[Gmecm] TBI Swap - Iron Duke Tech IV

[Gary]

> 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.  

> 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.

> ... which is usually after the spark event. 

:scratching head:

GAS


----- Original Message ----- 
  From: Andrew Gibson 
  To: gmecm at diy-efi.org 
  Sent: Friday, February 17, 2006 11:14 PM
  Subject: Re: [Gmecm] TBI Swap - Iron Duke Tech IV

  Yes Dave, it will run. But there is something else you have to consider. Some ecm's use 0 Degrees for the base timing setpoint. Some don't. The 7748 does not. So although a '7748 could in thoery run a distributor, it's timing tables are set up for DIS where crossfiring in the cap is not an issue. Another item to consider if you are going to modify the base timing in the ecm: The rotor still physically points at certain towers. So even if you modify the spark tables to compensate for your "case of tinkeritis," the rotor may not be close enough to the tower to fire correctly.

  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, which is usually after the spark event. If you get a fast enough sensor (GM fuel tank pressure sensors work well) and hook it up to a lab scope and manifold vacuum, you can see the rises and falls of manifold vacuum as each cylinder hits it's intake stroke. You want the injector to fire just as the rise begins to get good mixture in all the air.

  Here's where it get's interesting as to why. The injector has to stay open a certain amount of time to get the fuel in. 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. Why poor atomization? Because you missed the spot where the air is at it's highest velocity, and now the manifold is at lower vacuum, so the fuel condenses.

  Pretty cool huh? If you want to see the results, try swapping the wiring of two injectors on seperate banks of a MPFI engine, and see how the car runs.
  On an OBD II car it will trip a misfire code under the light throttle situations during warmup. Which touches on another point: Some engine management software (About 1994 on up mpfi) will change the injector timing depending on load and temperature of the air and coolant.
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