The N75 is the "Boost Frequency Valve". Electronic signals from the ECU tell the N75 how little or much to open the wastegate. The MAP (Manifold Absolute Pressure) sensor (ie boost sensor) tells the ECU how much boost is currently occuring, which (after some processing with some other variables) then determines what the ECU tells the N75 to do.

The N75, as noted above is a Frequency Valve, which is to say it is a solenoid turned on and off very quickly. It's function is to bleed air away from the wastegate. If the boost like from the manifold was connected directly to the wastegate, then once the boost pressure exceeded the spring pressure in the wastegate, it would push the rod, opening the flap, and diverting air flow away from the turbine, decreasing/limiting boost levels. On the S4, the spring pressure is around 4-5psi. When in limp mode (or APR Valet mode), this is what happens - all boost is directed right at the wastegate and thus it opens at 4-5psi and that's all the car ever creates. This is the N75 operating at 0% duty cycle. As the duty cycle is increased, air is diverted away from the wastegate (vented back into the intake path). This causes the wastegate to see less pressure. So maybe now the manifold is seeing 10psi, but because the N75 is diverting air away from the wastegate, the wastegate is only seeing 5psi, and thus it is opening there (maintaining 10psi). Increase that more and more, and you divert more and more air away from the wastegate. On a K04'd S4, when approximately 85% of air is diverted away from the wastegate, the wastegate then sees the 5psi it needs to open, and the car is seeing about 20psi.

An MBC functions in a very similar way. It has a spring and a ball valve. Depending how tight you tighten down the spring will dictate how much pressure is needed on the other side (manifold) before the ball starts to open, sending pressure to the wastegate to open. The N75 operates in high speed on/off pulses controlled by an electronic signal, whereas the MBC operates with a simple spring controlled by a knob tightening it down.

When a boost spike occurs, the MAP sensor tells the ECU the boost level, which it realizes is higher than it wants to produce. Therefore, it lowers the N75 frequency so that more air is going to the wastegate to open sooner. This might be more adjustment than what is needed, and thus the result is the car ends up running less boost than is desired (undershoot).

With an MBC in parallel set properly, the MBC acts instantly (ie as soon as the spring is acted upon with the boost pressure, opening the ball valve), sending more air to the wastegate, opening it to maintain the boost level. Therefore, the boost spike never occurs and the ECU never tells the N75 to reduce it's frequency, and thus no undershoot occurs. So the MBC in parallel is avoiding the overshoot (and subsequent undershoot) from ever occuring.

The best thing to do is log your particular car. See where the desired and actual boost curves are. Setting the MBC so it's about 1psi above the peak desired boost has worked well for me.

Mark P


Dyno: 469wHP / 451wTQ
93 oct - best 60':1.734 | best 1/4ET:12.239 | best 1/4mph:116.58
100+oct - best 60':1.698 | best 1/4ET:11.762 | best 1/4mph:121.72



ECUx template version 11.16 (3/12/07)
AWD Dynos by State - Race Gas in New England - B5S4 100k+ mile post

Current:
'02 Audi S4 Avant, Imola Yellow, APR Stg3 - imolavant website - 185k miles
'01.5 VW Golf 4dr 1.8T, Satin Silver, GT28R, EPL-tuned - 14.8 | 101mph - 195k miles
Previous:
'00 Audi A4 1.8T, Silver - Quattrobonium website