<snip>

TO THE POINT:  In the many helpful discussions on the MegaSquirt list,
one was how to "smooth out" the intake pressure pulses to avoid spurious
MAP readings at various RPMs.

Intuition leads you to implement a "surge tank" solution, where rapid
changes in pressure are "averaged out".  Turns out this is a bad idea,
due to opposing goals:
1) Larger surge tank = slower system response to throttle changes.
2) Smaller surge tank = inadequate "averaging" of pressure pulses.

#1 is the REAL problem, though.  A real long or fat vacuum line counts
as a surge tank, too.

It seems the best solution discussed is to put a restrictor in the
vacuum line to the MAP sensor.  A metal vacuum hose coupler drilled to a
specific size seems to be popular.  The only "surge tank" left is the
unavoidable minimal volume of the MAP vacuum line.  (The restrictor can
be at the manifold, the MAP sensor, or anywhere in between.)  The
restrictor effectively controls maximum rates of pressure change in the
MAP sensor side of the vacuum tube.  Drill to size to get your desired
response rate.

*******************************\__/*******************************
Manifold                        __                      MAP Sensor
*******************************/  \*******************************
                            Restrictor

This *is* RPM dependent.  The MAP tables are ultimately responsible for
factoring the fuel adjustments in.  We just want to eliminate
problematic MAP-sample-timing issues.

Simple.  Reliable.  Cheap.  Lightweight.  Reproducable.

What's not to like?

BTW - "taking many samples and averaging the data" is like adding a
surge tank.  Tracy probably wants to maintain system response.  (I'm
just guessing here...)

Vince Orton

----------------------------------------------------------
What about building and installing an integrator circuit using an op-amp and
place it between the MAP sensor and Tracy's pickup?
Del Johnson

------------------------------------------------------------------
Given the pulse characteristics of a tuned intake manifold I am now not
so sure you want to average the pressure with a accumulator or
restriction in the vacuum line. See the attached file.

An integration of the pressure on the other hand over the time the
port is open (IO to IC) sounds intriguing. Right now Tracy is sampling
the pressure just as the Exhaust Opens (EO) and the intake pressure
is dropping rapidly. Integration could be done by taking ten samples equally
spread over the time the intake port is open, multiplying each sample
by a time constant and summing the individual samples in software.
In other words a crude integration process.

Great idea Del. Thanks for thinking of integration. Hard to do with
just an op amp but it could be done using a gate. Easy to do with
a pressure sensor and a cheap ($1.00) single chip microcomputer
in software.  The output of the single chip would be a number
proportional to the area under the curve when the intake port
was actually open. Sounds like a patentable device to me except
it is not as you read it on here first :)

Paul Lamar



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