The stock turbine isn't efficent holding that much boost to redline.

 

I'm going to take an opportunity here to make some points on the limitations of the stock EJ20 and stock TD04 turbo, but I want to do it in plain english, conceptual terms rather than busting out the hardcore math in order to make it more understandable for those who aren't looking to re-engineer their cars. Those readers of this forum who do have a full grasp of turbo engine theory, feel free to elaborate on this later.

The first thing to remember is that you can't reduce your car's performance to simple, basic constants. "How much you boostin y0?" is not a very specific question at all. Boost depends on several other factors to be optimized.
The key to understanding boost is to understand that power output from your WRX is primarily related to total air flow through the engine. The more air you flow, the more fuel you can burn, and in turn, the more energy you can use to move the car. In an NA car, airflow is a near-linear relationship to RPM- spin the engine faster, move more air. Turbo motors behave partially in the same way, but with an added variable- boost pressure. The simple version of the airflow calc in a turbo motor is

airflow=RPM*displacement*boost*VE*atmospheric pressure

Displacement and VE (volumetric efficiency- usually expressed as the percentage of actual airflow divided by the theoretical airflow for displacement*RPM) don't change on a stock motor with a stock turbo, and you can't do anything about the weather, so we'll call this constant X. That leaves:

airflow=RPM*boost*X

We'll come back to this later, after a brief discussion of compressor efficiency. If you've ever seen a 2D compressor map,



you'll recall that the X axis is usually airflow, either by weight or volume, and the Y axis is boost pressure. Then there are those circle things in the middle that look sort of like countour lines on a relief map of a mountain, where the rings get smaller and smaller as the elevation rises. Only in the compressor map, it represents the turbo's ability to create boost without creating heat- which is, of course, bad for making power for several reasons. The closer to the middle of the contours, the less heat the turbo creates with boost.

Let's get back to the specific example of the EJ20. Let's say it flows 30 units of air, at 15 psi, at 4000 rpm in pure stock form. What happens if we raise the boost to, say, 17 psi? Well, since

airflow=boost*RPM*X

then airflow increases. Let's pretend it's 35 units of airflow now. Going back to our random example of a turbo map



you would look at 35 units of airflow at the X axis, then move up to 17 psi. This point must still fall within an acceptable level of heat efficiency in order to be safely run on your motor.

So why does everybody say that boost has to taper before redline on the stock turbo? Well, look at the right side of the map, where airflow is higher. Notice the round shape of the efficiency map? That means that as you flow more air, you either lose efficiency, or you have to lower boost. Being more efficient is always preferable to running more boost, as long as making the most power is your goal, and not saying you run more boost than your friends.

The simple fact is, the maximum amount of airflow on the turbo is almost a constant. Whether that airflow is reached through more boost and fewer revs, or more revs and less boost doesn't matter. If the turbo can only handle 30 units of airflow and you try to make it flow 35, you will have heat problems, which rob power and make the engine much more likely to break. The WRX stock turbo can't handle the amount of air that the motor can flow at redline on 17 psi. In order to keep this simple, I'm going to skip the specific calculations for the EJ20 and go straight to the normal limits observed by hundreds and hundreds of people who have done a lot of trial-and-error tuning on their cars.

The stock turbo can handle the amount of air that moves through it at 16.5ish psi at 4000-5000 rpm. About that point, maximum airflow for the compressor is reached, so as rpm rises, boost must drop. By the time you get to redline, the turbo can typically handle no more than 13 psi, with 12 being safer. The other thing to remember is that it's better not to be at the ragged edge of maximum airflow. By backing off of this somewhat, you put less heat into the air entering the engine, which allows the car to both run slightly leaner, and advance the ignition timing more. A small gain in ignition timing yields more power than a small increase in boost almost 100% of the time in a Subaru motor.A Mitsubishi Evo motor, on the other hand, doesn't run much advance, but suffers smaller timing penalties from higher boost, therefore more boost often makes more power than more advance, despite the heat penalty. That's a matter for more technical discussion though.