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Not exactly. Think of things in absolute pressure terms. We are walking
around in 15 psi pressure at sea level. That is 30 inches of mercury using
a manifold pressure gage. Our body would be crushed if it were not for
the same pressure being in our lungs.
Now what happens in the intake manifold due to an organ pipe tuning
effect is the PEAK pressure pulse may go up to 200% of
15 psi or 30 psi (60 inches of Hg). 100% more than wide open throttle.
When the port is closed at the RPM tuning point the pressure also goes
below 15 psi (30 inches of Hg) for awhile. Perhaps all the way to zero
pressure. The average is still 15 psi (30 inches of Hg). Remember
the throttle is still wide open. The key word is "PEAK". What we are
interested in is the amount or mass of air that flows into the engine.
That is proportional to the area under the peak plus the
the air flowing into the engine due to atmospheric pressure alone.
If the shape of the pulse were a right triangle the area would
be 1/2 base times height or 7.5 psi so to speak. That would
be 50 percent more air or the equivalent to 22.5 psi (45 inches of Hg).
In the real world we don't really see that much air. What we see is
about 20% more or the equivalent to 18 psi (36 inches of of Hg).
Using the term vacuum confuses the issue. A pure vacuum is just the
absence of pressure and what one would get in outer space.
BTW pressure at sea level expressed in psi is merely the weight of
a long, one by one inch, column of air up to the start of space
expressed in pounds. In other words the column weighs 15 pounds.
I don't mean to be condescending here I just don't know the level of
Physics education of every one on here. I am trying to explain things
from the most basic level possible.
I DO NOT disagree with you on merely changing the map table entry to
correct for this effect. It would be better however if we did not have
to do that 128 times. One for each .12 psi increment in the intake
manifold pressure. It would be better if the computer would
set the mixture on the basis of the actual mass of air entering
the engine at any RPM. That is exactly what a mass air flow sensor
does in an automotive EFI system or a carburetor does for that matter.
I think Tracy could do it by taking five or ten equally spaced pressure
samples during the time the port is actually open and integrating them.
Then add or subtract that number from the atmospheric pressure in the
intake manifold. The computer is fast enough to do this no problem. It
can probably get a sample in a few millionths of a second since it runs
at 33 million instructions per second more or less. I am sure it is now
twiddly its thumbs most of the time.
To Tracy credit there is an auto tuning mode where the
computer looks at the O2 mixture monitor installed in the exhaust
pipe and adjust the MAP table entries accordingly. I have not tried
that yet. That might solve the problem. I can see a scenario when
descending where that might not work. If it were me I would
also look at the EGT to get a better feel for what the
engine was doing.
Paul Lamar
Wish I had time to discuss this at length but alas, I don't. Working
on other stuff like stand-alone auto-tune, knock sensor handling and
have to actually build this stuff : )
The EC2 was designed with a plenum manifold design but FWIW, it would
work in this scenario just fine if the sample were taken at the peak
pressure point. Just need to know exactly what time it occurs. Will be
interested in the results of your tests if it happens. Knowing the
absolute value of peak would help to set the initial flow rate in Mode 3
but not a 'must have' item.
Tracy (replacing a failed LS1 coil on my plane today)
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Thanks for your input on this Tracy. The problem with two P-port
pressure taps "Teed" together like we are using now is the runner pressure
pulse data is 180 out of phase from runner one to runner two
thereby canceling some of the pressure.
I was just thinking about using two pressure sensors one on each
runner and an auxiliary PIC to process the info and send you corrected
pressure data when you need it.
So far I have not figured out how to do the second part.
..... Wait a minute.... I just got it!!! Use the first
RX7 gen vac solenoid valves to switch the hoses in time with the RPM.
That might work. I could condition and amplify your ignition
pulse to switch hose connections. Forget the aux PIC.
I'll have to check the speed of those valves. I have plenty
lying around.
The ultimate p-port solution is two separate pressure sensors. One for
runner one and one for runner two. Mount the sensors on
the manifold and run wires instead of hose for quicker response.
Day later ----------------------------------------------------------
I reworked the Mazda illustration. It now shows the intake and exhaust
curves for two rotors in sequence. The large blue dots represent the pressure
samples Tracy is taking currently. This is for the manifold on
song so to speak. At other RPMs other than the runner resonant condition
the curves will look different. Consequently so too will the manifold
pressure sample values be different. As I see it we have two problems
with the p-port
A: if two pressure taps are used with one pressure sensor and teed
together there will be some cancellation of the pressure signal.
B: if a true representation of the air going into the engine is
to be had several equally spaced samples should be taken while
the intake port is actually open and added together.
This in effect will give a number that is the integration of the
area under the pressure curve. This in turn should give a number
that is proportional to the actual amount of air going into the
engine.
C: it is possible only one pressure tap in one runner is needed
but it needs to be sampled in the right place on the curve. Namely while
the intake port is open.
BTW I checked the speed yesterday of the first gen vac air solenoid
valves and they are good up to 100 cycles per second square wave while the
engine is making 125 revolutions per second at 7500 RPM. However
they are not designed for that kind of abuse and may soon fail.
I am now thinking of un soldering Tracy's pressure sensors, moving
them over to the runners and electronically multiplexing the data
back into Tracy's computer.
Paul Lamar
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