[Mgs] Timing discussion

Richard Lindsay richardolindsay at gmail.com
Wed Apr 24 07:13:37 MDT 2019

   Paul, I agree completely with your assessment and recommendation for

> Completely agree with your reasoning behind centrifugal and vacuum
> advance, but:
>> "The other is pressure operated, offering advance when
>> the twin turbos raise the plenum pressure above ambient."
> Are you sure that isn't vacuum retard?

  Yea, I wrestled with that for ages when restoring my car. The early
Biturbo turbos blow through the carburetor, which is enclosed in the
plenum. When boost comes up, vacuum advance fails so the other capsule
takes over. Its a nasty, complex system with flow restrictors in various
vacum/pressure lines to retard reaction time. It almost destroyed my love
of dynamic systems! :-P

> If the turbos were increasing the pressure I'd expect that to
> make spontaneous combustion more likely, needing retarding to
> control.

   You are of course, correct. And to make things even more complex, the
problem you describe is handled by a knock sensor feeding the MABC
(Maserati Automatic Boost Control) computer. When preignition or pinking
occurs, the computer modulates the waste gates to reduce the boost.
   And to beat this dead horse a tiny bit more; The Biturbo's MABC does not
work like conventional boost control systems where boost is just dumped by
the waste gates when some maximum is reached. The turbos in the Biturbo are
small but there are two of them. And they make more boost at all engine
speeds - other than idle, I guess - than the engine can use. Therefore, the
MABC continuously modulates the waste gates to keep the maximum boost under
1 bar. They are always working at some level. The only true 'dump' occurs
when lifting from WOT. Yea, TMI. Now back to proper engines with proper
systems. :-) Thanks again Paul, for the technical insight.


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