'MAP' and Fueling

[peter.betts at nokia.com]

Hi,

I'm new to the group and have a few (probably) simple questions. (Sorry it's
a long Email)

What is the relationship between MAP and MAF. RPM and Air Temp are factors
in converting the MAP sensor output to match the MAF. What I am trying to do
is understand what the differences between MAP and MAF are.

I have a Toyota Supra Twin Turbo and they come with both MAF and MAP sensor
systems for the export and Japanese spec cars. These cars have a fuel cut
which operates as soon as the manifold pressure exceeds a preset limit of
13psi. I have designed a voltage hold circuit which clamps the output of the
sensor at 13psi (about 4.3V) and hence fools the ECU of the car into
thinking it is running much lower boost from the turbos.

The problem is that I am unsure if this will affect the fueling
charateristcis of the car if I have modified the MAP input the ECU, above
the 13psi limit. I may be having fueling problems but am unsure what I may
have done.

Thanks for the reply from Ric Rainbolt regarding the issue. I have
faithfully added his Email below.

*****P.Betts***********
The US Surpa site say modifying the MAP sensor output with a fuel cut
controller is a bad idea for the Jap spec MAP sensor cars but I agree with
Rob below. As the Supra runs rich at high boost the inability of the ECU to
correctly calculate the fueling above fuel-cut, because one of the inputs is
static, is small up to a point.

The next question is at what point (manifold pressure) does the MkIV (and
MkIII for interest) start to lean out? This is probably why water injection
makes such a difference because the fuel that is available is cooler and
thus able to deliver power to the engine more efficiently. (More
effieciently burnt)
(Time to get a second oxygen sensor or monitor the current one)

So as a summary.....

The Jap spec Supra **CALCULATES** the Mass Air Flow going into the engine
using the parameters described below, one being the Manifold Air Pressure
(MAP).
 
The Export (UK) Supra directly senses the MAF using some form of
butteryfly/flap type arrangement (I think) and uses this for fueling
calculations along with RPM (I suppose) The MAP sensor on the Export models
is only used to detect engine vacuum and pressure and doesn't directly
affect fueling.

Thoughts/Corrections anyone? Any guidance greatfully received.

Thanks
Pete


*****R.Rainbolt*************************************************
The fuel cut is used on both the Turbo Supra and Turbo MR2.  Since both 
cars run relatively rich at full (stock) boost, it is "safe" to use such a 
circuit to prevent the computer from hitting the fuel cut function, but 
only up to a certain amount of boost.  For the MR2, the FCD cuts in at 11 
PSI, but it is safe to run the car up to about 15-16 PSI, without fuel 
system mods.

To be safe, the best thing you can do is get an extra Oxygen Sensor 
installed and connected to a dash meter (HKS, Summit, Greddy, etc.)  If the 
FCD is set to 13 PSI, I would not go over 16-17 PSI. Make sure the car 
never turns from rich to lean under full power.

MAP (Manifold Air Pressure) is directly related to MAF (Manifold Air Flow). 
 The amount of air moving into an engine is refferred to as Mass Air Flow 
(MAF).  MAF is a function of 3 major factors (ignoring temp and humidity 
for a moment).  They are: Throttle Position (TPS), Engine Speed (RPM) and 
Volumetric Efficiency (VE).  VE is a complex component, being different 
values at different RPM and TPS levels.  Systems that only use TPS and MAP 
are referred to as Speed-Density (SD) systems.  They "approximate" the 
amount of air the engine is using computing MAF from the three above 
mentioned components.

Toyota and other manufacturers use MAF and MAP sensors, even though it is 
not absolutely necessary to have both. Either will work, as long as TPS and 
other factors are known.  On the MR2 and Supra, HKS sells a kit that 
actually lets you eliminate the MAF sensor.  The idea is to allow for 
better intake breathing and better throttle response.

Ric Rainbolt

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