How much boost can the stock turbo on a 04 wrx run efficiently

without a manual boost controller the stock wrx runs about 10 psi. if you hooked up a manual boost controller efficiently you could run around 14-15 psi.

Think about this, every pound of boost shortens engine life. Some writers believe that when running 15 pounds of boost the engine life is shortened by 50%.
So it is not just a question of how much efficiently, but for how long? A normally aspirated Subaru engine is good for about 200K, so with seven pound the engine will last a 150K and at fifteen only 100K.

First let me say that the only way to safely run more boost on ur car is to get a re flash or tune. Do not put a Manual or electronic boost controller & turn it up or u will blow ur motor. With that said, if properly tuned, the TD04 can efficiently run about 17 to maybe 18lbs of boost. If u have the turbo ported, it could push close to 20 lbs but it is pretty much out of it is efficiency above that. Stock, u r only getting about 10lbs in first & second & with more load about 13.5lbs in third & up. You will have to change the down pipe to be able to run 18 lbs but that will give u quite the bump in power so it is worth the money. Again do not mess with just a boost controller. The risk of full boost at part throttle is dangerous for the engine & also full boost during the open loop/closed loop is also something that can blow ur motor so only tune with proper engine management.

How much boost can the stock turbo take? Around 40psi
How much boost can your stock engine take before detonating and spitting pieces out? 14psi, but it wont last very long without tuning.

The stock WRX runs 12psi of boost on a TD04 tiny turbo. l believe the stock turbo will flow 16psi efficiently with in it is compressor range, mainly due to the size, and will tapper off to around 13psi or lower at higher RPMs without a ported exhaust housing. As stated a good reflash of the stock ECU will net better results and will make your engine last longer.