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Thread: Tuning help needed - '91 TBI 383 w/ Vortec heads

  1. #16
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    I tried out zeroing out the overspark and underspark vs RPM error as you said and it did hold my idle spark at a steady 27 degrees @725 RPM in Park, before that it would bounce around a couple of degrees.
    It didn't seem to work so well idling in gear at 575 RPM, the spark still moved around a bit (21-24) and where previously the RPM would stay pretty constant between 600-575 it was now occasionally dropping to 550 RPM. I guess that's part of the job of overspark/underspark vs RPM to try and help hold the RPM near it's commanded value, I'm also guessing there's more than just overspark/underspark trying to do that job as well.
    Great tip but once again when it looks like I might have finally solved something the final answer only ends up getting more complicated.

  2. #17
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    Quote Originally Posted by Kitch View Post
    I tried out zeroing out the overspark and underspark vs RPM error as you said and it did hold my idle spark at a steady 27 degrees @725 RPM in Park, before that it would bounce around a couple of degrees.
    It didn't seem to work so well idling in gear at 575 RPM, the spark still moved around a bit (21-24) and where previously the RPM would stay pretty constant between 600-575 it was now occasionally dropping to 550 RPM. I guess that's part of the job of overspark/underspark vs RPM to try and help hold the RPM near it's commanded value, I'm also guessing there's more than just overspark/underspark trying to do that job as well.
    Great tip but once again when it looks like I might have finally solved something the final answer only ends up getting more complicated.
    There is an idle speed table for both in gear and out. Even with the small cam in my van, I have the hot idle speed at 600 and it stays there without wavering at all.
    Last edited by donf; 3 Weeks Ago at 08:10 PM.

  3. #18
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    Quote Originally Posted by donf View Post
    I am not very familiar with the older gm computers. I don't know if you saw this in my thread but this has a lot of info on tuning the pre 94 tbi computers. The later ones are actually just a blurb in the back pages. http://www.gmcmi.com/wp-content/uplo...EFI-System.pdf
    Thanks for the link, ton of valuable info in there. Saved it to my iPad and am part way through reading it. It doesn't seem to cover my specific ECM (PCM actually since I have 4L80e) but still pretty applicable on the engine side.

  4. #19
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    Quote Originally Posted by donf View Post
    I am not very familiar with the older gm computers. I don't know if you saw this in my thread but this has a lot of info on tuning the pre 94 tbi computers. The later ones are actually just a blurb in the back pages. http://www.gmcmi.com/wp-content/uplo...EFI-System.pdf
    THank you for posting this. This has answered alot of questions and is now saved. I too am looking to build a 383 (for a 1988 pickup) and am wanting to keep the TBI. I have been cruising this board for awhile trying to learn what I can.

  5. #20
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    Quote Originally Posted by Redtires019 View Post
    THank you for posting this. This has answered alot of questions and is now saved. I too am looking to build a 383 (for a 1988 pickup) and am wanting to keep the TBI. I have been cruising this board for awhile trying to learn what I can.
    Glad to share, even with the 95 computer it's helpful. I actually tried to track the author down. I wanted to see if I could get all the pages. From the index its missing the tuned examples, I think that would be educational. The website mentioned in the pamphlet is still registered to someone since early 2000's but it's got the info blocked for privacy.
    Edit: I did some searching and actually found the complete pamphlet here zipped in docs, under CustomEfis Setup. The tuning examples were never in the info, probably why the others cut it off. It's just a link to a long gone website. It is a cleaner copy though.
    Last edited by donf; 2 Weeks Ago at 06:22 AM.

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