Older Cummins engines – especially Big Cam and NTC/N14-era platforms – used two different cooling system designs:
- Low-flow cooling systems
- High-flow cooling systems
These systems are not just minor variations.
👉 They are fundamentally different in how coolant moves through the engine.
And that matters when you’re trying to solve overheating – or considering a conversion.
What Is a Low-Flow Cooling System?
Low-flow systems were designed to:
- Restrict coolant velocity
- Increase dwell time in the radiator
- Improve heat transfer at lower flow rates
They typically use:
- Dual thermostat configurations
- Restrictors in the water rail/manifold
- Split or sectional radiator designs
- Lower-capacity water pumps
👉 The idea was controlled flow – not maximum flow.
What Is a High-Flow Cooling System?
High-flow systems take the opposite approach.
They are designed to:
- Move higher volumes of coolant
- Increase circulation through the engine
- Improve heat rejection under load
They typically use:
- Higher-capacity water pumps
- Different thermostat housings (often single thermostat designs)
- Larger or reconfigured radiators
- Revised coolant routing
👉 The focus is moving heat out faster, especially under heavy load.
Why People Consider Converting to High-Flow
Most conversions are not done for convenience.
They are typically driven by:
- Persistent overheating under load
- Increased horsepower or fuel rate
- Heavy-duty or extreme operating conditions
However, this is important:
👉 If a low-flow system is overheating and it didn’t before, something is wrong.
Common causes include:
- Plugged radiator core
- Restricted aftercooler
- Oil cooler contamination
- Airflow issues
- Internal coolant flow restrictions
Before converting anything: diagnose the system first!
What It Actually Takes to Convert to High-Flow
This is not a simple component swap.
A true conversion requires multiple system changes.
1. Water Pump
Low-flow pumps are not designed to move high coolant volume.
👉 A high-flow system requires a different pump with increased capacity.
2. Thermostat Housing and Thermostats
Low-flow systems typically use dual thermostats and restrict flow differently.
High-flow systems use:
- Different housings
- Different thermostat configurations
- Different flow paths
👉 You cannot mix these components and expect correct operation.
3. Water Manifold / Water Rail
This is one of the most critical changes.
Low-flow systems often include:
- Internal restrictors
- Flow control washers
High-flow systems require:
- Revised flow passages
- Removal or redesign of restrictions
👉 Leaving restrictors in place defeats the purpose of the conversion.
4. Radiator Configuration
Radiators in low-flow systems are designed for slower coolant velocity.
High-flow systems require:
- Higher-capacity cores
- Different tank design
- Increased flow capability
In many cases:
👉 A radiator shop can re-core or modify tanks for high-flow operation
5. Coolant Routing and Plumbing
In some applications, piping and routing must change.
This includes:
- Inlet/outlet configuration
- Thermostat bypass routing
- Aftercooler flow paths
👉 Incorrect routing can cause localized overheating even after conversion.
Common Mistakes During Conversion
This is where most problems happen.
Mixing Low-Flow and High-Flow Components
Installing a high-flow pump with low-flow restrictors or housings will:
- Create flow imbalance
- Reduce cooling efficiency
- Cause overheating to continue
Ignoring the Oil Cooler
Oil coolers can restrict coolant flow internally.
If plugged:
👉 They can cause overheating regardless of system type
Skipping System Cleaning
Contamination inside:
- Radiator
- Aftercooler
- Oil cooler
can restrict flow and reduce heat transfer.
👉 Cleaning or flushing these components is critical before conversion.
Assuming Flow Alone Fixes Heat
Cooling systems depend on two things:
- Flow
- Heat transfer
If the radiator or aftercooler is restricted:
👉 Increasing flow alone will not fix the issue
When Conversion Actually Makes Sense
A conversion may be appropriate when:
- The engine has been modified for higher output
- The application involves sustained heavy load
- The system has been verified to be clean and functioning correctly
- The correct high-flow components are available
However:
👉 Many overheating issues can be solved without converting systems
Practical Troubleshooting Before Converting
Before changing system design, verify:
- Radiator core condition (internal blockage or baffle failure)
- Aftercooler restriction
- Oil cooler condition
- Proper thermostat operation
- Correct coolant fill and air purge
- Fan and airflow performance
Even small issues – like trapped air or thermostat sealing problems – can cause overheating.
Final Takeaway
Converting a Cummins engine from low-flow to high-flow cooling is possible.
But it is not a simple upgrade. It requires coordinated changes across the entire cooling system
And more importantly:
👉 It should not be used as a shortcut for diagnosing overheating
If the root cause is not identified, the problem can persist – even after conversion.
Call 844-304-7688 or visit highwayandheavyparts.com to get the right diesel engine parts for your application.
From diagnosis through delivery, we’re Highway and Heavy Parts.
