Most people/sites suggest that to crank up the boost you
just turn in the 'P' screw in the ECU. Unfortunately this myth
was supported by a Winkler article in Nines. Based on
extensive testing on my 83T over many years, I believe the
proper way is to turn in the 'F' , and then turn in the
adjacent 'P' screw a little as a fine tune. The 16V's, up to
M93, have essentially the same system.
The screws (ie., pots, potentiometers) are as follows:
P = Percent................ solenoid 'default' duty cycle
F = Full boost ............ max boost vs rpm map
K = Knock ..................sensor sensitivity
Below about 5-6 psi and under 3100 rpm (and at idle), the
solenoid valve is 95% open, bleeding off the waste gate
actuator to keep it closed. Once either of these limits is
exceeded, the solenoid valve changes to a 'default' duty
cycle, set by the 'P' screw. This partly closes the valve,
and reduces the rate at which pressurized air is bled off
the actuator.
This allows the WG to crack open and slow down how
quickly the boost rises, all to avoid pressure spikes and
severe overshoot.
THE 'F' SCREW sets the max pressure allowed before
the solenoid valve quickly changes from the default 'P'
duty cycle to a more closed condition in an 'active control'
mode. This feeds more pressurized air into the actuator to
open the WG more and reduce boost.
The pressure map for my 83T is flat up to 3100 rpm, and
from there it tapers down 2.5 psi by 5500 rpm. If I turn F in
fully, the controlled max limit is 12.5 psi up to 3100 rpm,
dropping to 10 psi by 5500 rpm ( the 16V max F map
should be higher).
I'm currently going easy on the 2nd trans with F set to 10
psi, dropping to 7.5 psi at 5500 rpm.
THE 'P' SCREW trims the rate at which boost rises. I
found that turning in the P too much results in excess
overshoot, and an annoying run-away boost climb with just
slight pedal depression in upper gears. However, if F is
turned in (increased) without increasing P, then the
P-default will open the WG before the higher F limit is
reached, and no significant boost increase is achieved. A
relatively low 'P' setting limits higher rpm boost.
Only tweak an in-tune engine with good plugs and wires,
etc. First step is to be sure the basic boost is set at the
actuator rod. Set it per the manual to about 45-50% of
your desired max running boost. If you plan on a lot of time
at high rpm, intake and exhaust should be upgraded.
For 8V turbos with CIS injection, you can add an IC, but
the stock fuel system does not provide enough richness to
support higher boost without detonation.
For pre M89, the trans has a small pinion bearing that fails
often at stock boost levels.
For US 16V engines, the 2.5 FPR should be upgraded to
2.8 (or 3.0) if mid-red boost levels are desired.
Mark/note your starting position of the screws. If you
already messed with the P, turn it fully in and back it out
1/3 turn. Increase boost by turning in F a lot and P a little.
Hard core will turn in F fully, but the P is the boost-rise
rate, and should not be turned in too much for reasons
noted prevoiusly. The P takes some time to get right, and
winds up being a 'little nudge' type final adjustment. All
knock sensor control is left in operation.
Don't mess with 'K'.
If you max out F and want more, a Radio Shack 430-ohm
resistor, across the pressure sensor terminals, will get you
about 3 more psi. I put in a Mad-Max switch on the
console to kick in the resistor when I want to go from my
10 psi setting to 13 psi. A 530 ohm resistor is closer to a
2 psi kick. I also moved my pressure supply for the
solenoid valve from the turbo to the throttle body, but
before the throttle valve. This tends to slightly increase
high rpm boost, when in 'P default' mode, and is related to
the 1-2 psi pressure drop (at high flow) at the very
restricting stock 16V intercooler and piping
