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

Adam Wade espresso_doppio at yahoo.com
Fri Jan 31 14:02:30 GMT 2003


--- Marcell Gal <cell at x-dsl.hu> wrote:

>> The MORE difference bewteen ambient and MAP at a
>> given rpm, the LESS power the engine is making.

> nope. The less the MAP at a given rpm, the less
> power the the engine is making (less airflow to it,
> to be exact).

Assuming full throttle, a lower MAP would indicate a
higher velocity, or more air consumption per unit
time.  So at WOT, lower MAP = higher VE, like in your
formula (IIRC).

When dealing with a constant rpm, but changing the
throttle opening, the lower the MAP, the more closed
the throttle is, to the LOWER the VE would be.  So
closing the throttle at high rpm, for instance,
creates a VERY low MAP, which in this case means not
high intake velocity (and great power), but LOW VE.

> The MORE difference between ambient and MAP at a
> given rpm, the MORE air is flowing through the
> restrictions from ambient to the manifold OR the
> restrictions are greater (throttle closed, air
> filter dirty).

Yes.  In other words, a low absolute MAP either means
a lot of airflow, or very little airflow.  And unless
you know the throttle position, you can't tell which
from just the MAP reading...

> Now you can use your favorite SPICE clone (gnucap?)
> to simulate an engine ;-)

It was long, long ago when I realized that intake and
exhaust tract tuning was simply a bunch of L/C/R
circuits made of gases instead of electrons.  They
behave exactly the same.

>> 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

> gr8. Please share the logs if possible.

It will probably be a while, but I will share the logs
with anyone who thinks they might be useful.  I'm more
interested in sharing the concrete deductions and
results we get.

<knock sensor>

> The question is how can I collect those useful
> samples without destroying my engine. Maybe just
> drive 1 or 2 combustions to knock with advanced ign.
> (or a few lost water injections at high boost etc..)
> so that the engine survives the logging.

I realize you took my previous answer as being about
knock sensing instead of MAP readings.  My apologies
for the misunderstanding!

As far as testing knock sensing, and doing data
acquisition, I would think that to truly get an
effective picture of what you are "sensing" and how to
find its onset, you would have to be willing to
sacrifice an engine to the "greater good".  :D

> How fast does non-audible knock wear the engine?

A lot depends on the design of the combustion chamber.
 Silent knock doesn't seem to destroy engines very
often, from what I understand, but it can do
considerable damage to piston tops.  Anyone who has
done or read more concrete research please step in here.

=====
| 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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