2 pessure sensors ?

Phil Lamovie injec at ains.net.au
Tue Aug 10 18:42:28 GMT 1999

Hi All,

John wrote;

> Other vehicles with MAP use some sort of ambient air pressure
sensor.  I
> even dissected an after market fuel injection box and sure
enough,  they
> had two Motorola Pressure transducers inside: one for ambient
and one
> for MAP.

Please don't even mention that device. There has to be a good
why an absolute sensor with integral nitrogen reference is made.

Only one I can think of is so you can map engine vs absolute

Then if you fly or dive the fuel is corrected as the manifold
pressure is
always correct. if the full throttle vacuum is 30 kpa you could
use the measurement to check your altimeter or if it read boost
you would be holidaying by the dead sea.

If you reference all calculations to absolute values (kelvin kPa
you can correct with confidence.

How does the second pressure sensor (if they are both
differential ) know any better than the first.

Lack of first principals gives rise to many "interesting
products" but you wouldn't want to drive one.

You will find that the only pressure the ECU needs to concern
itself with is
the plenum pressure. This is proportionally linked to air mass
that is being CURRENTLY ingested. If you correct for absolute air
temp, absolute rpm and absolute pressure the job's done.

You could get creative and correct for fuel temp as was done 50
years ago
by Kugelfischer et al. This first principle was "lost" with the
advent of bad quality electronic fuel injection (read bosch D
Jetronic) and no body does it anymore except for a small
australian firm of no repute whatsoever and even then only for
Liquid LPG Injection where it  matters. more than the
 5 % that is does for petrol.

Remember all current pollution test are conducted after 24 hour
heat soak
at 24 deg C . means they can save money by not fitting a fuel
temp sensor
(the environment is not a customer of GM or Ford and thus doesn't

Page 188 of you guessed it gives;

Properties of Liquids/Gasoline(IC-Engine))

density = 0.72 - 0.75  g/cm3
melting point = -50  to -30 deg C
boiling point = 25 - 210 deg C (most be some heavy fractions in
thermal conductivity = 0.13 W/(m-K)
specific heat = 2.02 kJ/kg-K
volume expansion coefficient = 1.0 x 10-3/K

the last is a thought provoker isn't it ?

As the volume of fuel is a function of pressure diff across the
which should be a constant as the regulator is referenced to the
then the only variable is ms of injection time. If you know the
amount of air in mass  and add an appropriate ms value of  fuel
from a dyno load generated map then the vapor pressure is piffle
(is it ok to use strong words on this list ?)

> The number used in Grippo's document suggests 3.1 Kpa but
that's at >85F.  What would it be at -40F.

And when would it make a difference ? What does the engine think
50 kpa of vacuum  at sea level and 50 kpa of vacuum at 2000
ft/ASL and are the fueling requirements not met by the original

The engine has no mechanism for differentiating between a partial
sub atmospheric plenum at sea level  and a WOT at 10,000 feet
which also results in a sub atmospheric plenum.

The only thing that matters is the values in the table. They must
be derived
as absolutely as everything else. And dyno work is very absolute

your in obfuscation


 Phil Lamovie

 injec at ains.net.au

     cogito ergo zoom

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