Sounds like you need a bigger BOV or one that vents better or both.
Does this only happen at high rpm (high flow rates). ie. does it surge if you shift at 4000rpm? Cause that would be a similar flow rate to your stock turbo at 5000rpm (estimate)
As for the does a bigger turbo flow more than a stock turbo with 12psi in the plenum ??
Well it's 12psi in the plenum but the engine makes more power which means it has more flow. (ie. eats more volume of air per second) This is because the turbo is more efficient and uses less exhaust restriciton to make the intake pressure so it lets more air out of the engine for more air to go in.
Got it?
They would flow the similar in the midrange though where the stock turbo is still sort of efficient.
