Should I replace or eliminate the air separator tank on a Mazda RX-7 FD?

Whether to replace or eliminate the Air Separator Tank (AST) on the Mazda RX-7 FD is a highly debated topic, but there is one absolute consensus: do not keep the stock plastic tank. The factory plastic seams are prone to corrosion and splitting, which can suddenly dump your coolant and lead to catastrophic engine damage like melted seals ¹.

You have two safe options for your FD: replace it with a metal unit, or eliminate it entirely.

Option 1: Replace with an Aluminum AST

Upgrading to a metal tank (such as those from Pettit Racing, Mazdaspeed, Tri-Point Engineering, or Rotary Performance) maintains the factory cooling system design while removing the risk of a plastic rupture.

Pros of keeping an AST:

• Air Removal: The AST provides the highest point in the system to burp air and gives bubbles a place to come out of suspension, which is critical since air in the system does not cool as effectively as liquid coolant ¹.
• Pressure Management: At high RPMs, the water pump creates high coolant velocities and backpressure. If the AST is removed, the radiator cap is forced onto the high-pressure outlet of the water pump. This pressure can overcome the cap's spring, lowering the coolant's boiling point and increasing the risk of localized boiling or overheating ¹.
• Coolant Capacity: Retaining the AST allows the system to hold slightly more coolant.

Option 2: Eliminate the AST

Eliminating the AST mimics the cooling system setup used on earlier generations like the FC/Turbo II. This is typically done using an elimination kit, an FC plastic filler neck, or a billet aluminum filler neck ¹.

Pros of elimination:

• Simplicity: It removes a potential failure point from the engine bay entirely and reduces clutter.
• Cost: Bypassing the system is generally cheaper than purchasing an aftermarket aluminum tank ¹.
• Clearance: If you are modifying your intercooler setup, the AST often gets in the way and needs to be removed for clearance.

Real-world experience shows that many FD owners have run without an AST for years under heavy street and track conditions with zero overheating issues. However, if you choose this route, you must be diligent about manually burping the cooling system and monitoring your coolant health.

Verdict

• Choose Replacement if you want to maintain the maximum safety margin against coolant cavitation, localized boiling, and air pockets, or if your engine bay is mostly stock.
• Choose Elimination if you are upgrading your intercooler and need the space, or if you prefer a simplified, clutter-free engine bay and don't mind taking extra care when bleeding the cooling system.

Difficulty: Easy to Moderate. Upgrading the tank is a straightforward bolt-on swap. Elimination requires slightly more work to cap off lines and install a new filler neck on the thermostat housing.