MAF and SD (was re: Re: New member intro)

[Bohdan L Bodnar]

>With regard to speed-density vs. mass air flow, has anyone considered the 
>robustness (not in a control sense) of the two systems to errors such as 
>air leaks and sources of inflow downstream of the throttlebody?  Is a 
>speed-density system more resistant to or accepting of such disturbances?
>
>Tony
>

On a speed-density system, an air leak will result in an increase in idle
speed whereas on a mass air flow/vane air flow system this will result in
rough idle.  In fact, on a SD system running open loop a large air leak will
easily result in the engine screaming like a banshee trying to win a hollering
contest.  This occurs because the MAP sensor is used as the primary engine
load sensor.  A large air leak is interpreted as a WOT condition.  On a
MAF/VAF system this same air leak will result in enleanment of the a/f
mixture;  hence, the rough idle.

On my 1986 Le Baron (2.5 liters engine, TBI, SD controlled) running in closed
loop I can increase the idle rpm by about 100 rpm by opening up a moderate
leak at the power brake booster;  in open loop on a cold engine, this same leak
results in about 700 rpm increase.  It's also interesting to watch the
computer compensate for the air leak...the rpms gradually drop to the
programmed value.

These observations are standard diagnostic checks used in the field.

Cordially,

Bohdan Bodnar