what are the differences please.

Andy....

Atmos. (VTA) vents to the atmosphere, recirc, recirculates the boost back into the inlet.

which means what? `scuse the ignorance.

VTA is the type that makes the PSSSHHHTTT sound, and recirculating types don't. Recirc valves keep the pressure within the exhaust system, enabling the turbo to spin-up quicker when boost is required again. You'll find various people advocating both types of valve with equal conviction.

Some say that VTA valves are potentially harmful to Imprezas. Others say it doesn't matter a rat's a$$. Subaru supply the car with a recirc valve, and that's how I've left mine.

I don't need my car to sound like a can of coke opening every time I change gear anyway!