I know many folks on this board understand quite well
what Alpha-N is, but this is for those who don't understand it. I remember
how that felt, so I will offer my version of an explanation.
An engine management computer, regardless of how sophisticated
or crude, really only controls two parameters. The ignition timing and
the fuel delivery. The computer (DME or some aftermarket brand) uses
an array of input sensors to try and determine what state the engine
is in at any point in time. Is it loafing along on the freeway at low
rpm, or at max throttle climbing a hill at high rpm? Is it hot outside
so that the air is thin? Or is it butt cold and the air is real dense?
The list goes on, but the goal of the computer at
all times is to sample its input sensors and then determine the appropriate
output signals for ignition timing and fuel delivery (usually this determines
how long the fuel injectors fire, which is called the pulse width, but
it can also entail "when" to fire the injectors if the injection
The two most important things that the computer wants
to know in order to determine spark and fuel are: 1) engine load, and
2) engine rpm.
Engine rpm is easily determined from a crank sensor. But engine load
is a little more tricky. The best parameter to use in determining engine
load is the MASS of air entering the engine at any time. If you can
measure the mass of air directly then you are well off. Current BMW's
do just this with what is called a MAF sensor, or mass air flow sensor.
This is usually some type of heated wire or film, which is cooled by
the air flowing over it. In order to keep the wire at the same temperature
additional electrical current must be provided, and the measure of this
extra current gives a fairly direct indication of the mass air flow.
Our E30 M3's used a similar system, although it measures
the VOLUME of air flowing into the engine. The sensor is called an AFM,
which stands for air flow meter. The volumetric flow is determined by
how far a flapper (or barn door) is pushed aside by the incoming air.
The flapper is connected to a potentiometer (variable resistor, POT,
wiper, there are many names). But for the computer to know how much
fuel to mix with the air it needs to know how many molecules of air
are coming in, and that can only be determined by knowing the MASS of
the air, not just the volume. So, we need to combine the AFM air volume
signal with an air TEMPERATURE measurement Then the air mass can be
determined. So we are ok.
Note that in both cases the computer has some hard
data on how much air is entering the engine, so if you improve engine
breathing (header cams etc...), the basic code will still sort of work
as the computer is aware of the additional air. It's not perfect, but
it works ok most of the time for changes that are not too drastically
different than the baseline.
Alpha-N is different. Here there is no direct measurement
of either the mass of air, nor the volume of air entering the engine.
That way any possible obstruction from either a MAF sensor or AFM sensor
in the intake path is removed. The air can flow right into the engine
unobstructed. So how does the computer know what to do? It still has
rpm info from the crank, but it can't measure the mass of air coming
into the engine for fuel mixture determination?
The answer, in a crude sense, is that you "train"
the computer what to do in a given situation (on the dyno). For every
possible combination of throttle position and rpm, the tuner determines
the appropriate ignition timing and fuel delivery to yield max power
but not incur detonation. The computer just remembers all this (in the
form of maps which are stored on a chip), and when it sees a certain
combination of rpm and throttle position in the field, it just says
"what did they tell me to do in this situation?" and does
So now throttle position and RPM are the two dominant
input parameters to the engine computer. This is where the Alpha-N name
derives from. Alpha for the angle of the throttle plates and N for RPM.
Now if you have an Alpha-N system, and add a hotter
set of cams that allow more air into the engine, but you do not reprogram
the Alpha-N computer, then the computer has no knowledge of the extra
air now entering the engine and it just gives the fuel and spark that
it was told was appropriate for the milder set of cams. One can see
how this could be a problem. Your mixture especially, but also your
ignition timing will be way off. If your mixture goes lean you can potentially
damage the engine. That is why folks say that an Alpha-N system needs
to be retuned everytime you make a change to the engine. The computer
is "flying blind" to some extent, although it does at least
have an air temp sensor and a barro sensor to try to compensate for
density changes in the air due to temperature and elevation.
I have oversimplified some subjects to make it easier
to understand, and as usual there is probably more that I did not say
than that I did say about Alpha-N, but hopefully that gets some folks
a little closer to understanding it.