MAF Conversions

[FIScot at aol.com]

In a message dated 96-06-03 21:13:36 EDT, you write:

>'Everyone' talks about MAF conversions of existing EFI systems but can 
>anyone answer me a few questions about the whole exercise?

>1- what does a MAF sensor offer that a MAP and inlet air temp sensor 
>don't?  (or what do people think it offers?, ie what are the main 
>reasons for the conversion?)

I am not sure of the conversions you are talking about, but the MAF is a much
more accurate way to measure airflow.  The MAP sensor equipped engine relies
on a VE table, temp sensors and the MAP to *calculate* airflow.  It can work
quite well, and the MAP sensor has the advantage of being much cheaper than a
MAF sensor.  But the MAF is a better way.

If an engine modification alters the VE of the engine, the VE table in the
ECM is now incorrect.  The MAF really doesn't care about VE, it just measures
airflow.

>2 - are analogue (voltage) output devices used more commonly than 
>frequency output?

I have no idea which is used more.  The freq type may have some advantages.

>3 - how are they normally sampled in these conversions, synchronously 
>at a determined crank angle, synchronously at any angle at all, or 
>asynchronously?

Good question. The injected fuel quantity (indicated by injector PW) for a
cylinder should be matched to the air that the cylinder took in, and at the
desired AFR.  If one measured the number of freq style MAF pulses between
spark reference pulses, you would know the amount of air taken in per ref
pulse, which would then be the amount of air for one cylinder..  

One might also use a multi-toothed crank wheel, and measure the amount of
wheel travel against MAF pulses.

In using a DC type MAF, I guess a running average might give you an idea of
the airflow.

>4 - how do you know that the airflow you're measuring is actually 
>there?  Do people just slap them on and hope, or is some science 
>applied to make sure inlet manifold resonances are not upsetting the 
>signal?

I have no idea there.

>5 - How about reverse flow?  Is this any concern in the average 
>conversion?

The MAF can measure airflow both ways.  It may not be linear in the reverse
direction though.  This may be a problem if you use a cam with a lot of
overlap that causes reversionary pulses.

>6 - what kind of manifold filling compensation do people use?

I guess you are refering to transients?  By monitoring the TPS, you could
cover those
by adjusting fuel to suit the TPS delta.  The MAF could also be monitored for
a delta, indicating the need for more fuel.

Scot Sealander  FIScot at aol.com