Twin ECM Systems

[Frederic Breitwieser]

On the subject of dual ECMs, someone maybe a year ago suggested this to the
list, and I made a foolish attempt at it.  The idea was to utilize the
factory ECM for idle, day to day driving, and the secondary ECM to run as
the "race" computer compensating for the additional load, turbo boost, and
power needs.

I tried it several ways, an ended up having a strong desire to jump up and
down on all of the ECM parts, as neither was overly successful.  The test
bed was a rebuilt FWD 3.8L 1986 engine from a LeSabre, with the 148 factory
ECM attached "as-stock", i.e. oem injectors, oem sensors, etc.  Then, I
replaced the manifold with my dual-injector setup, and it ran fine, as long
as I plugged the second injector holes.  The next test was to wire up the
second injectors, connect to a secondary fuel rail/fuel pump, and try
again.  It started, but was was running waaaaaay to rich.  I simply wired
the injectors in parallel to the original ones, hoping the ECM would
realize through the O2 sensor that it was rich, and compensate.  It
probably tried to compensate, but was so far out of its range of
expectations/programing, it sputtered and stalled repeatedly.

The next rendition, was slightly more successful, but still, not good
enough to run on the street.  What I did was leech a second ECM out of a
junkyard, and run the two of them in parallel.  Two MAFs, two injectors,
etc, with each ECM thinking it was the only one.  Of course, there were
certain problems I quickly discovered (as the list suggested, which I chose
to ignore <G>), was that their wasn't enough airflow through the two MAFs
to give the ECM a real clue.  1/2 the air (wild guess) went through one,
and 1/2 the air through the second unit.  Therefore, the ECM(s) tried to
compensate for this "lack of air" and got real screwed up, and the engine
choked and stalled.  Then it wouldn't start at all.

So, we moved onto the next experiment.  We attached two turbos from a
Subaru XT-10 (or whatever the heck that wedged shaped car is called), and
forced more air.  The dual ECM thing is not the way to go, using only
factory, OEM parts.  Obviously a code change was necessary, but out of my
skillsets. So, I was left with two choices - either go aftermarket ECM, buy
two custom chips, or come up with another screwed up solution.  I ruled out
the chip idea because it would cost at least $100 a pop to have each chip
programmed and burned, and the chip makers would ask lots of questions I
couldn't answer.  I seriously doubt "Superchips" would create a chip for me
based on two OEM ECMs running in parallel, on a front-wheel drive motor.
So, I bailed on that idea.

Next, was to use the OEM computer for idle, local driving, and basic
cruising, and the existing Haltech unit I have for the second injectors.
It worked much better, though creating a map was a real pain in the ass.
The Haltach kicked in at about 3000 RPM when the turbos are humming nicely,
but I ran into one final problem - OEM mafs suck no matter how you look at
it under high boost.  I needed a MAP sensor, or maybe a MAF that can go
into the 3bar range of pressure, and accurately measure the flow.

If you remember a while back we discussed enclosing an existing MAF into a
larger tube, as to allow some of the air to bypass it.  This is what i
thought the dual MAF would do.  The problem is the dual-maf allowed too
much air to bypass either MAF, and required code/data changes.  In both
Corky Bell's "Maximum Boost" and I think Hartman's "Fuel INjection" book
discuss how to use an aftermarket ECM on top of an OEM ECM, thus ensuring
emissions compliance, yet enough fuel for additional power.  We moved the
injectors in the airstream of the main intake log, as well as on the
runners (where they are now) and we never got it working right.
Eventually, the FWD engine didn't like the abuse and threw a rod, so our
testing was never really completed.  Now that I have money sunk into an
4.1L RWD engine, I didn't want to blow it up by screwing with it.  I
figured a nearly-free FWD engine is a good test bed.

Now, the final revision I have, which actually works really well, is to run
an aftermarket ECM (Haltech in my case) as the only ECM, and the second
tier of injectors is run as a batch - therefore they all fire at the same
time.  By playing with the RPM levels and the maps internal to the unit, we
can somewhat control the amount of fuel that goes in.  Within reason.

As far as the OEM ECMs go, I wanted to "lie" to my ECM, and wire two
injectors in parallel to each other, with a one-shot timer attached to the
second injector.  As the intake manifold has increased pressure due to the
turbos, this pressure would in fact increase the second injector's "stay
open" time.  Thus, maintaining the same air/fuel ratio that the OEM ECM is
trying to achieve.  This way, I could utilize all OEM parts, a simple 555
timer and six MOSFETS, a pressure sensor, and a trimmer for adjusting.  I
actually built the circuit, but haven't tried it yet.

The goal was to achieve 500 HP out of a reasonably stock engine, yet pass
emissions COMPLETELY.  To pass emissions, all one has to do, if its an
issue, is electronically disconnect the second injector enable line (i.e.
disconnect the 555 timer one-shot) and cause the wastegate on the turbo
output to remain stuck open.  Viola, an instant, OEM naturally aspirated
engine.

Sorry for the long winded message, but I wanted to share some of the
failures with ya, and hopefully you would avoid them.  I still believe
lying to the ECM is the cheapest, easiest way of getting a high power
engine through emissions, just with a little lying to the ECM.

My fantasy V6 for my mid-engine car would be a twin-turbo'd weber-like
configuration with long runners :)


Frederic Breitwieser
Bridgeport, CT 06606

Homebrew Automotive Website:
http://www.xephic.dynip.com/

"When in doubt, use a bigger hammer"
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