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

[Perry Harrington]

> 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.

If you take the MAP reading in sync with the tach signal
every time you read it, the signal is consistent with
that point in time.

> 
> > 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?

Speed density is based on the air pressure in the intake
being the same as the pressure on the back of the intake
valve.  Once you hit atmospheric, which usually happens
at 100%VE, you have run out of that component.  Thus
it merely becomes a table where you are reading RPM and
looking up a fueling amount for that RPM.

With Alpha-N, you at least have a second component when
the MAP runs out of resolution.

> 
> 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?

I wouldn't be able to answer that from experience, but I
can make a hypothesis.  Since NA sensors are neccessarily
higher resolution (from a 0-5v perspective), it would
stand to reason that they have a higher absolute resolution.

If sensitivity is considered the same thing as resolution,
then you're right.

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

Alpha-N is purely a mapped system based on observation of
an actual engine on a Dyno.  MAP based systems allow the
controller to fuel the engine without running the motor
on a dyno and exactly tuning each load site.

Alpha-N is basically lower than MAP in terms of data
inferred from sensors.  MAP is lower than MAF, as MAF tells
you actual mass, whereas MAP infers it.

> 
> > 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?

I believe that above 100Kpa they are inferring mass airflow
from those throttle angles.  It only makes sense.

> 
> > 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.

In NA applications you are maxed out at ~100Kpa.  The
atmosphere is 100Kpa, and thuse your highest MAP reading
on NA motors.  Forced induction is a whole 'nother enchilada.

> 
> > 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.

That depends on the manuf.  All the Honda EFI stuff I've seen
has a vacuum tree tied to each TB below the plate.

> 
> > 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.

No, it's simpler than you think.  It goes to the old tuning
adage of "give it what it wants".  I have just recently
started messing with accel enrichment on my 5.0 motor with
the stock computer.  It's MAF and was most definitely suffering
from lean enrichment, so I fattened it up.  Right now I'm running
about 3x the enrichment at low throttle angles that the stock
setup was running.  I also have a TB that is 67% larger than
stock, a MAF sensor that is 72% larger, and the motor makes ~44%
more HP and ~25% more torque, all within 4CID displacement.

> 
> 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.  ;)

MAF should work equally well at all points in the engine
output.  You merely need a MAF with enough dynamic range.

> 
> > 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 only have to ensure it doesnt go back through the MAF.
If you place the MAF at the actual opening for the intake
ducting it'd go a long way .

> 
> > 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.

Nope, I'm merely saying that you don't want your pulsations
leaving the plenum.  If they do, you need to increase your
intake tubing length until the MAF is undisturbed.  Putting
large plenums before and after the MAF helps with this too.

> 
> Thanks for your insightful reply.  I look forward to
> learning more!  :D
> 
> | Adam Wade                       1990 Kwak Zephyr 550 (Daphne) |

--Perry

-- 
Perry Harrington			Data Acquisition & Instrumentation, Inc	
perry at dainst dot com					 http://www.dainst.com/

Those who would give up essential liberty to purchase a little temporary safety
deserve neither liberty or safety. Nor, are they likely to end up with either.
                             -- Benjamin Franklin

_______________________________________________
Diy_efi mailing list
Diy_efi at diy-efi.org
http://www.diy-efi.org/mailman/listinfo/diy_efi