Air Flow Measurement

[tom cloud]

>> Still seems to me (I'm still fishing for a definitive answer) that
>> TPS, BAP, MAP and TEMP (IAT) would give a very reasonable approximation
>> (after correction for non-linear TPS vs. opening area) of MAF.  No ???
>> 
>> Tom Cloud <cloud at peaches.ph.utexas.edu>
>
>Yes.  But don't forget the idle air bypass motor.

I don't think I have one o' those.  Actually, I know I don't, since I'm
using a throttle body from Holley.  But, if I decided to modify my oem
'91 F-150 EFI I guess I'd have that problem.

                                                Also, what the automotive 
>manufacturers do with their sensors is not necessarily just for air mass flow 
>calculations.  Even if they decided to do away with MAP in air mass flow
calculations, 
>they would still need it or another way of determining EGR rate for
diagnostics.

I don't got that (EGR) either.

>Here is another important point:  Under part throttle conditions, air mass
flow is 
>greatly affected by throttle angle and pressure drop accross the throttle
blade.  This 
>affect is the dominant factor in determining air mass flow.

yep, I can understand that

                                                By contrast, the closer we 
>get to WOT, the more actual air mass flow is affected by the dynamics of
the engine 
>(intake runner length, plenum volume, cams, etc), and the less it is
affected by 
>pressure drop across the throttle.  So it would make sense to use a
weighing system to 
>base mass flow calculation based on throttle angle at lower manifold
pressures, and base 
>it on MAP the closer we get to WOT. Right?
>
>I know this is getting too detailed.  The bottom line is that you CAN use
TPS instead of 
>MAP. and yes, you do need intake air temp regardless.
>
>Chow for now
>Mazda Ebrahimi

I guess I understand.  I wanted a system that would compensate for altitude
and barometric / atmospheric changes .... hence BAP.  I can see that MAP
might could be ignored and just BAP and TPS (plus IAT) used to give a
very workable efi system .... is that what you're saying?  Just seems to
me that BAP - MAP = delta P across a known opening (the throttle).  Why
is that any different at WOT?  And it looks like that would work quite
well at idle and low throttle, where the MAF sensor is at its weakest
performance point.  Course, I'm thinking about standard manifold, too.
I can see that runners and TPI could greatly affect the equation, as
there might not be a _real_ definitive MAP reading (would be different
at different places ???).

So, you're saying that speed density is _very_ adequate?  I think I knew
that??  I wanna know what's wrong with my assumptions -- why do oem's go
to all that trouble and expense to put in MAF if it can be calculated
from sensors already in place?  (I understand that there might be bypasses
that degrade the data -- but seems that could be overcome.)


Tom Cloud <cloud at peaches.ph.utexas.edu>