Fast Friday #4 is by Lucky Also!
Boost Pressure Variance
Boost pressure issues seem to be the #2 issue in the Volvo Performance world, beaten out only by vacuum leaks. Boost pressure control in the 1994-1998 models is a pretty simple system but for the ECU to manage boost correctly it relies heavily on known base values and settings. If these settings are off or if there are mechanical issues then the boost control system can be out of whack.
The primary adjustment is the wastegate setting. Factory setting of the wastegate actuator tended to not only be a quite a bit on the conservative side but also varied considerably. This inconsistency gets compounded when you are running a tuned ECU at higher than stock boost levels. For those running a manual boost controller (MBC) or an electronic boost controller (EBC) this might not be as much of an issue, however even with these aftermarket boost control devices the factory wastegate setting still required adjustment for them to work properly.
It wasn’t until 1999 that Volvo introduced a boost pressure sensor on US models such that the ECU could actually monitor and adjust boost based on the sensor input. In early models the boost pressure was not measured with a sensor but rather calculated by airflow, RPM, and throttle opening. The ECU was actually quite good at this calculation and with a properly set wastegate the boost target was pretty consistent. However as boost pressures increased the calculation that is made has more room for variance and hitting and holding a proper boost target can be a bit tough. That’s why both a properly adjusted wastegate and good condition boost control solenoid are critical.
You might not think that there’s much of a difference with respect to the various TCV/BCS available in the market place but you’d be surprised what you’ll find. The actuation time (called latency) of each of the various solenoids on the market varies quite a bit and needs to be accounted for in the tuning to provide the most accurate method of boost devleopemnt. The TCV duty cycle linearization map inside the ECU is what needs to be adjusted for whichever valve is being used. This allows for the most precise and consistent control of boost pressure from 1994 to the latest models. That’s why some cars react differently to different turbo control solenoids.
For 1999 and later models with a proper boost pressure sensor you’d think the issue of boost development might be resolved, unfortunately it’s not. The BPS in these models is more for control of max boost target (I.E. over boost or spiking) rather than as a feedback to determine if the boost pressure is low and subsequently try and increase it. So even with these cars the base wastegate setting and a good condition TCV is critical. For those applications really pushing the limit adjustment of the TCV duty cycle linearization map becomes more and more important and is something that should be requested from your tuner.
-Lucky
