What are the issues with an air separation tank delete on a Mazda RX-7 FD?

Deleting the Air Separation Tank (AST) on an FD RX-7 is a common modification, primarily done because the factory plastic tanks are notorious for splitting at the seams, dumping coolant, and causing catastrophic engine failure ¹.

However, eliminating the AST does introduce a few functional issues and debates regarding cooling system efficiency:

1. Difficulty Bleeding the System The primary function of the AST is to separate tiny air bubbles from the coolant. Air in a rotary engine's cooling system reduces heat transfer efficiency and can create localized hot spots, which lead to premature engine failure ⁴. Without the tank in place, it becomes slightly more difficult to fully purge or "burp" the system of air after a coolant change ³. You will need to carefully monitor the coolant levels for a few days after a refill to ensure all air is purged.

2. Radiator Cap Pressure at High RPM When the AST is deleted, the radiator cap is typically relocated to the thermostat housing, which sits directly on the high-pressure outlet of the water pump. At high RPMs, the pressure created by the water pump and the friction of the coolant moving through the radiator can push against the cap. This backpressure can overcome the spring in the radiator cap (e.g., causing a 16 psi cap to open at a lower effective pressure), which lowers the coolant's boiling point and theoretically increases the risk of overheating ¹. While this is a known issue in motorsport applications, many FD owners report running without an AST on the street and track for years without experiencing this pressure-relief issue.

3. Reduced Coolant Capacity Removing the tank completely removes its internal volume from the loop, resulting in a slight loss of total cooling system capacity ³.

How to Properly Delete the AST

Difficulty: Easy to Moderate

If you decide to proceed with the delete, doing it cleanly is critical to prevent leaks.

1. Use an 1986-1988 FC RX-7 radiator cap pedestal (Mazda Part # N326-15-17YA) or an aftermarket billet S4 filler neck to replace the stock thermostat housing neck ³.
2. Run the hose from the new filler neck directly to the coolant overflow tank.
3. You must block off the two small hoses that previously routed to the AST (one on the bottom of the radiator, one on the thermostat housing). You can connect them together using a 3/8" hose coupler, use rubber plugs, or ideally, have a welding shop weld the nipples shut permanently to eliminate leak points ².

Alternative: If you want to retain the air-separation function without the risk of the plastic tank splitting, you can upgrade to an aftermarket aluminum AST ¹. Ensure the aftermarket tank has a restrictor in the bottom nipple (matching the factory tank) so the coolant moves slowly enough through the tank to allow bubbles to separate ².