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I believe this phenomenon is what people refer to as "boost taper".
I think the ECU does have a certain amount of this built in. Possibly, sustained maximum boost was seen by Saab as being too stressful on the engine?
However, my understanding is that as the system keeps boosting, things happen that make the engine stop performing so well. e.g. the intercooler is not 100% efficient, so as the hot compressed air passes through it (and gets cooled), the intercooler heats up faster than the outside air can cool it. The cooling effect is reduced and the hotter, less dense air both produces less power (less mass at a given pressure) and promotes detonation (knock). Producing less power may affect the boost, but the ECU will have to respond to the detonation and when retarding the timing doesn't work, will reduce boost by allowing the wastegate to open further.
I'm sure I've missed a lot of factors here, but I believe the above to be true (and I'm open to correction). I'm sure this will apply even more to modified cars running higher peak boost, although perhaps modified ECUs will retard the timing further than the standard ECU before resorting to reducing boost (reducing and increasing boost takes a relatively long time, compared to altering ignition timing). Some heavily modified Saabs use larger intercoolers (9000 intercooler is a popular mod. for the classic 900, which has a smaller intercooler, and even larger intercoolers are available for the 9000) to combat this problem at very high boost levels, along, perhaps, with and water injection.
posted by 140.204....
, Tue, 11 Dec 2001 13:58:31
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