[Diy_efi] Newbie with a simplified Motronic project

Adam Wade espresso_doppio at yahoo.com
Sat Nov 2 03:37:53 GMT 2002


--- Eric D Byrd <klox at juno.com> wrote:

> One thing I don't get about MAF vs. MAP.  Everyone's
> making out like MAF is self-correcting, and never
> any longer any need for fuel trim tables. 
> How?

Well, I don't know if I'd say "no need", but certainly
it is self-correcting for changes in VE, as far as the
in-built tables allow...

Let's assume the valves are out of adjustment on this
car, so that there is less overlap at TDC between
exhaust and intake.  That will have the effect of
lessening scavenging, dictating a leaner mixture; it
will also reduce scavenging at higher rpms, again
dictating a leaner mixture.

While MAP can't "see" this, it can only try to
correlate that at a given rpm and throttle opening,
the engine might have a tad more air pressure in the
manifold than it did before and figure accordingly (or
not!), the MAF simply sees there is less air going
into the engine, and accordingly gives less fuel. 
Much simpler and more direct.  The ECU has to computer
the MAF based on throttle position and MAP, which is
fairly imperfect, IMHO.

The drawback to a MAP is that it only takes an average
reading, and is time-delayed from hitting the
throttle, so there's lag.  This is less the case with
MAF, but there are still times you need to alter
fueling RIGHT NOW based on changes in TPS, as the
MegaSquirt people can tell you.  ;)

> Both are sensors, that respond to a physical
> phenomenon by generating a voltage, and all such
> devices experience variations in response due to
> age.

They do?  Does the resistance of the wire in a hotwire
MAF change over time?  I've yet to see such a thing...
 And if the resistance characteristics of the wire
don't change, the reading doesn't change.  Physics
doesn't go "out of calibration".  ;)  That makes it
far superior to many other sensors, IME.

> and eventually the MAF will signal a slightly
> different value for the same air flow,

On what do you base this claim?  I think you are
wrong.

> Why don't MAFs require fuel trim tables?

See above.  It has nothing to do with aging of a
sensor.

> From the post below, it appears they do, and that
> they get them from feedback from the O2.

Sensors don't get feedback from ANYTHING.  The ECU may
or may not use O2 sensor feedback, and it may or may
not use it for any specific purpose.  Many vehicles
with EFI don't have O2 sensors, and they seem to run
just fine...

And I didn't see anything in the post you quoted
showing anything...  Or even stating anything.  ;)

=====
| Adam Wade                       1990 Kwak Zephyr 550 (Daphne) |
|   http://y42.photos.yahoo.com/bc/espresso_doppio/lst?.dir=/   |
| "It was like an emergency ward after a great catastrophe; it  |
|   didn't matter what race or class the victims belonged to.   |
|  They were all given the same miracle drug, which was coffee. |
|   The catastrophe in this case, of course, was that the sun   |
|     had come up again."                    -Kurt Vonnegut     |

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