re #3: MAF Conversions

[peter paul fenske]

>>1- what does a MAF sensor offer that a MAP and inlet air temp sensor 
>>
>>>MAF directly measures Air Mass. Fueling is simply based on AF ratio
>>>desired not extensive engine mapping. In other words a change in
>>>engine parms does not require a recalibration within limits.
>
>But you can calculate Mass Airflow from manifold temp, air density and 
>engine speed.  Why is this so inferior?
>
>>N = PV / RT is the map calculation. The fact is even using lookup
>>and base pulse fueling many adjustments are required. Digital systems
>>are not continuous but discrete. Thus while theoretically you could
>>have map work perfectly it requires a great deal of uP HP. As for
>>responding to changes, speed density is based on a load percentage
>>as related to manifold pressure. WOT has the highest pressure, idle
>>the lowest and fueling is based on where the engine is. IE base pulse
>>is modified by %load. It is this calculation that gets mucked up
>>when you change engine cam. With a lower vacume ,higher pressure PW
>>increases which results in too rich a mix. Thus the lookup table or
>>slope and base of the %load has to be changed when engine parms change.
>> 
>>>MAF is continuous. Just like MAT or CTC.
>>
>What do you mean by continuous?  Most ECU's are digital and require 
>some form of ADC, therefore you have to decide where (time-wise) to 
>sample the sensor output.
>
>>Guess its late. Oh well. More specifically response time is the key.
>>Mass airflow devices are either vanes or hot wire. These devices respond
>>relatively slowly to fluctuations and will average the airflow.
>>
>>6 - what kind of manifold filling compensation do people use?
>>
>>>Usually taken care of in PE corrector. sori not ve in maf.
>>
>VE? Volumetric Efficiency?  What sort of corrector/algorithm?
>
>>VE tables are part of the speed density system. The base pulse
>>gets modified by a value in a lookup table indexed to RPM.
>>In a MAF system there is an enrichment table. This allows some
>>variance in A/F ratio with changes in RPM.
>
>re: airflow fluctuations, I've data from a 1.8l 4-cyl that I'm working 
>on that at one speed-load point has airflow varying from 20g/s to 
>70g/s at twice crank speed!  This would not be uncommon.  How have 
>people overcome this in MAF conversions?
>
>>Once again instantaneous vs average airflow. You are interested in
>>the average airflow albeit over a short time period.
>>As for reversion, ram tubes, runners whatever usually run from the port
>>to a plenum. The throttle body feeds the plenum and the MAF feeds the 
>>throttle body. Thus the MAF is both far removed from the valve and
>>plenum airflow tends to average individual cylinders. If you are worried
>>about manifold pulsing look at the output of a MAP sensor sometime. 
>>The AD data from a MAP has a first or second order digital filter in
>>software usually. 
>
>Anyways if you want SAMS has a good book out on engine controls. In addition
>there are many SAE papers on the subject.
>>BEEN A PlEASURE: peter