AFR: air/fuel ratio. The AFR lookup table is where the PCM looks to decide
on a base injector pulsewidth when the PCM is in open loop. If you react
automotive gasoline and air in a laboratory, you'll find that the reaction uses up
14.7 units of air for each unit of gasoline. So the "stoichiometric" ratio
of air to fuel is 14.7 to 1, which is sometimes written as 14.7:1, and typically abbreviated as 14.7.
Note that this is the ratio of masses (which is the same as weight if you're not a
physics purist) of air and fuel, not the volume. Fuel density (the weight of
a particular volume, say 1 gallon) does not vary very much (IOW, it cannot be
easily squished), but air can easily be compressed,
so its density can vary a lot. This is why the MAF, or mass air flow sensor,
is so cool. <soapbox> Unlike speed density (SD) systems, which must estimate
the mass air flow based on MAP and IAT, the MAF measures the mass of the incoming air
directly, which gives it a much better chance of calculating the right pulsewidth than
the SD system. </soapbox>
A.K.A: also known as
BLM: The block learn
multiplier, aka long term fuel trim,
is a value the PCM stores to adjust the injector pulsewidth based on accumulated
(or "learned") oxygen sensor feedback.
BLM's usually change fairly slowly, if at all. The PCM limits the range of BLM from 108 to
160. Here's a frequent source of confusion: A BLM of 128 means that the PCM is calculating the right injector pulsewidth.
A BLM below 128 means that the PCM has to take out fuel to get the mixture right (the PCM calibration is too rich).
A BLM above 128 means that the PCM has to add fuel to get the mixture right (the PCM calibration is too lean).
One way to remember which is rich and which is lean requires that you can remember that 128 is ideal.
You get numbers less than 128 by subtracting - when the BLM is less than 128, the PCM is subtracting fuel.
You get numbers above 128 by adding - when the BLM is above 128 the PCM is adding fuel.
Another misconception: In
general there is nothing wrong with the BLMs not being 128. As long as
they don't reach their limits of 108 or 160, the PCM will be able to get the
fuel mixture just right. The exception to this is covered on the Split
BLM page. All that said, many of us like to tinker with the PCM until
we gat the BLMs as close to 128 as we can, just cuz...
CFM: cubic feet per
is a common way of measuring the amount (volume, not mass) of air the engine is inhaling.
ECT: engine coolant temperature.
Hot engines have different fuel and timing needs than cold engines, so the PCM needs to know.
INT: integrator, aka short term fuel trim,
is a value the PCM uses to adjust the injector pulsewidth based on the current oxygen sensor feedback.
INTs usually change very rapidly, as in many times per second. Here's that
same source of confusion mentioned in the BLM section : an INT of 128 means that the PCM is calculating the right injector pulsewidth for the current conditions.
An INT below 128 means that the PCM has to take out fuel to get the mixture right (the PCM calibration is too rich).
An INT above 128 means that the PCM has to add fuel to get the mixture right (the PCM calibration is too lean).
See BLM for a trick to remember which is rich and which is lean.
IAC: idle air
control. The PCM uses this gizmo (a stepper motor that precisely opens and closes
the idle air control valve, which is located in the bottom of the throttle body) to control the idle speed of the engine.
The control range is from 0 (?) (essentially closed, slower idle) to 160 (as open as it gets - faster idle)
IAT: intake air
temperature. This sensor, which measures the temperature of the air the
engine is about to inhale, is located in the elbow that attaches to the throttle body, unless you moved it with an IAT relocation kit.
If the MAF is broken, the PCM needs a backup method to measure how much air the engine is inhaling.
This backup plan, called Speed Density, uses RPM, MAP, and IAT to estimate air flow.
The IAT is also used to control ignition advance. Cold inlet air helps the
engine resist detonation (a Bad Thing), so the engine can tolerate a bit more
advance. Very hot inlet air is more
likely to cause the engine to detonate, so less advance must be run to avoid
that destructive detonation.
MAF: mass air
flow. This sensor, located somewhere between the air filter and the throttle body, measures how much air
(by mass, which is essentially the same as weight, not by volume) the engine is inhaling. The PCM needs to know this so it can add the right amount of fuel.
The MAF is considered to be an unnecessary restriction by those who prefer speed density fuel
control See the AFR definition for my spiel on this topic.
MAP: manifold absolute
pressure. This sensor, attached to the top of the LT1/4 intake manifold right behind the throttle body, gives a pretty good indication of the load on the
engine, and therefore the power it is putting out.
Stabbing the throttle in neutral (almost no load) will produce a very different MAP than stabbing the throttle in high gear (heavy load).
Similarly, cruising at a steady 40 mph on level ground results in a much different MAP than burying the throttle pedal.
There is a MAP-to-vacuum-gauge conversion table on the Units
page. If you are used to vacuum gauges, look at that chart or else you
will have everything exactly backwards!
N/A: normally aspirated, which
means no blower (supercharger), turbo (turbocharger), or nitrous. These 3
things are known collectively as "power adders".
Diagnostics-One is the name given to the first generation of engine control
computers containing software that tests sensors and control functions (like
EGR) for correct operation. Originally intended to satisfy emission laws,
some of the tests can help the hot rodder debug problems.
This tutorial only covers only OBD-I
PCMs. OBD-II showed up in f-body PCMs in '96, and brought with it some
new sources of pain. I think the basic fuel and timing control is the same
as the OBD-I PCM, but I'm not sure. However, I am sure that OBD-II added
tests for EVAP system function, added post-cat O2 sensors, and added a MAF limit
test that drove me nuts trying to get my supercharged engine to rev beyond 5000.
Here's what I found
- when the MAF signal exceeded 300 for more than 1 second, the PCM would decide
that the MAF was toast and would switch to speed-density (the "MAF
limit"). But in speed-density mode, the PCM does not handle engines
with blowers or turbos because the MAP sensor only goes to 1 bar (which is
atmospheric pressure, about 14.7 psi). Remember that MAP is more-or-less a
measure of power. Blown cars easily exceed 1 bar of manifold pressure,
which means that they are inhaling a lot more air than the PCM
realizes. This, in turn, means that the PCM is not going to supply enough
fuel for all that air.
With the original blown engine, I was
hitting the MAF limit at about 5000rpm. Right at that transition from
MAF-mode to speed-density mode, the injector pulsewidths would drop in
half. Naturally, the engine would fall on its face. The half-size
pulses would not even support combustion, so it was like hitting the rev
limiter. My solution was to convert to an OBDI PCM, which completely
solved the problem.
O2: O2 is the chemical symbol for oxygen,
but in PCM-land it refers to the signal from the oxygen sensors.
In theory, if the engine got exactly the right amount of fuel for the air that it inhaled, and if the engine managed to burn every last molecule of fuel, the O2 sensors would read 450 mV.
A rich mixture results in O2 numbers above 450 mV, and a lean mixture produces O2's below 450 mV.
The PCM watches the signal from the O2 sensors to find out if the engine is currently running rich,
or lean, or 14.7 exactly.
Another source of confusion: Some folks are surprised when they watch the O2 sensor signals with a scan tool during closed loop operation.
During open loop, the O2 sensor signals are fairly steady, but in closed loop they jump around wildly.
This is normal. The PCM can't maintain a steady 14.7 mixture in all the cylinders all the time, so it settles for an average of 14.7.
If your O2 sensors are healthy, they should be swinging back and forth passed 450 mV, spending
roughly an equal amount of time above and below it.
Another O2-related issue - it turns
out that the factory sensors are hopelessly inaccurate when the mixture gets too
far from 14.7. Where this hurts the tuner is at WOT, when the mixture is
probably around 12.5. You may have heard that tuning WOT for the
"upper 800s" or "low 900s" will give the best power, but
there are some guys out there with data showing that our sensors aren't reliable
when they're that far from 450. The safest approach is to tune on a dyno
that uses wide band O2 sensors, which are accurate for mixtures
PE: power enrichment.
The PE tables are used when the PCM you lean on the throttle pretty hard. While 14.7 is the chemically-correct AFR, engines
can make more power when they are run slightly rich, say 12.5. Cold
engines are also happier when they are richer than 14.7:1. The PE tables
are used to add the extra fuel when the engine
needs to make some serious power.
RPM: revolutions per
minute is the measure of how fast the engine is spinning.
TDC: top dead
center. TDC is the position of the piston when it has stopped going up
inside the cylinder, but hasn't yet started to go back down. Note that a
piston in a 4-stroke engine like the LT1/4 goes through 2 TDCs per combustion cycle.
The first TDC occurs between the compression stroke and the power stroke, and the second occurs between the exhaust and intake strokes.
If you're setting valve lash, you want the first TDC because that is when both valves are sure to be closed.
Both valves are slightly open at the other TDC (referred to as overlap).
TPS: throttle position
sensor. This sensor tells the PCM how far you've mashed the accelerator pedal.
0% is idle, 100% is full throttle. Note that the PCM is pretty tolerant of the actual TPS sensor output voltages.
The PCM will treat any closed-throttle TPS voltage below 0.9?V as 0%, and any
TPS voltage above 4.0?V as 100%. If you're having idle problems, one of
the first things to check with the scanner is to make sure that the idle TPS is really 0%.
efficiency. In engine theory, volumetric efficiency is a measure of how much air the engine is inhaling at any particular rpm compared to how much it can
Remembering that a complete cycle takes 2 full revolutions (and so it is filled
with air every-other revolution), a 350 cubic inch engine spinning at 6000 rpm can inhale 350 * 3000 = 1,050,000 cubic inches of air per minute, which works out to a little more than 600 CFM.
If the engine can only inhale 500 CFM at 6000 rpm (because of restrictions in
the air path), its "textbook" VE = 500 / 600 = 0.80 = 80%.
Note that blower and turbo engine volumetric efficiencies can go way over 100%, which exactly why they can make so much more power than a normally aspirated engine.
This happens because the blower or turbo is pumping more air into the engine
that it could inhale on its own. Think about your lungs, and what would
happed if you stuck a compressed air hose in your mouth and opened the valve!
WOT: wide open throttle, the only tuning condition that newbies seem to be interested in.