use of MAP in FI calcs

[Ludis Langens]

Heelandtoe at aol.com wrote:
> 
> I've noticed that in most ECM's all the tables that use MAP as an index  stop
> at 100 kPa.  Since its conceivable that MAP could go above 100 kPa on a high
> pressure day at WOT,

I suspect that GM's idea of 100 KPa doesn't actually match the NIST
definition of 100 KPa.  From the way that various ECMs handle the MAP
sensor, GM seems to have defined 100 KPa to be the highest pressure that
would be seen during a high pressure day.

This means that the tables won't ever actually max out during a WOT run
on a high pressure day.

> I'm betting that the computer normalizes it with the
> barometric pressure as in mapbar = 100 * map/baro.
> Have you guys seen any similar
> operations in your disassemblies?

A calculation similar to that actually is performed.  However, the
result is not used much.  It is mostly used for error bounds checking -
as in setting "codes".  A lot of the error limits (TPS High, etc.) seem
to be calibrated for sea level conditions.

> This would account for the
> day to day variation in barometric pressure and normalize mapbar so that it
> better represents true engine load.

You actually want _not_ to do a BARO correction in a lot of the
calculations.  Once you've measured a certain absolute pressure in the
plenum, you generally don't care what the pressure is on the other side
of the throttle butterfly.  It doesn't matter if you have the
supercharger from hell blowing into the throttle or not, a given MAP
reading will always mean the same amount of air in the cylinders. 
(Actually, barometric pressure will affect cylinder scavenging - and
there is a correction term for that.)


Dave Zug wrote:
> 
> If indeed the BARO is not re-calculated unless the car is turned off and
> back on, What happens when you start the car at the top of a mountain, and
> drive down to the bottom then drive 200 miles?  lotsa BLM correction?
> 
> food for thought.  Maybe why they changed it later?

Your software experience is with a turbo program.  The non-turbo ECMs
generally (always?) have the ability to recalc the BARO under specific
conditions during cruise.  With a turbo mucking up the intake air flow,
this is not possible (or much more difficult).  Hence, the turbo ECMs
don't recalc the BARO.

-- 
Ludis Langens                               ludis (at) cruzers (dot) com
Mac, Fiero, & engine controller goodies:  http://www.cruzers.com/~ludis/


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