Turbo control in a GM ECM

[Ludis Langens]

Shannen Durphey <shannen at mcn.net> wrote:
> > As I said, this engine is gasoline engine and it is a MAP (as opposed to a
> > MAF) system.  I can say with 100% confidence that the absolute position of
> > the accelerator (via the TPS) is never used to control fuel delivery.
>
> Interesting.  Those GM classes left me with the impression that TPS is one of the
> major players in fuel calculations.  But I have never gotten any specifics about
> the programming.  However, I could probably bet you succesfully that there is one
> time when TPS has a very direct result on fuel delivery.  During cranking, more
> than 80% throttle will cause the ECM to lean out mixture, as much as 50:1 on some
> applications.

Perhaps I wasn't quite clear about what I meant with my statement.  At no
point does the ECM do anything at all like this:

  InjectorPulseWidth = TPSPosition * MAGICCONSTANT;

There are however, many places where the TPS is compared with a value to
choose between different control methods.  Like this:

  if (TPSPosition > FOOCONSTANT)
    IdleAlgorithmA();
  else
    IdleAlgorithmB();

A similar question came up on a different list.  Several people thought that
they could make their engines run rich by bending the throttle to TPS link.
Even without O2 sensor feedback, that won't work in this case.

> So more about the turbo maps, please.  Would you say that a person could install a
> turbo on a 2.8l Fiero, and assuming he could determine appropriate wastegate
> control, have a running machine with no prom changes?

You could buy a kit with everything you need.  There is a guy/company that
has been selling them since the '80's.  One of the things included is a new
PROM.  While the original PROM/ECM has support for a turbo, it isn't
calibrated for one.  The turbo PROM has _lots_ of little changes as well as
a few large obvious ones.  For example, the main ignition advance table is
squeezed down to half width.  This makes space for the additional advance
data needed in the 1 to 2 bar range.

> Didn't someone say that all GM proms have turbo calibration maps included?  If so,
> does that mean I could rob a turbo from a 2.0l Pontiac and install it on my 2.2l
> Chev. without sacrificing my computer?

No idea.  I've looked at a very limited number of TBI w/MAP setups and they
don't have the 2-bar MAP sensor support.  And of course any MAF setup won't
have this support.

Jeffrey Engel <jengel at fastlane.net> wrote:
> Are you guys sure that vaccuum is used to actuate the wastegate?  
> It's possible I'm sure, but under boost there's pressure and more of 
> it until the 'gate needs to open up.  Sure would seem logical to use 
> pressure.  Of course, we are talking about GM here. . .

No, I'm not saying that.  From discussions with other people, and some
library research, the vacuum/pressure connection to the wastegate is more
of a sensory input.  It is not a "power source" like the vacuum used by
cruise controls and power brakes.

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