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  1. #1

    Default 1991 k1500 4.3 to 350 swap

    I hope im posting this in the right place, i was wondering if someone can shed some light on this, i recently totalled my 1991 c1500 2wd, and found a 1991 k1500 4wd with a 4.3, i already know the wiring harness from the 4.3 will work with some modifications, i know ill need new mounts, radiator and fan shroud, i know the exhaust may need to be modified to hook up, my question is being the 350 is out of a 2wd truck, and going into a 4wd truck, what will i need to replace on the engine itself for it to fit in the 4wd, i have the throttle body and pcm , and can i use the old exhaust from my 2wd on the 4wd, any help is appreciated thanks in advance
    1991 Silverado 2WD 5.7L

  2. #2
    Jr. Apprentice
    Join Date
    Apr 2014
    Location
    jenkin Township, Pa
    Posts
    23

    Default

    You will have to change virtually the entire electrical system, including the PCM..which requires changing the in dash harness as well. You have a LOT of work on your hands. The front of the frame is the same in the engine area whether it is a 4x2 or 4x4 so it will bolt in. The transmission may or may not be up to the task of the radical difference in torque, so there is another potential issue. Fortunately both of these are TBI injection and the pressure specs are the same, though I don't know if the flow (gallons per hour) are the same from the V-6 to the V-8, you have a lot of research to do! The exhaust Y Pipe will almost certainly be different, as will the catalytic converter(s) & diameter of the rest of the pipes as the V-8 will move a lot more air.
    Good luck. I've done a few change overs from old carb engine trucks to newer fuel injection systems...and that involved roughly 80-100 hours in a fully equipped shop and cost lots of $$$. I've done 2 of these...both are classics, one is really lifted.
    Kirk.
    ​Everything in life sucks, except the things that should!

  3. #3

    Default

    It is really no biggie. the harness is plug and play except for a couple places where the V6 harness won't reach to the new position with the V8 installed. All the connectors are the same. The downpipes should line up and bolt to the V8 manifolds. You will need either a donor PCM from a truck like yours that already has a 350 in it. Similarly, you can replace the chip inside the PCM you have. Simple to work with. Yes, you will need to change out the radiator and shroud and some other various stuff.

    The transmission will be fine and will directly bolt up to the new engine. It is ALMOST a direct swap.
    "It went together didn't it? Well then there has to be a way to take it apart!" - Me.

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  4. #4

    Default

    Thanks for the help, i basically have a whole donor truck at my disposal, im picking up a truck with what i suspect is catastrophic engine failure, runs like nothing is wrong, the 350 and pcm are coming out of the same truck, i was reading ill need an oil filter adapter to clear the front axle, and possibly an oil pan, if so would the filter adapter off the 4.3 work or will i need one off another 350, and can i get away with the 2wd oil pan

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