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Throttle Bodies, Built engines and manifold pressure

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by GUMBY, Jun 22, 2001.

  1. GUMBY

    GUMBY Registered Member

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    Will somebody explain to me what engine, TBI chip and cam manufacturers are using to determine when an engine will or will not work with a TBI? From what I can tell, it's dependent on cam lobe separation angle which affects vacuum which then affects the MAP sensor which then can cause your TBI to work or not to work. If the cam lobe angle is too small and vacuum is lessened due to a larger valve overlap, is this the major factor that determines whether or not the TBI will work? I understand that a chip has no effect on manifold pressure and that it's all up to the cam lobe separation angle being above 110 degrees. For example, I noticed that all the cams offered by comp cams that would work with a TBI had 112 to 114 degrees lobe separation angle. Am I totally off or am I in the right direction? I don't want to waste money on a slightly larger than stock crate motor if I can't get a TBI to work.
     
  2. HarryH3

    HarryH3 1 ton status Author

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    They're using the assumption that you are using the stock TBI and that the truck must still pass emissions tests in all states. With custom programming, you can make TBI work with nearly any engine. When you get into really rumpity cams, then the computer has to rely mostly on input from the throttle position sensor, which can really whack out your emissions. Since there isn't much demand for that type of system, then a custom (read expensive!) solution becomes your only choice. [​IMG]

    <font color=black>HarryH3 - '75 K5</font color=black>
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  3. CaptCrunch

    CaptCrunch 1/2 ton status

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  4. 502Burban

    502Burban 1/2 ton status

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    You are spot-on with your assumptions. I have done a couple of multi-port injection systems, and have found that lobe separation is the issue. This is due to excessive overlap decreasing the vacuum signal to the MAP sensor, as you suspected. Most DFI manufacturers recommend 110 - 112 degree lobe centers for cam selection. In addition to the cam manufacturers already posted, Crane makes some very good fuel injection cams.

    502Burban
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