I went to some of the RX 7 groups online to find out the best way to
control FI.I am looking at Megasquirt as a plan B or C if the 555 analog
system does not work out.I guess there is manifold pressure which is not
stable on a P Port. Ther is Mass air flow which is being proposed for the 555
system and is supported by Megasquirt. The RX 7 guys seem to think the MAF
sensor could impead the air flow to make big horse power.200 to 250 HP does
not seem to be big HP when they are getting a lot more out by turning the
engine faster.How does TPS or throttle position sensor fit into
this? Thanks for any help

Joe Berki Limo EZ 13b P Port

As long as the mass
airflow sensor is around 5.5 square inches it should be fine.That is
something about 2.625 or 2.75 inches in diameter will give around 250 HP.
The throttle position sensor will not work with the 555 system not is it
needed.

Paul Lamar

How would the mass flow sensor work with a turbo charger on the front
end? I Think that the MAF sensor would be calibrated for ambient
pressure N/A applications.

Doug in Japan

The 555 system has a provision for scaling the airflow signal.
The gain of the op amp can be adjusted.
It responds to mass flow only and not pressure per se.

Paul Lamar


 How will the MAF signal be steady in comparison to the manifold pressure?
 Where is the best place for the MAF sensor after the air filter?
 Joe

 The mass airflow sensor is a hot wire basically so it heats up
 and cools down rather slowly. It responds to the average flow unlike
 the manifold pressure sensor that changes in one one thousands of
 a second.

 The best place for it is after the filter ahead of the plenum
 so it measures all the air coming into the engine. Here is a 3D
 showing the old style 2nd gen trap door mass airflow sensor.
 The newer mass air flow sensors are mostly just a large tube
 about 3 inches in diameter and several inches long.
 The Ford Escort shown is a bit small.
 The air filter is not shown yet.

 Paul Lamar

How big is the plenum? Does that affect the runner length? Can injectors go
anywhere? Assume 4 and how would they be staged or would they be staged?

Joe

I would make it as large as possible.
Something about this size as shown in the attachments seems to fit
under most cowls. The exact shape is not important. Keep the volume up.
If the volume is too small it may act as a Helmholtz resonator.
Not good if it is tuned wrong. It may make the runner length
chosen so far invalid.

http://www.phys.unsw.edu.au/jw/Helmholtz.html

The engine gets the best BSFC if the injectors are out near the start of
the tubes. Bell mouths are essential. That allows the fuel to mix with
the air better. However the injectors can go almost anywhere along the tubes.
Make sure the box is sealed as raw fuel can accumulate in the box.
It is best if it drains down and into the engine with small tubes.

In the 555 system staging is not necessary. The only thing you care
about at less than 15 inches of Hg manifold pressure (half power) is that the
engine runs reasonably smoothly. Fuel burn in that mode is hardly
important. In any case if you run the engine at constant throttle opening
and RPMs for long periods of time, at any manifold pressure or RPM, you
can manually lean the 555 system.

BTW the mass airflow sensor and the injectors can be replace with a
500 CFM Holly or other two barrel carb. It is mounted where the mass air
flow sensor is shown in these dwgs. You will need to lean the carb some how.

Paul Lamar