Aftermarket EFI, speed density etc.

[Peter Wales]

I think we are getting a little mixed up here!  

>> 
>> What is the mapping for?
>
>The mapping is required since the user will not usually desire a constant
>air-fuel ratio.........

The user *will* require a constant air fuel ratio! Stoichiometric to be
exact. This allows the catalytic converters to work correctly by providing a
uniform gas mixture in the exhaust. The mapping is designed to take out the
vagaries of the engine and other mechanical wobblies and get the air fuel
ratio as close to stoichiometric as soon as possible. Then the slower
reacting oxygen sensor is used to generate a trim signal to get the mixture
to exactly stoichiometric under constant running conditions. "Learning"
computers generate a second map from the oxygen sensors' data to create a
faster and better approximation from the maps and then use the oxygen sensor
to do the final correction.

The EPA defined a test schedule which each manufacturer has to go through
and the total emissions allowed during this test are defined. The test is
usually very low throttle and accelerations, I suppose typical of a cruise
through rush hour traffic. Thus the manufacturer has the option of staying
stoichiometric just in this schedule, or as most have done, all of the time
the throttle is less than fully open. At fully open throttle the oxygen
sensor is out of the loop and the system is said to be running open loop
which means it is running on the map data only. This is preprogrammed in on
dyno testing and usually gives best power and performance. 


>> | MAP and MAF sensors are both useless for much serious performance work.
>> | Throttle position is the only reliable method in such cases.
>> 
>> Why is this?
>> 
>
>
>I suppose I should define the 'serious performance work' that I've dealt
>with.........  

Again Robert is getting mixed up. The reason racers use throttle angle and
engine speed is that it is very easy to set that up on a  dyno and then know
which row and column in the map to change. If you use a MAF sensor of any
type then you have to define the load and this is less easy to reproduce on
the dyno. Mapping is still fairly easy if you use the voltage output as a
reference. Getting a stable airflow reading at a given RPM depends on
throttle opening and dyno load. It is much easier to define the throttle
opening and use the load to control the engine speed to where you want it.

Lastly, flap air sensors are not notoriously unreliable. If they were there
would be ten trillion cars breaking down every minute because of them. They
are very reliable in service and if used correctly in race engines they work
adequately there as well. There is a school of thought which says they
should be replaced because the mechanical flap across the air stream
restricts the air flow and causes loss of power, but most racers are at full
throttle anyway so it is kind of academic.  



Peter Wales