[Gmecm] TBI Swap - Iron Duke Tech IV

[Gary]

>> 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. 

I failed to qualify the HP reference being a non issue when SEFI becomes 
batch, IOW above a certain RPM or at WOT.

> Why? Because more fuel is burned at lower rpms due to timing of 
> the injectors.

I'm confident you meant less in lieu of _MORE_ fuel, as this would
defeat the intent and have a detrimental effect on emissions. 

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.

Not a whole lot of air speed when the valve is closed.

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

>:scratching head:

At least you are 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.

At part throttle, I agree.  My thots were in reference to when injector 
sprays fuel at the back of a closed valve.  This puts injector opening 
B4 ignition!  No disagreement during part throttle.

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.

As long as the statement is qualified to represent part throttle operation.

GAS

----- Original Message ----- 
  From: Andrew Gibson 
  To: gmecm at diy-efi.org 
  Sent: Saturday, February 18, 2006 10:14 AM
  Subject: [Gmecm] TBI Swap - Iron Duke Tech IV
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