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Compression Ratio and The Pontiac Engine

Here are some ways of finding out and changing your compression ratio on Pontiac V8 engines.

On a Pontiac engine the compression ratio's were change by using different heads. Below are some popular heads and chamber sizes (always be sure to have yours checked before any milling).

cc year casting disp comments valve size
71cc '68 R96A 400 RAII R-PORT 2.11/1.77
71cc '69 722 400 RAIV 2.11/1.77
71cc '70 614 400 RAIV 2.11/1.77
72cc '69 48 350/400 HO/RAIII 2.11/1.77
72cc '70 12 400 HO/RAIII 2.11/1.77
72cc '67 670 400 GTO 2.11/1.77
72cc '67 97/997 400 Ram Air 2.11/1.77
72cc '68 62 400 std 2.11/1.77
72cc '68 16 400 GTO 2.11/1.77
72cc '68 16 428 HP 2.11/1.77
72cc '68 31 400 RA 2.11/1.77
72cc '69 16 400/428 Std 2.11/1.77
72cc '69 62 400 Std 2.11/1.77
72cc '70 12 400 RAIII 2.11/1.77
72cc '70 13 400 GP/F-bird 2.11/1.77
75cc '68 62 428 Std 2.11/1.77
87cc '70 15 455 B-body 2.11/1.66
87cc '70 64 455 GTO/GP 2.11/1.77
92cc '78/'79 6x-#4 400 F-bird 2.11/1.66
96cc '71 96 400 GTO/F-bird 2.11/1.77
96cc '72 7K3 400 A-body/F-bird 2.11/1.77
98cc/114 '73-74 4X 400 Std 2.11/1.66
101cc '75 5c 400 Std 2.11/1.66
101cc '76-79 6X 350-400 Std 2.11/1.66
111cc '71 197 455 HO 2.11/1.77
111cc ''72 7F6 455 HO 2.11/1.77
111cc '73-74 16(orX) 455 SD 2.11/1.77
114cc '71 66 455 Std 2.11/1.77
114cc '72 7M5 455 Std 2.11/1.77
98/114cc '73-74 4X 455 Std 2.11/1.66
124.51cc '75 51 455 Std 2.11/1.77
124.51cc '75-76 6H 455 T/A 2.11/1.66(?)
124.51cc '75 6S 455 T/A 2.11/1.66(?)

Now that you know what it started out as you can subtract 1cc for every .005 milled off the head. It is not recommended to go farther than .060 off the head and for anything over .015 you must mill the intake side (your machine shop will have to determine how much).

Be sure to have your heads cc'ed and do not assume yours to be this size.

HERE IS A COMPRESSION RATIO CALCULATOR

To get the compression ratio here are the formulas:

  •  
    CR= Cylinder swept volume + Compression area at TDC
    Compression area at TDC
    Deck height (cc) = bore x bore x .785 x deck height x 16.387

    Cylinder swept volume= bore x bore x .785 x stroke x 16.387

    *Pontiac 400 Compression area=25.45 + head cc (this is assuming 6cc for valve relief, 9.7cc for head gasket, 2cc ring area, and 7.75cc deck height volume.)

    *Pontiac 455 compression area = 25.57 + head cc (assuming 6cc for valve relief, 9.7cc for head gasket, 2cc ring area, and 7.87 for deck height volume)

  • *blocks are assumed to have a .030 over bore and heads cut .010.

     

     

    1998-2010

    78blackbird@thepontiactransampage.com
    Feel free to drop me a note. * I am looking for some good articles for the third and fourth gen birds. If you have any that you would like to see on this site send them to me and I will post them on my site.     

    • The information in here is just an outline.
    • The author claims no responsibility for the info contained in these pages. 
    • If unsure of any tip or modification seek a professional for the job.
    • In some states this may violate pollution laws
    • Check with your state on the laws.
    • Not affiliated with Pontiac or General Motors