One of the reasons for this is that Saab used smaller turbos than they might otherwise have done in order to reduce turbo lag and make the cars more driveable - a big plus-point in the marketplace for Saab's turbocharged cars. However, turbos have a certain efficiency range as far as airflow capacity is concerned. To keep within the efficiency range, boost is reduced at higher RPM (when airflow is greatest) to reduce the total airflow through the turbo. Increasing airflow beyond the efficiency range is possible, but tends to result in higher turbine speeds and excessive heat generation.

Due to the same principle, when Saab wanted to produce a higher-torque, higher-power version of the 2.3T engine for the Aero, they used a bigger turbo at the expense of increased turbo lag. The Aero tends to suffer less from boost taper than other 9000s (at least mine seems to).

A second reason is that as the intercooler heats up, inlet air gets hotter and the risk of detonation increases. When detonation is sensed, one of the car's resorts is to reduce boost pressure.

posted by 62.253....

• why does boost slowly taper off as revs increase?, paulmanchester, Tue, 18 Jun 2002 15:11:26
  • thanks, another 1 tho'!, paulmanchester, Tue, 18 Jun 2002 15:41:14
    • Depends-, Ari , Tue, 18 Jun 2002 15:54:02
      • Clean power >, Bill Davies, Tue, 18 Jun 2002 16:59:19
        • poor performance, pag, Tue, 18 Jun 2002 18:44:03
        • poor performance, paulmanchester, Tue, 18 Jun 2002 18:21:27
      • It has been very warm here the last couple of days..., BillJ, Tue, 18 Jun 2002 16:58:34
        • Re: It has been very warm here the last couple of days., steve, Tue, 18 Jun 2002 17:22:05
          • Found a fix, BillJ, Wed, 19 Jun 2002 19:12:35
        • These temperamental Aeros... (n/m), Bill davies, Tue, 18 Jun 2002 17:00:15
          • Perhaps I'll wash it this weekend (n/m), BillJ, Tue, 18 Jun 2002 17:12:17
  • Re: why does boost slowly taper off as revs increase?, Craig, Tue, 18 Jun 2002 15:31:55
    • Turbo sizing is a compromise, BillJ, Tue, 18 Jun 2002 17:10:43 <-- Viewing This Message
      • Re: Turbo sizing is a compromise - can i fit..., paulmanchester, Tue, 18 Jun 2002 18:59:32
        • Re: Turbo sizing is a compromise - can i fit..., Craig, Wed, 19 Jun 2002 14:41:26
          • Quite right. Why do something difficult and >, Bill Davies, Wed, 19 Jun 2002 16:38:09
        • What I know (and what I don't), BillJ, Wed, 19 Jun 2002 03:33:32

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