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Technical GMC 270 engine measurements and specs

Discussion in 'The Hokey Ass Message Board' started by Devin, Nov 18, 2014.

  1. Devin
    Joined: Dec 28, 2004
    Posts: 2,351

    Devin
    Member
    from Napa, CA

    I recently picked up a 1956 GMC 270 to replace the rusted hulk of a 216 in my '50 Styleline. My goal is a smooth and torquey engine with a little extra zip from stock.The engine seems in decent condition but it is a core. In the interest of rebuilding this, I've been wanting to do some of the preliminary engine math. I'm trying to plan static and dynamic compression ratios in order to pick an appropriate cam, pistons, carburetion etc. I've been able to find stroke, rod and bore info easily, but am having a hell of a time finding out conclusively what the actual deck height is as well as the deck clearance. The best info I could find was that the deck height is 11.0625", but this info was posted as a question and was not verified with any authority. I'm assuming the piston compression height is 11.0625" - 2" (half the stroke) - 7" rod length - whatever the deck clearance may be. Last, it's been very difficult coming up with actual cam grind specs as well. Any cam advice from personal experience is definitely welcome. Any info and insight would be a great help and is sincerely appreciated.
     
  2. Devin
    Joined: Dec 28, 2004
    Posts: 2,351

    Devin
    Member
    from Napa, CA

    Bump for the inliners!


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  3. need louvers ?
    Joined: Nov 20, 2008
    Posts: 12,906

    need louvers ?
    Member

    As far as I know, the 270 is a 4" stroke X 3 7/8" bore. I have one too which is going to be about the same engine, so I'll watch what you are up to here.

    I do know that compression ratios and piston choice are determined by what head you intend to run, as some were of an open chamber design and others closed or wedge chamber. The open Chambers if memory serves correct are almost absolutely relegated to using a specific, pop up type piston to get into the 9:1 range.
     
  4. RichFox
    Joined: Dec 3, 2006
    Posts: 10,020

    RichFox
    Member

    With out looking it up, as I remember they were 3 25/32 bore X 4 inch stroke. I bored mine to 3 15/16 for 292 cid. I used a 302 D shaped combustion chamber head. Used SBC stainless valves + .100 length. Chuck Potvin ground my cam. Gives you an idea of how long ago this happened. Was a pretty mild "RV" type cam. Ran good for several years. My pistons came from Arias. Had a small pop up. 9 to 1 as i remember. I had two Corvair turbos and a single Holley 780 vacuum secondary. I liked the 270. Still holds an El Mirage record.
     
    Last edited: Nov 19, 2014

  5. Devin
    Joined: Dec 28, 2004
    Posts: 2,351

    Devin
    Member
    from Napa, CA

    Thanks for the replies. I have the small port 819 head which is fine for me as this will be a cruiser. Anyone know the deck and piston compression heights?


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  6. fsae0607
    Joined: Apr 3, 2012
    Posts: 871

    fsae0607
    Member

    I'll be watching this thread. This is the same engine I want for my truck! Anxious to see this engine build!
     
  7. sunbeam
    Joined: Oct 22, 2010
    Posts: 5,274

    sunbeam
    Member

    The GMC is about 2 inches longer than a 235
     

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