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'91 350 conversion to late 70's 400 KEEPING the EFI?

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by KRAZIE87K5, Nov 10, 2003.

  1. KRAZIE87K5

    KRAZIE87K5 1/2 ton status

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    \'91 350 conversion to late 70\'s 400 KEEPING the EFI?

    I've got a '91 350 that has... well... some issues. The heads heads, intake, TBI, sensors, etc etc are ok though. How hard would it be to swap the short block 400 for the short block 350? Basically, just doing a heads-up swap from one to the other?

    Issues I can see:

    1. drilling the heads for the 400 coolant passages
    2. cam? can the roller cam be saved? Or will I need to go to std flat tappet cam?
    3. do all 400's have cooling issues? How to prevent this?
    4. sensor locations... can the sensors all be installed into the new motor? Oil pressure (behind the distributor), knock sensor (coolant drain plug), CTS (intake and head - should be ok those parts are staying)?
    5. engine mount locations? Are they the same from 350 to 400s?

    What am I missing? Any help is greatly appreciated. Also, how much power can I expect to gain from a decently cammed 400 from a stock roller 350?

    Thanks all!

    -Dan
     
  2. BurbinOR

    BurbinOR 3/4 ton status

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    Re: \'91 350 conversion to late 70\'s 400 KEEPING the EFI?

    Dan, some input -
    1. If I remember right, the block is the determinating factor in using the roller cam, so you probably cannot use the roller cam. Also have to make sure the cam is 'FI friendly'. You will use more fuel so some tweaking/reprogramming on the PROM is needed (not required per se but you will need it).
    2. Not all 400's have cooling issues, just make sure the cooling system is up to snuff. I had a 4 row radiator in front of my 406 for 6 years and never had it get hot. Ever.
    3. Sensors should be fine - I had all the same ones in my 400 block when I went TPI. Knock sensor right in front of the starter in the block.
    4. Same mount locations. Might be able to use your serpentine setup directly on the 400 block. Did that on my 406, all bolt on.
     
  3. 84_Chevy_K10

    84_Chevy_K10 Banned

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    Re: \'91 350 conversion to late 70\'s 400 KEEPING the EFI?

    What about the fact that his intake is setup for the 87-95 style bolt pattern?

    Dan,

    Are you planning on using the 350 heads on the 400? I suppose it wouldn't be such a bad idea as all stock 400 heads are pretty small AFAIK.

    Are you sure that engine is roller cam? I was under the impression that only 96 and newer truck engines were roller.
     
  4. rjfguitar

    rjfguitar 3/4 ton status GMOTM Winner

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    Re: \'91 350 conversion to late 70\'s 400 KEEPING the EFI?

    Go with a set of vortec or aluminum aftermarket heads. I made the mistake of using stock 400 heads. some serious HP can be achieved with different heads. you will need a new prom custom burned. Like burbin said, make sure you have a good cooling system to handle the 400. You can use an aftermarket roller cam with new lifters if that is what you want. Only vortec motors after 95' had factory rollers. Good luck, I have a built 406 and I love it over my old 350. They are mean motors with big block torque if "torquey" after market parts are used. /forums/images/graemlins/smile.gif
     
  5. SUBFAN

    SUBFAN 1/2 ton status

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    Re: \'91 350 conversion to late 70\'s 400 KEEPING the EFI?

    Check the chamber size in the 350 heads, they may be to small and make to much compression.

    If you can reuse the heads, be careful drilling those steam holes, or have it done proffessionally - not that much $.

    With a small Computer compatible cam, the TBI should work OK, not great, but driveable and liveable. Obviously more power can be had in the future with tweaks.

    The power increase with what it sounds like you are planning will be slight, but noticeable. A dedicated 400 build-up will show big power gains.

    I have some 400 heads that might be for sale....
     
  6. dyeager535

    dyeager535 1 ton status Premium Member

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    Re: \'91 350 conversion to late 70\'s 400 KEEPING the EFI?

    If the cam is an aftermarket retrofit roller cam, then yes, it will work.

    If it is a stock roller, then no it won't work. Only roller cams in trucks were *'96+ Vortecs*. Don't believe anyone can point to any chevy literature calling '95 model year truck 5.7's "Vortec".

    Roller cams were in all Chev V8 cars starting 1987 IIRC.
     
  7. BurbinOR

    BurbinOR 3/4 ton status

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    Re: \'91 350 conversion to late 70\'s 400 KEEPING the EFI?

    [ QUOTE ]
    What about the fact that his intake is setup for the 87-95 style bolt pattern?

    [/ QUOTE ]

    He said he was using his block, with heads and tbi going onto it as a package............ /forums/images/graemlins/smile.gif
     

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