Understanding the Cooling Demands of RB Engine Builds

The RB series, whether it's the RB20, RB25, or the legendary RB26, is a powerhouse of Japanese engineering. These engines are celebrated for their ability to handle enormous power increases, but they come with a significant thermal challenge. Under hard driving, especially on a road course or during spirited backroad runs, an RB engine can generate extreme heat. An inadequate cooling system leads to detonation, head gasket failure, and warped cylinder heads. Upgrading your radiator and fan setup isn't just a performance mod—it's a necessity for reliability.

For street-driven RBs, the cooling system must handle stop-and-go traffic, extended idling, and occasional spirited driving. Track builds face sustained high RPM and high load, often in hot ambient conditions. The stock Nissan cooling system was designed for factory power levels and moderate driving. Once you increase boost, add a larger turbo, or install an intercooler, the thermal load skyrockets. A well-matched radiator and fan combination is the cornerstone of a reliable high-performance RB build.

In this guide, we will break down the best fans and radiators for both street and track applications, discuss supporting components like shrouds and coolant, and provide actionable advice for selecting the right parts for your specific RB setup.

Choosing a Radiator for Your RB Build

The radiator is the primary heat exchanger. For RB engines, the most popular upgrade path is moving from the original copper-brass radiator to an all-aluminum unit. Aluminum dissipates heat more effectively and is significantly lighter. Three brands consistently stand out in the RB community.

Koyo Radiators: The Gold Standard

Koyo has been a mainstay in the Japanese performance scene for decades. Their radiators are manufactured using a brazed aluminum core with a high-density fin count. The 53mm and 56mm core options balance cooling efficiency with fitment simplicity. For street and light track use, the Koyo VH (Ventura) series is a drop-in replacement that offers significantly more cooling capacity than the stock unit. For serious track builds, the Koyo B-Series radiator with a thicker core (up to 64mm) provides maximum heat rejection. Koyo radiators also feature a durable finish that resists corrosion. Koyo's official website offers detailed fitment guides for RB20, RB25, and RB26 engines.

Mishimoto Radiators: Performance and Aesthetics

Mishimoto has earned a strong reputation for producing high-quality cooling products with a focus on both function and appearance. Their RB-specific radiators feature a 50mm or 60mm dual-core design with welded aluminum construction. Mishimoto radiators include a high-pressure radiator cap and a lifetime warranty, which provides peace of mind. The company also offers the “Race” version with a more aggressive core and no provision for the factory expansion tank, making it ideal for track-only cars. For street-driven RBs that see occasional track days, the standard Mishimoto radiator is an excellent choice. Mishimoto's product pages include detailed dyno-tested cooling data that can help you decide.

CSF Radiators: Race-Bred Innovation

CSF is a leading OEM supplier that also produces aftermarket radiators for high-performance applications. Their RB-series radiators utilize a unique B-tube technology that increases tube surface area for superior heat transfer. CSF radiators are available in single-pass and dual-pass configurations. The dual-pass design forces coolant through the core twice, which improves heat rejection in applications with high water flow rates, such as track cars with high-RPM operation. CSF radiators often feature a built-in oil cooler option, which can be a space-saving advantage for turbocharged RBs. CSF's website provides technical white papers on radiator selection for high-heat applications.

Radiator Thickness and Fin Density: What to Choose

For street builds with a turbo upgrade up to 400-500 whp, a 50mm to 55mm thick radiator with medium fin density is sufficient. Track cars aiming for 600+ whp should opt for a 60mm to 64mm core. However, thicker radiators may require modifications to the fan shroud or the removal of the factory A/C condenser. Fin density is a trade-off: higher density (16-20 fins per inch) cools better at speed but may restrict airflow at low speeds. For track use, a lower fin density (12-14 fins per inch) is often preferred because it allows air to pass through more easily at high RPM, and it is less prone to clogging with debris.

Cooling Fans: The Key to Low-Speed and Idle Cooling

While the radiator handles most of the heat rejection at speed, fans are critical when airflow is limited—such as in traffic, on the starting grid, or during autocross. Stock RB fan clutches are known to fail and often produce parasitic drag. Upgrading to an electric fan setup is a common and effective modification.

Spal High-Performance Electric Fans

Spal is widely regarded as the premier electric fan brand for automotive cooling. Their fans use a curved blade design that moves high volumes of air with low noise. The 16-inch and 11-inch Spal fans are popular choices for RB engines. The 30103020 model (16-inch, 2,450 CFM) is a favorite for larger radiators, while the 30102000 (11-inch, 1,180 CFM) works well for dual-fan setups. Spal fans are also sealed against moisture and dust, making them durable for engine bay environments. They are a direct fit for many Koyo and Mishimoto radiators when used with a proper shroud. Spal's website provides detailed airflow and amp draw data.

Flex-a-Lite Electric Fans

Flex-a-Lite has been a mainstay in the American muscle and import performance markets. Their fans are known for their slim profile—many models are less than 3 inches thick—which helps clear tight engine bays. The Flex-a-Lite Black Magic series offers adjustable thermostatic control, which can be set to turn on at a specific coolant temperature. This adjustability is valuable for track cars that need to keep the engine cool between sessions. Additionally, Flex-a-Lite fans often include a high-quality mounting kit with nylon ties that reduce vibration. Their dual-fan “S-Blade” design provides even airflow across the entire radiator core.

Derale Performance Fans

Derale offers a wide range of fan sizes and configurations, including pusher and puller fans. For RB builds, the puller configuration (mounted behind the radiator) is typically more efficient. Derale fans feature a high-strength motor housing and balanced blades for smooth operation. The 16-inch high-output fan moves around 3,000 CFM and is well-suited for heavy track use. Derale also produces fan controller kits that include a variable speed module, which allows the fan to run at lower speeds during normal driving and ramp up when temperatures climb. This reduces noise and electrical load on the system.

Fan Shrouds: The Overlooked Upgrade

No fan is effective without a properly designed shroud. A shroud ensures that air is drawn evenly across the entire radiator core, rather than just through the area directly in front of the fan. For RB builds, aftermarket shrouds are available from companies like M&I (Mizumo) or can be custom-fabricated from aluminum. A well-made shroud also protects the radiator core from physical damage and provides a mounting point for the fans. A shroud with a depth of 1.5 to 2 inches is ideal. When selecting a shroud, ensure it matches the radiator core dimensions and provides a seal around the fan periphery.

Supporting Cooling System Components

Even the best radiator and fans will underperform if the rest of the cooling system is neglected. Here are critical supporting upgrades for RB builds.

High-Pressure Radiator Cap

A factory RB radiator cap typically opens around 0.9 bar (13 psi). Upgrading to a 1.3 bar (18-19 psi) cap raises the boiling point of the coolant, which reduces the chance of cavitation under extreme heat. Mishimoto, Koyo, and Stant all offer high-pressure caps that fit the standard RB filler neck. However, ensure your hoses and heater core are in good condition and can handle the increased pressure.

Performance Water Pumps

The water pump must circulate coolant effectively. Stock RB water pumps are adequate for most street builds, but high-RPM track cars can benefit from a high-flow pump or an electric water pump. Companies like Stewart Components and Davies Craig offer high-performance electric pumps that eliminate parasitic drag and provide controllable flow. For a street/track dual-purpose car, a mechanical high-flow pump (such as one from NISMO or a modified stock pump with a billet impeller) is a good balance.

Coolant and Additives

Using a quality ethylene glycol-based coolant mixed with distilled water (typically 70% water to 30% coolant for track use) is best for heat transfer. For track-only cars, some builders use pure water with a lubricant additive like Water Wetter or Purple Ice. These additives reduce surface tension, allowing the coolant to transfer heat more efficiently. Avoid using tap water, as minerals can cause scale buildup and reduce cooling efficiency over time.

Thermostat and Bleeding

It’s common to install a lower-temperature thermostat (e.g., 160°F instead of the stock 180°F) to keep engine temperatures down. However, the ECU relies on coolant temperature for fueling and timing adjustments. If you lower the thermostat opening temperature, the ECU may not reach its closed-loop operating window, which can hurt fuel economy and response. For track cars, it's acceptable to use a 160°F or 170°F thermostat, but be aware that the engine may run rich. A proper coolant bleeding procedure is critical after any cooling system work. RB engines are prone to air pockets that cause hot spots. Use a vacuum filler or a conical funnel to burp the system thoroughly.

Installation Tips and Common Pitfalls

Upgrading the cooling system on an RB car requires attention to detail. Here are practical tips to ensure a successful install.

  • Verify fitment before ordering: R32, R33, and R34 chassis have slightly different radiator mounting points. C34 Stagea and Laurel RBs also have unique configurations. Always cross-reference the radiator dimensions (core width, height, and inlet/outlet positions) with your car.
  • Use silicone hoses: High-heat environments can cause rubber hoses to degrade. Silicone hoses (SAMCO, Mishimoto, or GReddy) resist heat and cracking. Replace all coolant hoses while the system is drained.
  • Wire the fans correctly: Use a relay and a fused circuit. A direct switch to the battery without a relay can overheat the wiring. Install a temperature sensor in the radiator or a thermostat cap to control fan activation automatically. For track cars, consider a manual override switch inside the cabin so you can run the fans even when the engine is off.
  • Consider an oil cooler: For track builds, engine oil temperature often exceeds coolant temperature. A thermostatically controlled oil cooler (e.g., Setrab or Mocal) with a sandwich plate adapter can prevent oil breakdown. Some CSF radiators include integrated oil coolers, which simplify plumbing.
  • Do not skip the undertray: The factory undertray guides air to the radiator. Removing it for aesthetic reasons reduces cooling efficiency, especially at speed. Aftermarket aluminum undertrays are available that maintain airflow while being more durable.

Radiator and Fan Combo Recommendations by Build Type

Street Daily Driver (300-450 whp)

Radiator: Koyo VH 53mm or Mishimoto 50mm
Fans: Single Spal 16-inch puller with a 1.5-inch aluminum shroud or a dual 11-inch Flex-a-Lite setup
Supporting: 1.3 bar cap, silicone hoses, 170°F thermostat, Water Wetter additive. This combo handles stop-and-go traffic and the occasional WOT pull.

Street/Track Dual Purpose (450-600 whp)

Radiator: Mishimoto 60mm Race or CSF 55mm dual-pass
Fans: Dual 11-inch Spal pusher/puller or Derale 16-inch high-output fan with variable speed controller
Supporting: High-flow water pump, oil cooler with thermostat, 160°F thermostat, dedicated fan controller. This setup keeps the engine cool during 20-minute track sessions and is still tolerable on the street.

Full Track/Race Car (600+ whp)

Radiator: CSF 64mm dual-pass or custom-built radiator (e.g., PWR or C&R Racing)
Fans: Dual 12-inch Spal or high-output Derale with a fully sealed aluminum shroud. Use a fan controller with a coolant temperature override.
Supporting: Electric water pump (e.g., Davies Craig EWP115), oil cooler, transmission cooler, and a coolant expansion tank. Remove A/C condenser, use only water with Purple Ice, and ensure ample ducting to the radiator core. A grille duct or bumper cutouts may be necessary to force air into the radiator.

Conclusion

Cooling an RB engine build is not a one-size-fits-all task. The street-driven car that sees occasional traffic requires a different approach than a track-focused machine that lives at high RPM. By selecting a high-quality radiator from Koyo, Mishimoto, or CSF and pairing it with a robust electric fan setup from Spal, Flex-a-Lite, or Derale, you can dramatically improve thermal performance. Do not overlook the supporting components—a good cap, properly bled coolant, and a fan shroud can make the difference between a car that runs strong all day and one that heats up on the third lap.

Investing in a proper cooling system is the cheapest insurance for your RB engine. Whether you are building a drift car, a street monster, or a time-attack machine, take the time to research and match the components to your specific power level and usage. When in doubt, consult with a reputable RB shop or look at forums like Nissan Performance Magazine for real-world feedback on specific parts combinations. With the right setup, your RB will stay cool under any demand, delivering reliable power for years to come.