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

[Adam Wade]

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

> 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.

Does not speed density require an assumption about VE
in any case?  Since all it does is measure the
velocity of the air into the cylinder by pressure
difference, there has to be a "VE table" built into
the fueling somewhere, yes?

Ideally, you would want to measure the MAP in the
throttled volume right before the intake valve opened,
if I have the hang of all this.

> Synchronous MAP sampling should go a long way to
> solving the "bouncing MAP" problem.

Yeah, that seems to be the ingenious yet logical
consensus.  ;)

> I think it would be trivial to add that to the
> MegaSquirt, so I'll look at it.

The trick is going to be finding the right point to
read the MAP, although I suspect that immediately
before intake valve opening would be a great place to
start.

>> 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.

Ahh, gotcha.  Assuming you read the throttled volume,
and read at intake valve opening if there are
fluctuations that read on the sensor, yes...

Now, what if you are reading the plenum, with the
throttle on either side?

> 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.

That would be my sucpicion as well.  I wonder if
anyone make a logarithmic MAP sensor, so you could
have high resolution at one end of the range, but more
overall range...  Or even if that is feasible.

> 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.

Which is, what, about 60-70% total possible airflow?

So the problem with alpha-n at slammer throttle
openings bcomes one of resolution?

> 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).

I was meaning tuning the AE.  Certainly you can dump
in a whole pile of fuel and the thing will jump, up to
the point that you cause it to misfire, or the fuel
doesn't atomize meaningfully.  What would be the best
way, though, to deliver an AE squirt that would give
close to best power all through the delta rpm?  Or
does it end up being a non-question in practice?

> 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.

Because fuel won't fall out of suspension very much if
intake velocity falls somewhat from opening the
throttle...  And the engine picks up speed quickly,
thus raising the velocity again.

> Case and point are CV slides used on pretty much
> every 4 cyl bike motor made in the last decade.

Well, those indicate there is still some issue left,
at least when fuel is being drawn up through jets by a
vacuum...  Essentially, the slide acts as a
self-limiting throttle, preventing the rider from
opening it fast enough to prevent fuel pickup.  This
is less of an issue with fuel injection, as the fuel
is under pressure in the rail.  However, there have
been a number of creative schemes in motorcycle fuel
injection to handle small throttle-opening
transitions...  Suzuki used stepper-motor secondary
butterflies, actuated by the EFI computer.  And
Yamaha, in a funny turn, opted to add CV slides onto
their throttle bodies!  :D  I wonder if Suzuki and
Yamaha perhaps are not using speed density at low
loads and engine speeds (although the ECU for the
Kawasaki and Suzuki sportbikes are very similar and
made by the same manufacturer...  That doesn't
necessarily mean anything, though).

> 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. 

*grins*  I can see where being on the lean side at
cruise might be useful from a number of standpoints. 
Fuel economy and emissions, to name a few.

On motorcycles, anyway, my experience has been that
you get better acceleration if you tune for best-power
(i.e., slightly rich) cruise.  Of course, that's not
necessarily happy for fuel mileage...

>>> 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.

Veeeeeery interesting...

> 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.

And yet very important.  A lot of sportbike guys are
yanking the resonators off their intakes.  I have to
wonder if they are taking away some intake air, and
thus richening the bike closer to best power from the
stock lean condition...

> 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.

So IOW, anywhere you can add more plenum area is going
to help a MAF, because it's going to make the airflow
more consistent and stable...

> 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.

That would be a tricky one, largely due to the shape
of the intakes.  Something like a Ducati 748/916/996
would be close to ideal.

> A MAF doesn't have to be round or huge, it just has
> to measure MAF.

True, but I imagine it would cost a bit to get one in
a really funky shape for a limited production run.  ;)

> 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).

An increasing number of bikes are going to a single
large intake.  It almost seems to make more sense to
use two smaller ones, and put a MAF on each one.

=====
| 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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