[Diy_efi] Speed-density vs. MAF/MAP...

[Ryan McCormick]

i am way over my head in most conversations in this
list.  but i have a question.  i have followed along
on the debate on where to take samples of MAP (eg, pre
TB, post TB, by intake valves, in air box)

Is there a prefered length of vacuum hose from that
point of refence to the actual sensor?  i am asking
because the further away from the valves it seems the
air flow is smoother.  would sampling closer but using
longer vacuun line be any thing worth pondering over?
Ryan (first post to this group)

--- Adam Wade <espresso_doppio at yahoo.com> wrote:
> --- Marcell Gal <cell at x-dsl.hu> wrote:
> 
> > (I reroute the discussion to the list if you don't
> > mind)
> 
> Not at all.  I actually thought my original reply to
> you was to the list.  ;)
> 
> >> In addition, you can read MAP before or after the
> >> throttle.  That would give two very different
> >> numbers.
> 
> > you're probably right.
> 
> Even if you're at full throttle, you have a
> restriction in the throttle body itself, plus the
> plenum acts as a capacitor for the air...  So unless
> you're at a sufficient rpm, steady-state, to get
> relatively smooth airflow through the throttled
> volume
> (taking into account the intake valve opening and
> closing), you should have pretty notable differences
> between PAP (as you call it below) and true MAP.
> 
> > However since MAP is Manifold Air Pressure I would
> > not call pre-throttle Air Pressure as MAP.
> 
> Well, there are speed density systems out there
> using
> both techniques, so obviously there are ways to make
> both workable, although I imagine they differ.  I'd
> definitely like to experiment with that, or hear
> from
> someone who knows the differences from experience.
> 
> >> If you read MAP from the throttled volume
> 
> > that's what I call MAP
> 
> Yes, I agree that that is truly MAP.
> 
> >> right before the intake valve closes
> 
> > I agree with that, good point.
> 
> I think I meant right before it OPENS.  If there is
> some supercharging going on from tuned intake length
> and reflected pressure pulses, that is where you
> would
> "see" it with a MAP sensor.
> 
> Of course, VE cannot be inferred from that alone, as
> you also have valve overlap, engine rpm, and exhaust
> length as factors in the mix.  This is one area
> where
> I believe mass air has the advantage when reversion
> is
> not an issue; mass air automatically takes into
> account any changes in VE as long as there is no
> backflow through the MAF.  More VE means more air
> ingested, and you can read that directly.  In fact,
> you should be able to very easily compute VE with a
> solid MAF signal and rpm.
> 
> Speed density CAN take into account changes in VE,
> but
> it depends on how and when the MAP is measured.  If
> MAP is relatively steady (high engine speed), then
> it's trivial, and speed density should be as
> accurate
> as mass air.  It's when the signal is not steady
> (closer to idle) that issues might arise from how
> the
> MAP sensor is sampled.
> 
> >> If you have a higher throttled volume MAP before
> >> the intake valve opens, at a given RPM and
> throttle
> >> opening, then you're going to get better filling.
> >> If the engine itself suddenly becomes more
> >> efficient (tuning the exhaust to that particular
> >> RPM, for example), then the MAP should drop
> >> slightly as more air moves through.
> 
> > OK. So we can confirm the appr. 
> 
> > VE= Constant * (pAmbient - MAP)/RPM
> > relationship, ( only applies, when MAP(RPM) is
> > measured at WOT, seems similar to an airbox
> pressure
> > sensor config)
> 
> Fair enough.
> 
> I think the big advantage of the airbox MAP, then,
> is
> that you can close the throttle(s) and still use the
> same formula as above.  While MAP drops like a stone
> at idle with the throttle(s) closed, that gives the
> opposite of the above formula...  The MORE
> difference
> bewteen ambient and MAP at a given rpm, the LESS
> power
> the engine is making.
> 
> > which is quite inverse of what could be thought
> > or some previous email suggested (though not
> stated
> > explicitely).
> 
> I think I am seeing now where the confusion arose in
> our communications.
> 
> >> I'm led to wonder, since the most obviously
> useful
> >> MAP numbers seem to come from the throttled
> volume,
> >> what the real-world difference is in plenum MAP
> and
> >> throttled volume MAP.  
> 
> > Is there a better name for Plenum Air Pressure,
> like
> > PAP?
> 
> Not yet.  Maybe I should invent a new standard in my
> book.  ;)
> 
> I think I answered most of my own question in this
> email, as well.
> 
> >> Does the plenum and throttle(s) act as a filter
> for
> >> low-rpm pulsation to some degree?
> 
> In answer to my own question:  More than that, it
> allows you to see the ACTUAL pressure drop from
> ingested air at ANY throttle opening, while you
> would
> need to know the throttle opening with a true MAP
> sensor to know what the pressure reading was telling
> you.
> 
> > I think so. The further away from the valves, the
> > smoother the airflow should be -- generally.
> 
> And you have the big capacitor of the plenum.
> 
> > Why don't we take 32..64 MAP samples per
> revolution?
> > (at least to get datalogs to support the 
> > pre-intakevalve-opening theory).
> 
> I am working with someone who is going to be doing
> some significant datalogging, and we are going to
> outright test a lot of the ideas that have come up
> in
> this threat to find strengths and weaknesses, useful
> and less useful methods of doing things.  I've
> become
> immensely curious, and it's time to find out the
> correct answers.
> 
> > Even processing that realtime should not be too
> > prohibitive with fixed point math.
> 
> I'd even be interested in taking oscilloscope
> traces,
> synced to crank timing, and exploring different
> ideas
> for sampling times in crank degrees to again find
> what
> would be more or less useful in a practical sense.
>  
> > (definitely less resource than the pure wav data
> > from a knock-sensing mic)
> 
> Heh, I almost think it makes more sense to look at
> synced analog initially to get some ideas, and then
> explore the actual application of those ideas in a
> digital realm, once we know what our sampling
> parameters need to be.
> 
> =====
> | 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     |
> 
> __________________________________________________
> 
=== message truncated ===


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