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

[Perry Harrington]

> And on an inline-four, where you get an intake event
> every 180 degrees of crank rotation...  *light dawns*
> 
> At lower rpms, the sample rate isn't even very high.
> 
> *why do you 'need' to switch from speed-density to
> alpha-n when VE goes over 100%?*

The question comes down to what type of MAP sensor you
have and where you take the MAP reading.

If you have a MAP sensor that reads > 1BAR absolute, then
you could do SD above 100% VE.  Also, at > 100% VE, where
is your high pressure point?  I'd presume that the
intake tract prior to the Cylinder is your high pressure
point and thus that's where you'd need to put the MAP
sensor pickup.

Syncronous MAP sampling should go a long way to solving
the "bouncing MAP" problem.  I think it would be trivial
to add that to the MegaSquirt, so I'll look at it.

> 
> > 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.
> 
> Doesn't that hold true for any speed density reading? 
> You have to know the VE or the intake pressure doesn't
> mean a lot, yes?

Correct.  Basically 100Kpa = 100% VE.  If you run an
engine at WOT and record the MAP values for a given
RPM, you'll get the VE curve of the motor.  You can
then take that info and plug it into a program like my
VE fitter for the MS and get a VE table that is linearly
scaled by MAP values.

> 
> > With Alpha-N, you at least have a second component
> > when the MAP runs out of resolution.
> 
> I would think that would more be a function of
> properly "sizing" the sensor, or is some lighbulb
> still not screwed in all the way here?

That's true.  If you peg the MAP sensor then you're stuck
with A-N.  I would assume that Kwak does it that way so
they get a high res MAP signal at the driveability portion
of the range and just run WOT A-N for the high end.

> 
> > 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.
> 
> Yep, that much I had pretty much gotten, but I was
> under the impression that some ranges of operation
> were more "reasonable" for one system or another.

All depends on the resolution of the application.

> 
> > I believe that above 100Kpa they are inferring mass
> > airflow from those throttle angles.  It only makes
> > sense.
> 
> Let's compare a situation where the engine is making
> the same amount of power, and thus the vehicle is
> running at the same speed...  In once case, at a
> higher rpm but with a smaller throttle opening; in the
> other, a lower rpm at a greater throttle opening.
> 
> The VE of the engine with the smaller throttle opening
> is likely going to be lower (even taking into account
> cam setup), yes?  So to be making the same power with
> a higher VE, there's a higher air velocity, and thus a
> lower pressure, yes?
> 
> Since the VE of the engine changes based on throttle
> position, where's the compensation for differences in
> actual air mass ingested vs. MAP (assuming you are
> reading the MAP from the airbox, as a goodly number of
> the vehicles I am thinking of actually use)?

You've pointed out the obvious deficiency of A-N, load
sensing.  But in the situation you've described, load
sensing isn't important because you are at WOT.  Tuning
for WOT is trivial in comparison to part throttle with
varying load.

At WOT and VE >=100%, you can think of the power output
as a simple relationship between throttle angle and RPM.

Note that in EFI terms, WOT is usually >= 30% throttle.

> 
> > That depends on the manuf.  All the Honda EFI stuff
> > I've seen has a vacuum tree tied to each TB below
> > the plate.
> 
> I unfortunately haven't had a lot of chance to look at
> Honda stuff yet.  I've spent most of my time with
> Kawasaki and Suzuki, which do not.  I'm interested to
> see how all the different manufacturers do it.
> 
> *accel enrichment*
> 
> > 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.
> 
> That's pretty respectable.  I suppose my question is
> then, how important is best power for anyone creating
> these settings?  It's certainly possible to make it

Well, I can tell you that no-one in the 5.0 world does
a thing to their motor for "drivability", it's always
about more power.  I built the motor a year and a half
ago, and I'm still tuning it for drivability.  You can
always add more power later via forced induction or
artificial respiration (N2O).

> BETTER than it was, and definitely to make it more
> DRIVABLE.  I tend to think in terms of "If you're not
> using something capable of achieving best power, then
> why go in that direction?"  Certainly there are many
> ways of making a vehicle drivable when you mash the
> pedal (accelerator pumps on carbs are quite drivable),
> but many of them are going to introduce fuel that is
> not appropriate over the course of the acceleration
> event, or not in the best manner...  Again
> acceleration pumps come to mind, which clearly are not
> the most efficient way of delivering fuel for an
> acceleration event, and can obviously be improved
> upon.

There have been many discussions about AE on this list.  In
the end it all comes down to velocity.  Bike motors have
the potential for insanely high intake velocities.  Also,
they have a huge dynamic range of power.

Having the tb right next to the intake port like how bike
engines do it makes things a lot simpler in terms of AE.
Because the velocity is usually very high at all throttle
angles you don't need much AE, if at all.  Case and point
are CV slides used on pretty much every 4 cyl bike motor
made in the last decade.

My understanding about tuning is that to achieve best power
and response, tune it lean and throw lots of fuel at it
when you mash the loud pedal.  Cro-magnon, yes.  I've not
tuned any race motors, but I can tell when you get close
to right by listening.

> 
> > MAF should work equally well at all points in the
> > engine output.
> 
> I was more thinking of reversion issues.  They can be
> minimized with a large enough airbox, of course...  I
> wonder if airboxes that are designed to current
> thinking would be big enough to make MAF practical at
> low engine speeds on motorcycles, and maybe use the
> alpha-n to run JUST an acceleration enrichment map...
> 
> > 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.
> 
> I now wish I had an appropriate MAF and a sportbike
> sitting around to play with.  ;)
> 
> *intake exploiting large pulsations*
> 
> > 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. 
> 
> It's inevitable that SOME energy will leave that way. 
> But yes, you can reduce it to almost meaningless
> levels.
> 
> > Putting large plenums before and after the MAF helps
> > with this too.
> 
> Many manufacturers are fond of putting resonation
> chambers right inside the intake opening to lower
> intake noise levels...  I would think that in between
> that resonator and the main airbox would be a good
> spot.  It's also convenient physically.

I will tell you one thing.  I had a "cold air" intake on my
5.0 before and after the build.  Prior to the build it
would make more torque and flowed a bit better with the
stock heads and Explorer intake.  After doing the buildup
though, it was the pits.  There was no plenum area in front
of the MAF and I was totally unable to get a reasonable idle
from the motor for a long time.  It did run like a raped ape
on the highway, but driving through a parking lot was
near impossible.

So I did what most people wouldn't, I put the stock airbox
back on.

Boy, did that make a night and day difference.  Right away it
smoothed out the bottom end like nobody's business.  I estimate
it picked up at a *minimum*, 20lbft of torque and probably 5HP.
This is low RPM, the range where tuners won't touch.

The point being is that the extra plenum area the stock airbox
provided really helped out things.

This is also the principle behind those "power pipes" you may
have seen.  They look like overgrown intake pipes, but in
reality they add extra plenum area between the TB and the MAF.

AFA adapting MAF to a sportbike, they should have plenty of space
for one.  Consider a CBR 600 F3, it's ram air from below the fairing.

A MAF doesn't have to be round or huge, it just has to measure MAF.
If manufacturers were interested, they could make the MAF into the
air intake pipe.  Pre-filter I'm sure the MAF would need cleaning,
but it would be ideally located (eg, CBR 600 F4i dual snorkels).

> | 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

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