re: Why Calibrating AFMs? One AFM bump that works.

[JKEROUAC@xxxxxxxxxxxxx]

re: Why Calibrating AFM's works.

Just like with OEM carburetor jettings of the past, 
the factory calibration of AFMs and other emissions 
related components is set so that no car will run bad, 
just that many if not most cars can be dialed in 
exactly at a tooth or two from the initial oem 
setting.  Also, as an engine ages, it tends to lean 
out.  Also timing chains stretch.  That causes a 
retarded cam timing that allows greater spark timing 
advance to be used before the onset of detonation.
  So even if your AFM was dialed in right from the 
factory after the engines gets miles on it it may want 
a tooth or two leaner or richer.

We're not trying to bump anything up here. The goal is 
to get the AFM's pointer position correct in the first 
place, so that the ECU does not have to take 
performance robbing steps to then correct the mixture.

     Trap door AFM's have been on BMWs for over 25 
years now, and I haven't heard any new ideas in at 
least the past ten or so.  but the discussion 
continues on anyway.

     When the AFM is out of calibration, then the ECU 
has to take steps to try to correct the mixture.
There are many combinations of spark advance, injector 
timing, and injector duration that can produce a given 
exhaust gas measurement.  But does that mean its 
maximizing either efficiency or power?  Absolutely 
not.  It just means that its keeping the emission laws 
happy, and any other effects are secondary.
       So if the initial signal says the ECU has to 
make a correction, well, what does it do?
It can retard the timing, for one thing.  Bad for 
performance, no doubt.  It can reduce the injector 
shot at the current timing, or a combination of the 
two.
     The optimum performance will occur when the ECU 
needs to do none of these things.  Ergo, let's get the 
pointer on the AFM correct in the first place.

What happens when the AFM is giving max signal, the 
revs are up, and there just isn't enough fuel, the ECU 
is in max power mode, etc?

    That's where higher fuel presssure comes in.  
Adjsutable fuel pressure regulators come in all sorts 
of overpriced fancy shmancy styles.
     But hey, what about that OEM manufacturer, Bosch.
Well if you take that part numbe rof your regulator, 
replace the last three digits with a 1, for one 
outlet, or 3 for 3 ouitlet, there it is, adn for 
enormously less than these aftermarket gougers charge.
     Also, a higher pressure produces a finer 
atomization from the injector.  Same as turning up the 
pressure on the garden hose.  So even fbefore you're 
getting a full injector shot, you're getting a more 
efficient burn at lower revs too.
     Only after the rest of the system is optimized, 
and you are up to 60psi, and still not getting enough 
fuel, would you go to larger injectors.  This only 
normally is needed with displacement increases at 
least 10% or more, or with much higher revving race 
motors.

     Here's a way to up the max output signal from any 
factory AFM:  Notice the bump stop that limits the 
pointer arm movement?
Notice there is more carbon band to go after the point 
where the stop limits the pointer movement to?
     The track marks of wear on the carbon band makes 
this easier to see.
Does two plus two equal four?
     Bend the bump stop so that it still does make 
contact but just barely at the full end of pointer 
movement.  Now the pointer makes it to the end of the 
carbon band, and you are getting more signal from the 
AFM to the ECU.

     Notice:  Its likely that most first timers will 
get the AFM too rich til they get the hang of it.

The best description of setting an AFM appeared in 
some Roundel articles in the mid eighties, written by 
Metric Mechanic.  their old book, "An Enlightened 
Approach" really told the straight story about all 
this stuff.  by the mid eighties, trap door AFM were 
pretty much all figured out.
     So some of this stuff may not seen intuitive at 
first, but if you take the basics on faith and go from 
there, you will come to see the how and why of it all 
just like countless other tuners before you have 
discovered.
     For those of you with L Jetronic free of oxygen 
sensors, you can plug in a 75 or so Datsun 280 Z 
computer directly into your system for a power 
improvement too.

     Believe it or not, everything presented here I 
learned from others, none of it is anything I came up 
with myself.
Tune with your head, not over it. lol.

'jk

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