How about no MAF or MAP?

Tony Bryant bryantt at stimpy.fp.co.nz
Wed Jun 5 05:09:40 GMT 1996


My pet project at the moment, is a EFI (& Elec. Ign.) system for
my Datsun 510 club race car. I've taken one look at your average
flap type mass airflow sensor (280ZX), and decided that

1) It will severly restrict air flow
2) The plumbing problems are a real hassle (same applies to hot
 wire types)
3) The extra plumbing makes the second intake harmonic unusable. 

Since I'm using twin dellortos as the throttle bodies, there is
no plenum chamber to take a stable manifold vaccum reading off.
So MAP is out.

I've decided therefore to use a throttle position vs. engine speed
calculation.

A map based lookup is a problem, because it is hard to set while
driving(!), and resolution is limited due to ROM space.

The solution I have used is to solve two simultaneous equations 
to find the air flow:

AirFlow = M.A.Pressure * VE(RPM) * RPM  { Just like in a MAP system }

&

AirFlow = CriticalFlow(Throttle Pos) * compressible gas 
                                           function(M.A.Pressure)

I have no false hope about getting emissions quality control from 
this scheme, but:

1) Since New Zealand is a clean green country , we don't have such
   silly pollution laws (yet :-).
2) It's (mostly) a race car.
3) I want everybody to smell my Avgas.

The question I have, is does anybody have a general equation for
the critical flow for a given throttle position, with real
co-effecients?

p.s. It is now running, but without knowing the critical flow above
closed throttle, the mixture at part throttle is "interesting".

p.p.s  Has anybody here had any experience with measuring mixtures
by measuring peak spark voltages? Any comments?
***********************************************************
* "Insanity is the only sane response to an insane world" *
*                 >> bryantt at fp.co.nz <<                  *
***********************************************************



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