Engine Coolants Explained: Types, Maintenance and Racing Applications

Engine coolant is often treated as a simple coloured liquid that only prevents an engine from overheating. In reality, modern coolant is a carefully formulated chemical package responsible for temperature control, corrosion protection, freeze protection, water-pump lubrication and the long-term health of the entire cooling system,therefore types of engine coolant have been developed and improved.

Using the wrong coolant, mixing incompatible products or neglecting maintenance can damage radiators, water pumps, thermostats, cylinder heads, seals and internal cooling passages.

For performance and racing vehicles, correct coolant selection becomes even more important because higher engine loads, increased boost pressure and repeated high-speed operation generate considerably more heat.

What Does Engine Coolant Do?

Engine coolant circulates through passages in the engine block and cylinder heads, absorbing heat produced during combustion. It then travels through the radiator, where airflow removes part of that heat before the coolant returns to the engine.

However, temperature control is only one part of its job. A correct coolant must also:

• Protect aluminium, cast iron, steel, copper and other metals against corrosion

• Prevent freezing in cold conditions

• Increase resistance to boiling

• Lubricate the water-pump seal

• Reduce deposits and scale

• Protect rubber seals and plastic cooling-system components

• Help control cavitation around pumps and hot internal surfaces

A modern cooling system contains several different materials, so the coolant’s inhibitor package must be chemically compatible with the vehicle. Coolant technology has evolved alongside engine materials and cooling-system design, which is why the correct manufacturer approval matters more than the colour printed on the bottle.

Coolant and Antifreeze: Are They the Same?

The terms are often used interchangeably, but there is a technical difference.

Antifreeze concentrate normally contains glycol and a corrosion-inhibitor package. It must be mixed with the correct amount of suitable water before use.

Ready-mixed coolant has already been diluted by the manufacturer and can normally be poured directly into the cooling system.

A common mixture for road vehicles is approximately 50% concentrate and 50% water, although the exact ratio must follow the vehicle and coolant manufacturer’s instructions. BASF recommends a 50:50 dilution for its concentrate products to provide a balanced level of corrosion, overheating and frost protection.

Adding extra concentrate does not automatically improve cooling. Excessive glycol concentration can reduce heat-transfer performance and increase viscosity. The correct concentration is a compromise between heat transfer, freeze protection, boiling resistance and corrosion protection.

The Main Types of Engine Coolant

Coolants are usually classified by their corrosion-inhibitor technology. The main categories are IAT, OAT and HOAT, although several specialised variations also exist.

IAT Coolant

IAT stands for Inorganic Additive Technology.

It is the traditional coolant chemistry commonly associated with older vehicles. IAT formulations normally use inorganic corrosion inhibitors such as silicates and may also contain phosphates or other traditional additives.

Advantages of IAT Coolant

IAT coolant provides rapid protection to metal surfaces. Its inhibitors form a protective layer over internal cooling-system components, making it suitable for many older engines containing cast iron, copper, brass and traditional radiator materials.

It remains the correct choice when required by the vehicle manufacturer.

Disadvantages of IAT Coolant

Traditional inorganic inhibitors can become depleted more quickly than modern long-life technologies. This generally means shorter replacement intervals.

Deposits can also form if the coolant is neglected, incorrectly mixed or combined with an incompatible product.

IAT should not be dismissed simply because it is older technology. In a classic vehicle designed for it, the correct IAT coolant may be safer than installing a modern long-life fluid with the wrong chemical compatibility. Valvoline continues to classify IAT coolant for older vehicles that specifically require this technology.

OAT Coolant

OAT stands for Organic Acid Technology.

OAT coolant uses organic corrosion inhibitors rather than relying primarily on traditional silicates. It is widely used in modern aluminium engines and extended-service cooling systems.

Advantages of OAT Coolant

OAT formulations can provide long service life and effective corrosion protection without covering every internal surface with a thick protective layer.

They are widely used in modern vehicles with aluminium cylinder heads, radiators and other lightweight cooling-system components.

Disadvantages of OAT Coolant

OAT protection may not be suitable for every older cooling system, gasket material or soldered radiator construction.

Some OAT formulations also use different organic-acid packages from others. Two coolants described as OAT are therefore not automatically identical or approved for the same vehicles.

OAT coolant must be selected according to the required specification, not simply because the bottle says “long life.”

HOAT Coolant

HOAT stands for Hybrid Organic Acid Technology.

As the name suggests, it combines organic-acid inhibitors with selected inorganic additives. The objective is to combine the long service life of OAT chemistry with the rapid surface protection associated with traditional inhibitors.

HOAT is not one single universal formula. Several subcategories exist.

Silicated HOAT and Si-OAT

Si-OAT coolant combines organic-acid technology with controlled silicate protection.

It is widely used in European vehicles and is commonly associated with manufacturers such as Mercedes-Benz, Volkswagen, Audi and Porsche, depending on the exact approval.

Silicates can provide fast protection for aluminium surfaces, while the organic-acid package supports longer service life. BASF describes Si-OAT as a hybrid technology combining the benefits of silicate-containing and silicate-free coolants.

Phosphated HOAT and P-HOAT

P-HOAT combines organic-acid inhibitors with phosphates.

These formulations are frequently used by Asian vehicle manufacturers, although the exact chemistry and specification vary between brands.

Again, the colour is not enough to identify compatibility. Blue, pink, red or green coolants may all use different formulations depending on the manufacturer. Valvoline lists P-HOAT, phosphate-free HOAT and Si-OAT as separate coolant technologies with different vehicle applications.

Ethylene-Glycol Coolant

Ethylene glycol is the most common base fluid used in automotive antifreeze.

When correctly mixed with water and the appropriate inhibitor package, it provides freeze protection, raises the boiling point and supports corrosion control.

Its main disadvantage is toxicity. Ethylene-glycol coolant is harmful if swallowed and must be kept away from children and animals. Spilled or discarded coolant must be handled responsibly and should never be poured onto the ground or into drains.

The presence of ethylene glycol does not determine whether a coolant is IAT, OAT or HOAT. Those terms describe the inhibitor technology, while ethylene glycol describes the primary base fluid.

Propylene-Glycol Coolant

Propylene glycol is used in some lower-toxicity coolant formulations and specialist applications.

It can be selected where reduced toxicity is important, but it should not be installed merely because it sounds safer. The cooling system must still be compatible with the complete product formulation.

As with ethylene-glycol coolant, the correct concentration, inhibitor technology and manufacturer approval remain essential.

Waterless Coolant

Waterless coolant is a specialist product designed to operate without conventional water content.

Its main advantage is an extremely high boiling point. One current waterless performance coolant specifies a boiling point above 190°C, considerably higher than plain water at atmospheric pressure.

Advantages of Waterless Coolant

Waterless coolant can reduce the risk of local boiling and vapour formation around extremely hot areas of the cylinder head.

It may also reduce corrosion processes that depend on water and can operate at lower cooling-system pressure in certain applications.

This makes it attractive for some classic cars, specialist vehicles, high-temperature engines and vehicles that spend long periods in storage.

Disadvantages of Waterless Coolant

The cooling system must be thoroughly prepared before conversion. Remaining water and conventional coolant can reduce the intended performance of the waterless product.

It is not necessarily the best choice for every racing engine. A very high boiling point prevents boil-over, but it does not automatically mean that the fluid removes heat from the engine more efficiently.

Waterless coolant is also more expensive, and topping up with conventional coolant or water compromises its properties. The manufacturer states that significant water contamination reduces its boiling-point and corrosion performance.

Can Coolant Be Chosen by Colour?

No.

Coolant colour is not a reliable universal standard. Green coolant is not always IAT, red coolant is not always OAT, and blue coolant is not automatically suitable for every Asian or European vehicle.

The same colour can be used for completely different chemical formulations. Different colours may also be used for products that meet similar specifications.

Coolant should be chosen using:

• The vehicle manufacturer’s specification

• The official coolant approval

• The required inhibitor technology

• The correct glycol base

• The correct concentration

Valvoline specifically warns that modern coolant colours depend on the manufacturer and are no longer a dependable method of identifying compatibility.

What Do G11, G12, G12+, G12++ and G13 Mean?

These names are commonly seen in European vehicles and are strongly associated with Volkswagen Group coolant specifications.

They are often treated as universal coolant categories, but they are not a complete industry-wide classification system.

Different generations can use different inhibitor technologies, including traditional silicated formulas, OAT, Si-OAT and other hybrid technologies.

A bottle displaying a familiar G-number should still be checked for the precise manufacturer specification and approval required by the vehicle.

This is particularly important on Porsche, Audi, Lamborghini, Bentley and other Volkswagen Group platforms where cooling-system materials, engine design and approval requirements can vary between model generations.

Can Different Coolants Be Mixed?

Mixing coolant should be avoided unless the compatibility is confirmed.

Some products may be chemically compatible enough to prevent an immediate reaction, but that does not mean the mixture will retain the intended service life or corrosion protection.

Incompatible mixtures can potentially cause:

• Reduced corrosion protection

• Deposits or sludge

• Restricted radiator passages

• Water-pump seal damage

• Reduced heat transfer

• Shortened service life

• Incorrect freeze protection

When the existing coolant cannot be positively identified, the safest professional approach is normally to drain and thoroughly flush the system before filling it with the correct approved product.

Simply topping up according to colour is not acceptable maintenance. Valvoline advises checking the owner’s manual for the correct coolant formula and explains that colour alone cannot establish compatibility.

Concentrated Coolant Versus Ready-Mixed Coolant

Concentrated antifreeze gives the workshop control over the final mixture. This can be useful when adjusting freeze protection for a specific climate or when a complete cooling system has been drained.

However, concentrate must be mixed accurately using suitable water.

Ready-mixed coolant provides a controlled concentration prepared by the manufacturer. It reduces the risk of incorrect dilution and is convenient for topping up or routine servicing.

The important point is to understand what is being added. Pouring undiluted concentrate into a system that already contains a correct mixture can create an excessive glycol concentration.

What Type of Water Should Be Used?

When coolant concentrate requires dilution, demineralised or deionised water is normally the safest choice unless the product manufacturer specifies otherwise.

Hard tap water can contain calcium, magnesium and other minerals that may contribute to scale and deposits. Water quality varies greatly between regions, so a clean controlled water source removes unnecessary uncertainty.

This is particularly relevant in high-performance engines with small cooling passages, aluminium components and tightly controlled operating temperatures.

Coolant Maintenance

Coolant is not a lifetime fluid unless the vehicle manufacturer specifically says otherwise.

Its corrosion inhibitors gradually degrade, and contamination can enter the system through repairs, incorrect topping-up, internal component failure or poor maintenance.

Check the Coolant Level Correctly

Coolant level should normally be checked with the engine cold and the vehicle parked on level ground.

Never remove a pressurised coolant cap from a hot engine. Hot coolant can escape violently and cause serious burns.

A falling coolant level should never be dismissed as normal. A sealed cooling system should not regularly consume coolant.

Possible causes include:

• External hose or radiator leaks

• Expansion-tank cracks

• A defective pressure cap

• Water-pump leakage

• Heater-matrix leakage

• Thermostat-housing leaks

• Turbocharger or charge-cooler leaks

• Cylinder-head gasket problems

• Internal engine leakage

Repeatedly topping up without locating the cause can eventually result in severe engine damage.

Inspect Coolant Condition

Coolant should be inspected for:

• Oil contamination

• Rust or brown discolouration

• Floating deposits

• Thickened or gel-like fluid

• Metallic particles

• Incorrect concentration

• Unusual pressure or bubbling

• Exhaust gases in the expansion tank

Clean colour alone does not prove that the coolant is still serviceable. Freeze protection, concentration and chemical condition may require proper testing.

Follow the Correct Replacement Interval

There is no universal coolant-change interval.

Traditional IAT coolant, long-life OAT coolant and modern HOAT coolant can have very different service requirements. The correct interval depends on the coolant, vehicle, operating conditions and manufacturer specification.

A vehicle used for circuit driving, high-speed testing or repeated dyno operation should be inspected more frequently than a normal road car.

BASF states that coolant service life after dilution depends on the vehicle manufacturer’s specifications.

Flushing the Cooling System

A proper coolant change involves more than emptying the expansion tank.

Depending on the vehicle, old coolant can remain inside:

• The engine block

• Cylinder heads

• Radiator

• Heater matrix

• Turbocharger cooling lines

• Charge-cooler system

• Auxiliary pumps

• Low-temperature cooling circuits

Modern performance vehicles may use more than one independent coolant circuit. For example, the engine, intercoolers, hybrid system, battery or transmission may each have separate temperature-control circuits.

When changing coolant type, the system should be flushed sufficiently to prevent the old chemistry from contaminating the new fluid.

After filling, the system must be correctly bled. Air trapped in the circuit can cause poor heater performance, unstable temperatures, localised overheating and water-pump cavitation.

Some modern vehicles require vacuum-filling equipment or a diagnostic procedure to operate electric coolant pumps and bleed valves.

Coolant for Performance Road Cars

A tuned road car still needs year-round freeze protection, corrosion resistance, long-term stability and compatibility with all original cooling-system components.

The best coolant is normally the correct manufacturer-approved formulation at the correct concentration.

A performance coolant should not be selected based only on marketing claims about lower temperatures. If it does not meet the required approval, it may compromise the water pump, seals, radiator, heater matrix or electronic cooling components.

Before changing coolant, the complete system should be assessed:

• Engine operating temperature

• Radiator capacity

• Thermostat control

• Fan operation

• Pressure-cap rating

• Water-pump condition

• Coolant concentration

• Airflow through the radiator

• Intercooler and air-conditioning heat load

  
  

Lowering coolant temperature is not always desirable. Modern engines are calibrated to operate within a specific temperature range for fuel efficiency, oil control, emissions and component clearances.

Coolant for Racing Applications

Race vehicles can require a very different coolant strategy from road cars.

Plain water has strong heat-transfer characteristics, which makes water-based systems attractive for circuit use. However, plain water provides little freeze protection and inadequate long-term corrosion protection.

For this reason, racing systems may use water combined with an approved corrosion inhibitor and wetting additive.

Wetting additives are designed to improve contact between the liquid and hot metal surfaces, reducing the formation of bubbles or vapour barriers. Red Line describes its WaterWetter additive as providing improved metal wetting and corrosion protection when used with plain water or glycol coolant.

Why Some Race Series Restrict Glycol

Many race organisers and circuits dislike glycol-based coolant because it becomes extremely slippery when spilled onto the track.

A damaged radiator or burst hose can distribute coolant across the racing line, creating a serious hazard and requiring extensive track cleaning.

Some competition regulations therefore require plain water or an approved non-glycol racing coolant. Motul specifically advises competitors that some racing regulations permit nothing other than pure water and recommends checking with the relevant organising body.

Never assume that normal antifreeze is allowed. The championship, circuit and event regulations must be checked before the car arrives at technical inspection.

Water-Based Racing Coolant

A typical circuit-racing setup may use:

• Demineralised or distilled water

• A corrosion inhibitor

• A wetting or surface-tension-reducing additive

• An approved coolant catch tank

• The correct pressure cap

• Regular inspection and replacement

This type of system can provide strong heat transfer and may be safer to clean from the circuit after a leak.

However, it has serious limitations. Water-based racing coolant cannot be left in a vehicle exposed to freezing temperatures unless it contains sufficient antifreeze protection.

It may also require more frequent maintenance because water and oxygen can contribute to corrosion when the inhibitor package is weak, depleted or incorrectly mixed.

Glycol-Based Racing Coolant

Not every race category prohibits glycol.

Rally cars, endurance vehicles, hill-climb cars, motorcycles and vehicles used in low-temperature environments may still require freeze protection and a raised boiling point.

Specialist racing coolants can use ethylene glycol with high-performance organic inhibitor packages. Motul’s Factory Line racing coolant, for example, is a ready-to-use monoethylene-glycol product using organic anti-corrosion technology.

The correct choice depends on:

• Competition regulations

• Ambient temperature

• Engine operating temperature

• Cooling-system pressure

• Race duration

• Vehicle storage conditions

• System materials

• Spill and safety requirements

Cooling-System Pressure in Racing

Cooling systems are pressurised because increasing pressure raises the boiling point of a water-based coolant.

However, installing a higher-pressure cap is not automatically an upgrade. Every hose, radiator, expansion tank, seal, water pump and connection must be capable of safely handling the additional pressure.

A higher-pressure cap can hide a marginal boiling problem while placing additional stress on the cooling system. It should only be used when the system has been designed and tested for it.

Race cars should also use a properly installed expansion or catch tank to prevent coolant from reaching the tyres or track surface.

Common Coolant Mistakes

The most common cooling-system errors are surprisingly basic:

• Selecting coolant by colour

• Mixing incompatible technologies

• Using concentrate without proper dilution

• Adding tap water with high mineral content

• Ignoring small coolant losses

• Failing to bleed the system correctly

• Installing racing coolant in a road car without considering frost protection

• Leaving plain water in the system during winter

• Using glycol where race regulations prohibit it

• Assuming a higher boiling point means better overall heat transfer

• Treating an overheating engine by changing coolant instead of finding the fault

Coolant cannot repair an undersized radiator, blocked airflow, damaged water pump, incorrect thermostat, failed cooling fan or poor engine calibration.

Final Thoughts

There is no single coolant that is best for every vehicle.

IAT, OAT, HOAT, Si-OAT and P-HOAT formulations use different inhibitor technologies for different cooling-system materials and manufacturer requirements. Waterless coolant, conventional glycol coolant and water-based racing fluids also serve very different purposes.

For road cars, the manufacturer’s specification and official approval must come first. For racing applications, coolant selection must also consider circuit regulations, ambient temperature, heat transfer, corrosion protection and spill safety.

At Torque Tuning, cooling-system performance is evaluated as part of the complete vehicle package. Increased engine output creates increased thermal load, and that heat must be controlled through correct coolant, adequate radiator capacity, proper airflow and disciplined maintenance.

The correct coolant will not make a badly designed cooling system good. But the wrong coolant can damage even the best-engineered system.