[Gmecm] TBI Swap - Iron Duke Tech IV

[Andrew Gibson]

>> 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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