These cars use speed density tuning - the car uses Volumetric Efficiency tables to hit boost targets. Cobb's tuning guides can tell you all you need to know about the ECU's strategies.

 

Could do it the old fashioned way. Up boost 1psi and see how the power curve looks. Log IATs and once you see increased IATs with diminishing power gains, you're past the efficiency island.

 

Yeah, I suppose that would work, but the influence of the IC really impacts that method, would it not? Would be nice if I could l log temps coming out of the turbo itself and then if I see it rise dramatically I'll know I'm in the wrong spot!

 

Reading through this thread http://www.focusst.org/forum/focus-st-performance/19802-frpp-tune-question-boost-25-lbs.html

Admittedly, I am really dumb on turbos, so a lot of this is new so please forgive me. Let's pretend that the compressor map is ours. Would the surge line being at about 27lb/min be the reason why around 5000rpm mass air flow seems to come to a slow in gains(assuming it's the turbo somehow limiting airflow, not the engine ve)?

 

I think you got the map backwards. Going up in airflow, but maintaining steady boost, you eventually hit the choke line. The choke-line is the region of the turbine where the air velocity at the tips of the compressor actually hits mach 1! The pressure loss near mach 1 for the compressor blade sky rockets, and we can't compress the air anymore. Essentially the air starts flowing out of the compressor before we can squish it into other air.

Once you hit choke, the speed lines you see would essentially drop straight down from where they are on the curve. So as we force more airflow out, we essentially just rapidly raise the speed of the turbo. (And as we're doing this efficiency is going down big time)

The reason we die at around 27lb/min is because our turbo starts to choke, not surge. We end up in a situation where more airflow means we need more pressure, but that higher airflow means more choke and less pressure, so we kind of stabilize at 27lb/min.

Running in choke is harder on the turbo (by how much? I can't say for this specific case) as we spin it up faster and warm it up some more.

I can tell you this. Choking typically occurs in the 55-60% efficiency range, so we know that at 15ish PSI (about 2.0 pressure ratio) and 27lb/min, we're at about 58% efficiency.
You can try to do some math with the turbo equations to derive the efficiencies further down the 2.0 pressure ratio line, but it's not going to be much help. You could look at the speed limits in ATR, and try to work those backwards as well. You'll notice they go airflow vs inlet temperature and give you a pressure ratio. You can bet that pressure ratio is around 60-65% efficiency for the airflow, as they're essentially trying to limit choke with this table to prevent compressor over-speed.

There's a little more to it than this table shows (and the actual limit always ends up being much lower than this ratio due to some fancy math only Braden has the equations for ).

On the stock turbo, it'll be a fact of life that choke happens up top. I haven't tried playing too much with trying to run lower boost with higher efficiency and trying to crank timing to see if I can get more power that way, but I somewhat doubt it'll help too much.

Down low (2k-4k RPM 10-18lb/min) you're running in the peak efficiency zone. By extrapolating the surge line that Ford has for the turbo (in the bypass valve section), You'd be able to push 10-18lb/min up to 27-30lbs of boost without surging if you had the fueling and rods for it (we don't have the fueling and we'd be talking like 470wtq at that point!). The turbo would be spinning pretty quick though there too, and the heat would be INTENSE.
Just guessing on the line, you can pretty much bet your money that Ford runs the stock tune right into the 75-80% efficiency zone in that area normally. A shot from the hip would put 22lbs at about 70% efficiency, 25 at 65% and 30 at 55-60% (and near if not on the surge line!)

It's really tough to say as we don't have one published really. You can get the surge line from the tables, and guess the choke line on the speed limit area, but other than that, you have to do as XRJoe suggests and compare CATs from run to run.

 

Strap it down on a dyno and start playing around. It's honestly the best way to really see what the car likes and what it doesn't. It's much safer than street tuning and it offers a controlled environment that shows the difference between tuning changes not road conditions(Vdyno is bad for this.)

 

This is what I figured Braden already did and what influenced his comment that over 22psi had "negligible" gains. Would be nice to get my hands on a dyno though and figure this out one day.