Faulty dump valve?
 

Recently bought my first scooby (05 wrx blob). And I love it! Looked around, and I think I might have a dodgey dump valve. When I accelerate hard there is a lull in power for a few seconds after changing up, along with a sort of hissing sound coming from the engine, like a slow quieter release of air.
The car is standard apart from having the PPP out of factory and a previous owner has put on an aftermarket dump valve. How can I know what dump valves would be suitable for the car?

Fit the standard valve back on the car.

I would, but it didn't come with :(

The breakers on here will have plenty of the standard ones. New gaskets can be got from importcarparts.co.uk