing
- Systems filled too quickly
- Systems without a high-mounted fill point or expansion tank
The fill point should be the highest point in the system. High sections of plumbing may require additional bleed points.
2. Thermostat Problems
A thermostat that does not open can stop hot coolant from reaching the radiator. A removed thermostat can also create control and circulation problems rather than fixing the original issue.
Confirm that the thermostat is installed, is the correct part, and opens at the expected temperature.
3. Insufficient Airflow
Insufficient airflow can be caused by:
- Weak electric fans
- Fans that do not perform under radiator restriction
- Misleading advertised CFM ratings
- An incomplete shroud
- Hot engine air recirculating through the radiator
- Mud, dirt, or debris blocking the core
- Poor radiator positioning
Not all fans are created equally, and they are not all tested under the same conditions.A fan that looks adequate on paper may not pull enough air through a real radiator core.
4. Radiator Capacity Is Too Small
Calculate the usable core area and compare it with the engine’s horsepower. If the radiator is below the recommended WOD capacity, fan upgrades alone may not solve the problem.
For endurance racing, use the next larger radiator package whenever possible.
5. Incorrect Coolant Circulation
Possible circulation problems include:
- Water pump rotating in the wrong direction
- Thermostat not opening
- Air trapped in the system
- Improperly routed LS steam ports
- Restricted coolant path
- Weak water-pump performance
- A radiator core with an internal restriction
Before changing parts, verify that coolant is actually moving through the system.
Test #1: Check Coolant Flow and Thermostat Operation
Warning: Hot coolant can cause severe burns. Wear protective eyewear and protective clothing. Never remove the radiator cap from a hot or pressurized cooling system. Begin this test with a completely cold engine.
Step 1: Start With a Cold Engine
Allow the engine to cool completely.
Remove the radiator cap or open the fill point only while the system is cold and not pressurized.
Confirm that the system is full.
Step 2: Start the Engine
Start the engine and observe the coolant at the fill opening.
Depending on the system design, you may see light coolant movement while the engine warms.
Step 3: Watch for the Thermostat to Open
As coolant temperature rises past the thermostat rating, coolant flow should increase noticeably.
On the systems we commonly work with, the movement may initially resemble a gently moving or “babbling” stream. Once the thermostat opens, the flow should become much stronger.
You are looking for a clear change that confirms the thermostat opened and coolant began circulating through the radiator.
If the flow does not increase:
- Shut the engine down before it overheats.
- Allow the system to cool.
- Inspect the thermostat.
- Verify water-pump rotation.
- Check for trapped air.
- Inspect LS steam-port routing.
- Check for coolant restrictions.
Once flow is confirmed, reinstall the cap before continuing to the Delta T test.
Test #2: Perform a Radiator Delta T Test
“Delta T” means the difference between the coolant temperature entering the radiator and the coolant temperature leaving it.
A functioning radiator should receive hot coolant at the inlet and discharge cooler coolant at the outlet.
For this test, you will need an infrared temperature gun.
Prepare the Vehicle
- Bring the engine completely to operating temperature.
- Make sure coolant temperature is above the thermostat’s opening temperature.
- Confirm that the thermostat is open.
- Run both radiator fans.
- Take one set of measurements at idle.
- Repeat the measurements while holding the engine at approximately 2,000 RPM.
Do not perform the test before the engine is fully warm. A closed or partially opened thermostat can produce misleading results.
Record These Four Temperatures
Write down the readings in this exact order:
- Temperature shown on the vehicle’s gauge
- Temperature measured at the thermostat housing
- Temperature measured at the radiator’s upper inlet
- Temperature measured at the radiator’s lower outlet
Where to Measure the Radiator
Do not take the primary inlet and outlet readings from the rubber hoses. Use a flat area of the radiator’s metal tank near the upper inlet and lower outlet.
Take several readings from the same location to make sure the measurement is consistent. Marking the measurement areas with painter’s tape can make it easier to hit the same spot each time. The upper inlet should normally be hotter than the lower outlet.
Calculate the temperature drop:
Upper inlet temperature − lower outlet temperature = radiator Delta T
Delta T Interpretation
| Temperature Drop | Initial Interpretation |
| Less than 10°F | Very little heat is being removed. Inspect airflow, fan performance, coolant circulation, thermostat operation, radiator capacity, test conditions, and measurement accuracy. |
| 10–20°F | The radiator is removing some heat, but the result may be marginal. Watch whether engine temperature stabilizes or continues climbing. |
| 20–30°F | Normal target range showing good heat rejection through the radiator. |
| More than 30°F | Inspect for restricted coolant flow, trapped air, a partially opening thermostat, weak water-pump circulation, hose restriction, or a restricted radiator core. |
A Delta T reading does not identify one failed part by itself.
It helps answer a more important first question:
Is heat entering the radiator, and is the radiator removing that heat?
A low temperature drop can be caused by poor airflow, inadequate radiator capacity, incorrect coolant flow, or a test performed before the engine and thermostat were fully warm.
A very large temperature drop can indicate that coolant is moving too slowly through the radiator because of a restriction, weak circulation, trapped air, or a thermostat that is not opening completely.
The results need to be considered alongside the vehicle’s temperature trend and operating conditions.
Choosing the Right System From the Beginning
The most reliable cooling system is the one sized correctly before the chassis and plumbing are finished.
Begin with the engine’s actual horsepower. Calculate the required usable core area. Decide whether the vehicle will be used recreationally or in endurance racing. Then choose the radiator package that provides enough capacity and fits the chassis.
From there, complete the system with:
- Quality SPAL puller fans
- A full-coverage shroud
- A thermostat suited to the application
- Properly designed -16 AN plumbing
- Correct LS steam-port circulation
- A high-mounted fill point or expansion tank
- A 16 PSI cap
- A coolant-recovery system
- A 50/50 coolant mixture
- Proper bleeding at every high point
Cooling is not one component. It is a system. When every part of that system is selected and installed correctly, the engine can maintain a stable operating temperature through slow crawling, long climbs, high horsepower, and demanding off-road use.
Shop Wide Open Design Cooling Systems or contact the WOD team for help matching radiator core size, fan package, thermostat, and plumbing to your engine and chassis.
Questions or Comments?
We'd love to hear your thoughts or answer any questions.