[Diy_efi] Override/Passthrough EFI system?

[Brian Dessent]

> Xudong Yan wrote:
> 
> Hi folks,
> 
> Has anyone thought off, has interest in, or has done something along
> the following lines:
> 
> Let's say I want to build a "piggyback" EMS, on a car that already has
> a working EMS system.

I have never tried this, but here's my impression.  It would make sense
for simple things, such as adding fuel or retarding timing at certain
operating points.  However, I think it would be a bit harder to shorten
things, i.e. reduce fuel or advance timing.  To lengthen pulsewidths or
delay firing the coil is pretty easy.  But to shorten the pulses or
advance the timing you have to know ahead of time what the original
ECU's pulsewidth  or timing degree was.  Certainly it's possible -- one
way might be to have your controller observe the ECU's output for one
cycle and then modify the values appropriately after that, as long as
the operating point (rpm/maf/load/whatever) doesn't change radically. 
But you'd be forever playing catch up, "what was the ECU going to do
here?"  During transients (such as acceleration) it would be hard to do
this.

Alternatively you could have your controller derive/compute its own
values from all the sensors, and then override the ECU's at certain
operating points.  But this involves a lot of work and a whole lot of
tuning since you're more or less building in standalone functionality. 
You might be able to do this by just mapping the ECU's output based on
all inputs, essentially a large lookup table, but there are so many
various correction factors that go into the mix that you would go crazy
or use a LOT of ram if you tried this.

Brian

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