The screw in either the stock housing, or the Lexus V8 unit only adjust "leaked" air past the Karmann sensor.
The more leaked air you have, the higher the point before the TCCS will "see" a need to "Fuel Cut."
From what I know, at 1500hz signal from either AFM, the TCCS goes into FC. The Lexus is just larger, so more air flows through it before the 1500hz signal is reached.
The more you back out the air screw on either, the more air either will flow before FC is reached. (In most cases.)
So, backing it out of the stock AFM raises the FC point slightly because more air can flow in before 1500hz is reached. Same with the Lexus setup.
Screwing either air screw "closed" more will cause the 1500hz reading sooner, and you can fine tune FC points in this manner. Reg's longer screw did this for the Lexus/550cc mod, so you could restore stock FC at about 12 to 13psi with the larger flowing meter.
With the stock one backed out, and on a CT-26, I've seen people run 17psi before FC is hit. Larger turbo setups flow more air, so at lower psi, they tend to hit FC at lower psi, not because of the higher pressure, but because they are flowing more air at lower pressure, and the 1500hz signal is reached, triggering the FC.
If you use a AFC, or other fuel computer/modifying device, you can reduce the Hz signal, or even clamp it out, defeating FC in the process, but also leaving your engine open to higher boost pressure than it can supply fuel for. (recipe for disaster in short order.)
You can also scale down the AFM signal, using larger injectors, to "raise" FC as a result. (IE: your new larger injectors need less signal to flow the same amount of fuel, so you adjust the mainscale of your fuel computer get the right amount of fuel for the new lower AFM Hz reading. (Your TCCS thinks less air is coming in, but because of the larger injectors, you are really adding the right amount of fuel with the shorter injector pulse...)
To tune, a Wide Band meter is highly reccomended if your going to go this route, or you risk a very short engine life indeed. (And the TCCS will adjust timing when you scale back like this, and it advances the timing.. Not good if your going to run higher boost pressures...)
Ok, so much for that AFM screw discussion
