“Tightening” the wastegate does what Matt says - you move the boost curve to a different RPM range - it doesn’t actually increase boost.Gmstewart22 wrote: ↑Sat Apr 13, 2019 11:06 pmPerhaps this is just my ignorance, but when I turn up my wastegate pressure I get an overboost code and my computer compensates by cutting fuel my tuner guy says that's what he tunes for.. he adjusts the computer to accept a higher value.. man now I'm really confused! Again pardon my ignorance I've only been into volvos for a few years and just recently started nodding them
As you mention, it can create boost spikes. This is due to the ECU not expecting the boost level, or in your car (and mine), the volume of air coming in. It will then (over)correct, usually to the detriment of performance in favor of its own safety, such as with cutting fuel.
The boost levels are controlled by the ECU via the TCV (turbo control valve) duty cycle table/map, which also interacts with the Load request map and the fueling map in the ECU. Most of the calculations are based on MAF (mass air flow) sensor input to the ECU, coupled with throttle position, etc. You can absolutely not control boost levels with the wastegate (alone).
As Matt says, for 98 (and older), as well as some 99s, you can use a manual boost controller, but you also need to be aware that an MBC creates the risk of too high boost levels at low RPM, as well as lean conditions. Both of those can be catastrophic to the engine. I.e. no more engine. Done correctly, an MBC is excellent. Done incorrectly, you need a new engine.