Accurately checking compression ratio

Discussion in 'Small Block Tech' started by Mark Demko, Jul 15, 2018.

  1. Mark Demko

    Mark Demko Well-Known Member

    I want to know EXACTLY what my compression ratio is before I fully assemble my engine.
    Whats the best way to do this?
    Have the machine shop do this for me?
    Buy the equipment from Summit and do it myself?
    If I buy the tools from Summit, what all do I need? I know I need the buret, what else?
    I spec'd the Auto Tech pistons 11 to 1, but I want to check what EXACTLY it is in the engine, it may be less, or more.
    Im curious, and like to know stuffo_O
     
  2. Stevem

    Stevem Well-Known Member

    Buy the complete kit with a plastic burette if you can as the glass ones are hard to keep safe when your working, ask me how I know!
    Not included in the kits which are needed is clay and a good single edge rasor blade.

    The kits tell you have to do every check you need to know how to do , everything but how to level the darn head itself which can take up the most time in some cases!
     
  3. 436'd Skylark

    436'd Skylark Sweet Fancy Moses!!!!!

    Respectfully this is a bit out of order. You ought to know what it is before you order parts, not on assembly!

    The cc burette is your best bet with a dial indicator or piston stop to find tdc...
     
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  4. Mark Demko

    Mark Demko Well-Known Member

    All the required specs were sent to Auto Tech before the pistons were made, I requested 11 to 1 comp. ratio.
    Im just wanting to know what it actually is before heads are bolted on:cool:
     
  5. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

    All you need is how much the pistons are in the hole, the head gasket thickness, the chamber size, and how many cc’s are in the piston dish/valve reliefs. (Didn’t Autotec furnish this?). Have the heads been milled? Chambers should be around 58cc. Head gasket , .040. Were the Autotecs ordered with the right CD to make them 0 deck?
     
  6. Mark Demko

    Mark Demko Well-Known Member

    Yep there damn close to zero deck, I just wanna measure for the hell of it:D
    Auto Tech did furnish all piston specs.
     
  7. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

    What is the piston dish?
     
  8. Mark Demko

    Mark Demko Well-Known Member

  9. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

    I get 10.89 with a quick calculation.
     
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  10. Short_Crank

    Short_Crank Platinum Level Contributor

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  11. alec296

    alec296 i need another buick

    Put a straight edge over cylinder and use a feeler gauge get a guesstimate if piston depth. If your at zero deck , I got about 10.98 with 58 cc head.
    If block is not milled, I’m guessing you are at .038 below deck at 1.855 ch.
    That’s using summit racing compression calculator.
     
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  12. Mark Demko

    Mark Demko Well-Known Member

    Thanks guys!
     
  13. Jim Weise

    Jim Weise GOT AIR?

    The most accurate way to determine compression ratio, with the least amount of "formulas" involved, with an assembled shortblock, is to use a dial indicator to get the piston to tdc. Then use grease to seal between the cylinder wall and piston.. do it neatly, so there is a solid seal there, but no extra.. then you need a 4" square piece of 1/8" plexi-glass.. Drill a hole in it to feed the burret thru, and a bunch of small holes to allow air to escape... Position the block about 1- 2* off of level, and fill that cylinder. Fill cylinder and obtain CC number.. Use this number as your Piston CC, and that along with head cc and head gasket thickness and head gasket cylinder diameter, will give you exact static compression ratio.

    Look for a compression calculator online that asks for both head gasket thickness, and the actual diameter of the opening of the gasket over the pistons.

    For instance, the 8494 PT 455 head gasket that we typically use on the 4.350 455 based motors, actually has a diameter of 4.385. Now .035 over the bore size, in that application is not a big deal, but consider this is the same gasket that goes on a 4.040 Bore 400 ci motor, then you can see the importance of considering that number, if your wanting exact static compression.

    If you really want to know what the gasket is without knowing the exact bore size, then seal it up with some light grease on a flat surface, and CC it with your burrett.. then just add that number to the number you generated above.

    Put zero's in the formula's for everything but piston CC and head CC when you do this.

    JW
     
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  14. Mart

    Mart '71 350 GS, almost stock

    Yep, you can get an exact head cc using same plexiglass method with valves closed and spark plug installed.
     
  15. Mark Demko

    Mark Demko Well-Known Member

    Thanks Jim!!
     
  16. 436'd Skylark

    436'd Skylark Sweet Fancy Moses!!!!!

    Or you can make your high school math teacher proud and do the simple math. Add the numbers up do some dividing...
     
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  17. Jim Weise

    Jim Weise GOT AIR?

    Ya, next your going to want me to actually dial a phone number... :eek::p
     
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  18. sean Buick 76

    sean Buick 76 Buick Nut Staff Member

    I’m with ya Jim remove as many variables and possible and get the real compression ratio with less math and more fact.
     
  19. Mark Demko

    Mark Demko Well-Known Member

    I cc'd a couple head chambers, got 57 cc's, that along with the other numbers I get 11.02 to 1
     
  20. Mart

    Mart '71 350 GS, almost stock

    You gonna be able to run pump premium? I imagine those numbers were using .042 head gaskets?
     
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