Air Flow Meter discussion (WAS: "I'm missing something...)

[Kevin _]

>
>Kevin _ wrote:
>
> > Unfortunately, in the automotive world, all the aforementioned sensors
> > besides the MAP are VERY slow.
>
>Here's a post from GMECM a few weeks ago:

*text omitted*

> > On the other hand, a general MAP sensor reacts in about 1ms.
>
>Maybe so, but how fast do you actually sample it?

For what I would consider a proper system, you would sample throughout each 
power stroke, taking an average of all the values during that power stroke 
(think of it as a 2-revolution filter).  Then, calculate every power stroke. 
  I'd want to see 2 separate circuits, one that is triggered by cam timing 
every other revolution and is super-fast to tell the average MAP value for 
that pair of revolutions.  This is obviously a pretty simple circuit and I 
would want to see it sample the MAP maybe every 0.1 ms or so to oversample 
and develop that running average.  Then, have the 2nd circuit polling that 
1st processor for its averaged value and to restart its timing sequence 
every power stroke.  That one would sample every power stroke, so, at 9k 
rpm, every 13.3ms - lower rpms would of course be slower.

> > This is by far the superior way to go for making a drivable EFI setup.
>
>Sounds like opinion to me, but I do believe you are entitled to it.  I
>would like to know some of your background and how you formed your
>opinion.

Well thank you, saying I'm entitled to that opinion probably means that 
something I said was at least a little coherent (I sometimes miss that mark 
:)  That opinion was formed by a decent amount of experience with the EFI 
system of DSM's (talons).  According to one guy who knows the code of these 
cars well (Todd Day), the ECU recalculates injector pulsewidth every 
injector firing, which is every power stroke.  I know that going to a 
slightly slower airflow sensor and adding about 10-20ms of processing time 
on top of that completely screwed up all the transients.  Making the 
adjoining hardware faster and faster made it better, but never truly fixed 
it.

Another thing I don't like about airmeters and forced induction is they 
cannot handle throttle lift.  If the turbo is spooled up and the entire 
intake tract is at 20 psi, then you lift throttle, that 20 psi worth of 
intake tract air makes its way back out the airmeter backwards.  The stock 
sensor in the dsm's has a reset wire where the ECU sends it a signal saying 
a throttle lift condition has occurred, and the stock airmeter then just 
kicks back a preset signal, not the real airflow.  This phenomena becomes 
more and more obvious when you go to a larger and larger turbo and 
intercooler.  At the point where I had a 24x6x3" core with a large length of 
2.5" piping and a T4 54-trim compressor wheel, the car was almost undrivable 
during light transients because the backflows were so strong every time 
you'd get off the throttle from any boost (I inadvertantly removed the lift 
throttle - output reset feature with the new airmeter).  I would see a 
higher voltage from the meter after a throttle lift than I would during a 
WOT pull (and yes, this is with a stock style recirculating blow-off valve). 
  I also have a 5-spd dsm and if you hit the shifts good enough in that car 
you can beat the airmeter's response (or the ECU's safeguards of it) and get 
a HUGE bog into the next gear from completely wrong fueling.

The car with the big turbo has become a drag race car.  It now has a Haltech 
E6k and has run a best of 11.40 at 124 mph (~500hp from a 2 liter).  I've 
been chasing demons in it all year and hope to have them stomped away and be 
well into the 10's next year.  Anyway, I think I've taken up enough 
bandwidth now.  If you sample it fast enough, I'm thoroughly convinced the 
speed density setup is the better way to go.

Thanks for reading,

Kevin

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