[Q] EFI computers on different engines

[thomas walter x5955 apprvl-Clark Meier]

> From: Craig Pugsley <c.pugsley at trl.oz.au>

> He wants me to make up a box that tweaks various inputs to get the
> fuelling correct.

[rest snipped just to save bandwidth]

Will this be an analog box to precondition the signals?

Have you thought about a uC box to replace the original? 
Have you done any uC work?

[wants to remove air box vane meter... and replace it with
a map sensor]

The early Nippon Denso should be the BOSCH L jetronic analog
design. Contact switches for idle and full throttle should be on
the throttle body. 

If the air box is an 7 pin style, two of the pins are just for
a relay contact to turn the fuel pump on. Safety feature that
if the engine stalls, no air flow, hence shutting down the fuel
pump. I'd look at using a oil pressure switch to control that
function if you replace the air box.  Safety first!

> 
> My understanding of these trapdoor type systems is
> that almost all of the fuelling calculation is based on the trapdoor
> potentiometer, along with idle curcuit (switched by AFM or throttle
> position), 

Yes.  Ideal would be to "bench test" the donor computer, with all the
parts to get an idea of the functions of how much the air vane
position modifies the amount of fuel injected.

> accellerator pump type enrichment (again from throttle position), 

  I take it you mean WOT (wide open throttle) switch.

> cold start enrichment (Water temp sensor) and air temperature
> sensor (for air density)

If it is a early analog box there should be two water sensors:
one for normal running, the other is the "cold start-time switch"
to control the enrichment injector for cold starting. 

Later uC systems have done away with the enrichment injector, just
open the standard injectors on cold starts!
 
> Questions:
> i/ If I have a MAP sensor and alter this signal ONLY and feed it into
> the trapdoor potentiometer input on the ECU (ie use a MAP instead of a trapdoor
> AFM), and leave all of the other sensors STOCK (unmodified), is this
> system likely to function successfully?

If possible, you may want to make a series of measurements to get an
approximate idea of the voltage vs. airflow from the air box from the
donor system.  Once you know that function, then it would be NICE to
have an actual system to compare manifold pressure vs. air vane voltage
readings.  This should give an idea of the function between the two.

PROBLEM: If the engine has a radical camshaft, with much pulsing of the
intake track it will be more difficult to monitor the air with a MAP
sensor. In that case I would be tempted to look at adding a throttle 
position sensor (linear pot on the throttle shaft) to give a more 
repeatable idea of the throttle position. [Throttle position AND a
MAP sensor will give a better idea of the engine's fuel requirements].

> The concept I had planned to utilise here was to have a series of LM3914
> bar graph drivers arranged with some pots to make a 'graphic equaliser'
> so the input voltage can make a custom tailored output:
> 
> _____          __________________
> |MAP|         |    LM 3914 (x3)  |
> |   |---------|                  |
> |___|         |__________________| 
>                |  |  |  |  |  |  |  }=pot that varies output between
>                }_ }_ }_ }_ }_ }_ }_   0&5v when that output is on.
>                 |  |  |  |  |  |  |
>                 |__|__|__|__|__|__|____________> Output voltage to ECU 
> 
> I figure that this should allow a MAP sensor to approximate a trapdoor
> AFM, as it is essentially fully adjustable.
> Assume for this question that the computer is off a similar type of
> engine.

Neat way to do it! ;-) But fails on the KISS (Keep it simple,
Silly) principle.  Once you know the function,  it maybe easier to 
use a op-amp or two. Function would be a matter of air vane vs. 
throttle position (or just MAP, if the cam is mild).

For initial testing, may not be a bad idea to hook up a 3 wire
oxygen sensor and monitor it to ensure you are usually in the rich
conditions. Nothing worse then helping out a friend, only to melt
down their engine from too lean conditions. [I'd love to have
a wide range sensor for road testing... just out of my budget, too!]

There are those on this list who are much better qualified than I,
but thought I would at least toss out a few ideas to get the ball
rolling. 

Cheers,

Tom Walter
Austin, TX.