Performance thermostats like the IAG 156° Low Temperature Thermostat offer real benefits for heavily modified or track-driven EJ engines—but they’re not always the right fit for every setup, especially daily-driven street cars. We’ve seen a few customer reviews raising valid questions about how this upgrade affects engine warm-up, fuel economy, and overall performance. If you're wondering whether a low-temp thermostat is right for your build, this article will help break it down.

Designed for demanding conditions, the IAG 156° Thermostat activates sooner than the OEM unit, enhancing cooling efficiency when the engine is under heavy load.

The thermostat in your Subaru’s cooling system controls when coolant begins circulating through the radiator, directly impacting how quickly and efficiently the engine regulates temperature. The factory thermostat doesn’t begin to open until around 172°F (78°C), which helps the engine warm up quickly—but can also delay cooling under heavy load. In contrast, the IAG 156° Low Temperature Thermostat starts opening at 156°F (69°C), allowing coolant to circulate sooner. This earlier activation helps reduce the chance of sudden temperature spikes and improves overall cooling efficiency, especially during aggressive driving or track use.

In a tuned or track-driven Subaru, high cylinder pressures, aggressive timing, and elevated turbo temps can push your cooling system to its limits. A lower-temp thermostat helps combat this by allowing earlier coolant circulation, keeping temperatures more stable and reducing the risk of heat soak or detonation. This is particularly important during repeated hard pulls or lap sessions where managing heat is a constant battle.

A 156° thermostat isn’t ideal for every build—on street-driven cars, it can prevent full warm-up and impact efficiency, emissions, and long-term wear.

This is where context matters. On a street-driven WRX or STI, especially one that sees shorter trips or colder ambient temperatures, a 156° thermostat may prevent the engine from reaching full operating temp. This can lead to:

• Poor fuel efficiency
  
  
• Increased emissions
  
• Reduced heater output
  
• ECU compensations that limit performance
  
• In extreme cases, accelerated engine wear due to oil not reaching optimal viscosity

If you're not seeing consistent hard driving or engine temps above normal, a factory-range thermostat might be the better option for longevity and drivability.

The IAG 156° Thermostat is best suited for:

• Modified EJ engines making more power than stock
  
• Vehicles with upgraded cooling systems (radiators, fans)
  
• Track day or autocross cars
  
• Street cars in hot climates with ECU tuning to match

It's not recommended for:

• Daily drivers in colder climates
  
• Stock or lightly-modified Subarus
  
• Builds without supporting cooling upgrades

Optimized for performance and track use, the IAG 156° Thermostat should be paired with supporting mods—not your average daily driver setup.

If you're pushing your Subaru to the limit, you need components that can keep up. IAG’s 156° Thermostat is designed with longevity in mind. Constructed from 304 stainless steel to resist corrosion and endure high temperatures, it includes a quality gasket for a reliable seal. The laser-etched IAG logo reflects the precision and craftsmanship behind every part we make.

The IAG 156° Low Temperature Thermostat is a proven upgrade for drivers who push their EJ engines hard and need faster coolant circulation to manage temps. But like any performance part, it’s important to match it to your goals. If you're building a high-horsepower street machine or prepping your WRX or STI for track use, this thermostat is a smart investment. For more casual driving, sticking with OEM specs may offer a better balance of performance, economy, and reliability.

Selecting the right thermostat means matching it to your driving goals—choose the IAG 156° to enhance cooling under heavy use, or keep OEM for consistent, everyday reliability.