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

[Adam Wade]

--- Perry Harrington <pedward at apsoft.com> wrote:

> I'm rarely a dick about details, but...

>> From my thoughts and reading, I had always been
>> under the impression that it made more sense to use
>> speed-density for SMALL throttle openings and
>> lower engine speeds, as MAP sensors bounce around
at
>> small

> I believe you meant MAF in the above sentence?

My understanding is that MAF sensors are slow to
react, and read both incoming air, and air that
backflows into the airbox during overlap at or near
idle with aggressive cams.

My understanding (especially from watching a MAP
sensor on a motorcycle in use on an airbox) is that
raw MAP output from the airbox at anything but high
airflow situations leaps around like a prancing elf. 
;)  How any meaningful measurement could be made from
such an airbox measurement, I do not know.

> Speed density is MAP vs RPM, Alpha N is TPS vs RPM. 

Ahh.  I was using wrong terminology.  Shoot me now. 
;)

> I would believe that they are using Alpha-N when
> they peg the MAP sensor to atmospheric.  Bike
> engines have VE ranges well over 100% for thousands
> of RPM of operation.  For this reason it becomes
> impractical to use MAP.

I'm not sure I understand why, as long as your fueling
map compensates for that.  Is not some actual
measurement of the actual air mass being ingested the
best way to determine accurate fueling (we'll assume
steady-state, since acceleration is a whole other can
of worms).  If you factor in VE when making a fuel
map, why switch?  Is there something magic about 100%
VE?  How can someone inject a turbo, if that is the
case?

Am I correct in my assumption that a small-ranged MAP
sensor is more sensitive to small variations in MAP,
and you end up with the same kind of compromise as
when you size injectors (small injectors are better at
controlling fueling at small amounts, but run out of
ability to deliver fueling sooner than large
injectors)?  Would that be the reason?

And what about alpha-n?  Is that a bad way of doing
things at low rpm, compared to the problems of
speed-density?

> Basically you'll have a bunch of bins that are
> 100Kpa vs RPM.  They can then safely assume that
> when they max MAP, they can go to TPS vs RPM, as
> this has more resolution at that point. 

I see.  Is it useful resolution, though?  Do small
differences in TPS really reflect well what the engine
is doing, as opposed to speed-density setups?  I would
think that at higher rpms and wider throttle
positions, a good measure of the actual airflow could
be made using MAP (and very responsive, too), which
should in theory be more accurate than alpha-n.  What
am I missing in my mental picture?

> If you hit 100Kpa at say 7000 RPM and the motor
> redlines at 14,000, you've got a span of 7000 RPM
> where the TPS goes from possibly 1/4 open to full
> open.  So it makes sense to use TPS instead of
> MAP in that instance.

I guess I'm still missing why MAP has to be limited to
100Kpa, or any other arbitrary number.

> Alpha-N is used where a MAP signal is not reliable
> or has too little resolution.

And from my own observations, it would seem that
there's very little useful data in an airbox-mounted
MAP at low airflows (small throttle openings and low
rpms).  Thus my feeling that alpha-n would be the
choice.

> The primary reason people use Alpha-N is on big
> cammed motors that have very little vacuum and their
> vacuum signal bounces at idle.

Motorcycles sure meet that specification!  :D  They
are cammed like racing car motors much of the time,
and with the advent of FI, they have been taking to
more and more aggressive cams.  Variable valve timing
is not really here yet (a few exceptions to the
contrary).

> 5 inches of vacuum isn't alot of resolution when you
> consider 30 inches is full scale.  Hence here's
> Alpha-N again.

It would make a lot more sense to read actual manifold
vacuum in situations like this, but motorcycles read
it off the inside of the airbox, before the throttles.
 It looks pretty scary on an oscilloscope at idle, the
output of that sensor, with the airbox being a series
of Helmholtz resonators with varying restrictions
along the path...  etc.

> Now, bikes need to stay small.  But MAF would likely
> be the solution to both problems.  MAF may be "slow"
> to respond (the air column must accelerate after the
> throttles open, but with generous accel enrichment
> you have less problems.

I suppose the question becomes, how would you set up
an accel enrichment that would be effective?  that
itself would be pretty complex, I think.

Bike manufacturers like cheap and simple.  They go for
the least effort-intensive solution possible for
production bikes, so it might be easier to design,
tune and get to market a "lesser" system.

> MAF would eliminate the resolution issue on bike
> engines, since you have an accurate measure of air
> at all times. 

Except at or near idle on engines with generous
overlap (and I think even modest cam numbers for bikes
would make a lot of car people blush).  But I agree,
at higher airflows, it should be a nearly ideal
solution in a lot of ways.  The trick would be making
one big enough to fit in the fairly cavernous intake
tracting, as well as the funny shapes most
manufacturers choose.  ;)

> You need to make sure no reversion is taking place
> due to intake valve pulsations.

You mean at or near idle, with overlap?  I believe
that's a major problem with bikes, from the scope
traces I've watched on the airbox.

> You need an intake that is tuned to exploit the
> large pulsations.

If I'm not mistaken, though, that tends to run counter
to the objective of tuning the intake to make best
ram-air/top-end/high-flow power.

Thanks for your insightful reply.  I look forward to
learning more!  :D

=====
| 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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