[Gmecm] TBI Swap - Iron Duke Tech IV

Sat Feb 18 17:00:47 UTC 2006

Hi there!

There's lots on this in the archives, and I (obviously) need to go do
some reading, but I'm a bit confused about why this (triggering DIS ECM
with a distributor, or vice versa) won't work on the TBI case being
discussed (or for MPFI systems either). I'll quickly concede that SFI's
a different beast.

Let's use a V6/60 like the 2.8V6. The distributor will generate one
pulse per firing event, spaced 60 degrees apart. This goes to the ECM,
which then modifies the signal and sends it back to the EST module to
fire the plug. Still, the signal from the EST is one per cylinder, all
60 degrees apart. These pulses to the ECM do not change with ignition
timing.

The DIS is similar on this engine family. The ICM sends a pulse to the
ECM once per firing event, spaced 60 degrees apart. These pulses to the
ECM do not change with ignition timing.

I *assume* for the 4 cylinder case that the ICM generates one pulse
every 90 degrees, just like the distributor. If not, that could be a
show-stopper (since the distributor puts out a different pulse train)
but I'd be surprised if the DIS for the 4-cylinder did that.

The only difference appears to be the reference angle for the firing
event. The ICM (in the V6 case, anyway) is referenced at 60 degrees,
and the distributor is referenced at 0 degrees. For an application
using a distributor to trigger an ECM designed for DIS (i.e. 1227730
running $A1 for MPFI), couldn't the distributor just be installed at 60
degrees instead?

Or for the other case -- DIS module running a TBI ECM (i.e. a newer
V6/60 engine's DIS module hooked to a 1228062 TBI ECM), couldn't this be
handled by choosing how the plugs are wired?

Could the base timing value in the ECM be adjusted to compensate for
either case?

The whole problem of when the ECM is triggered relative to spark event
(base timing) seems pretty easy to overcome.

I understand that with DIS you can have much more advance because the
physical constraints of the rotor/terminal relationship are removed, but
that's a matter of tuning the tables to keep the spark timing in check,
right? Don't command too much timing and it's not an issue.

Jay Vessels
1982 Chevrolet S-10 Sport, 2.8V6 TBI
1984 Chevrolet S-10 Blazer Sport, 2.8V6 (TBI pending)

Andrew Gibson wrote:
>>> the rotor may not be close enough to the tower to fire correctly.
>>> 
>>> 
> 
> 
>> In regards to the rotor contact area, it is better to actually 
>> observe the rotor of a given application, as I've seen rotors with 
>> contact terminals 15/16" wide.
> 
> 
> Some are 15/16" wide. But the rotors used on TBI engines are about 
> 1/4" wide. So if it's not close enough to the tower, you have a wide 
> gap, which means more voltage required to jump the gap and less burn 
> time at the plug. AKA-misfire
> 
>>> One other issue I want to share, is the injector timing. It's not
>>>  enough
> 
>>> just to fire an injector when the spark fires. The injector MUST 
>>> fire when the air is just beginning to be sucked into a 
>>> cylinder....
> 
> 
>>> The window to get the fuel in decreases as the RPM go up. So if 
>>> the injector timing is off you may miss that window, and get a 
>>> misfire from poor fuel atomization.
> 
> 
>> You're making this out to be more technical and precise than it 
>> really is.  After ~4000 RPM it is common for SEFI to become batch 
>> fire. While not 100% DC, this does imply spraying fuel to the back 
>> of a closed valve.  This explains why there is virtually no HP 
>> difference
> 
>> from batch fire to sequential, or bank to bank, FTM.  Sequential's
> 
>> strong suit is lower part throttle emissions and improved fuel 
>> economy.
> 
> 
> Sequential injection does have better emissions. Why? Because more 
> fuel is burned at lower rpms due to timing of the injectors. If you 
> throw off the timing it is common to lose 35-50 HP. A good example is
>  if a Ford (I know this is the gm section) wipes out it's cam sensor.
>  The injectors switch to batch fire and my clients usually complain
> of a check engine light and a gutless car. Another good example is
> the Mazda 2.6L 4cyl. The distributor can be put in at 3 different 
> positions. Even though it will run at all three positions, only one 
> will work right. If wrong the gas milage drops to about 6 mpg, and 
> the truck is flat out gutless. All becasue the synch pulse for the 
> injectors is in the wrong place. I made that mistake once and the 
> owner was quite upset.
> 
> As for batch firing above 4000rpm: There is not enough time to fire a
>  injector with the precision you can below that speed. And besides, 
> with the air velocity you have at that speed, atomization of the fuel
>  is not an issue.
> 
>>> ... which is usually after the spark event.
> 
> 
>> :scratching head:
> 
> 
> At least you arre thinking. Ignition timing is typically running 
> between 6 to 32 degrees BTDC. The intake valve does not start to open
>  until 1 to 2 degrees BTDC. So if you fire the injector too early air
>  is just compressing behind the valve, not moving. So you miss the 
> rush of air that will mix the fuel in well.
> 
> One other thing you may be interested to know. GM TBI fires the 
> injectors when the ecm recieves the signal pulse from the distributor
>  or ignition module. However I'm sure you know that EST come into 
> play. But only for the spark. The ecm sends a signal back to the 
> module to modify the ignition timing, without modifying the injector 
> timing. When is the injector fired? At 0 Degrees BTDC. Right where 
> you set the timing.
> 
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