STATIC COMPRESSION RATIO CHART FOR PONTIAC ENGINES AND HEADS
50 55 60 65 70 75 80 85 90 95 100 105 110 115
303 9.35 9.18 8.67 8.23 7.83 7.47 7.15 6.86 6.60 6.36 6.14 5.93 5.74 5.57 326 10.00 9.78 9.25 8.77 8.34 7.95 7.61 7.30 7.02 6.76 6.52 6.30 6.10 5.91 354* 11.23 10.55 9.97 9.45 8.98 8.57 8.19 7.85 7.54 7.26 7.00 6.77 6.55 6.34 366 11.55 10.86 10.25 9.71 9.23 8.80 8.42 8.07 7.75 7.46 7.19 6.95 6.72 6.51 389 12.20 11.48 10.83 10.26 9.75 9.30 8.89 8.51 8.18 7.87 7.58 7.32 7.08 6.86 400 12.53 11.77 11.11 10.52 10.00 9.52 9.10 8.72 8.37 8.06 7.77 7.50 7.25 7.02 411# 12.86 12.08 11.40 10.79 10.26 9.77 9.34 8.95 8.59 8.27 7.97 7.69 7.44 7.19 421 13.15 12.36 11.65 11.03 10.48 10.00 9.54 9.14 8.77 8.44 8.13 7.85 7.59 7.34 428 13.31 12.50 11.79 11.16 10.60 10.10 9.65 9.25 8.87 8.53 8.22 7.94 7.67 7.42 439% 13.54 12.71 11.99 11.35 10.78 10.27 9.81 9.40 9.02 8.67 8.36 8.06 7.79 7.60 455 14.15 13.28 12.52 11.85 11.26 10.72 10.24 9.81 9.41 9.05 8.71 8.41 8.13 7.86 469~ 14.52 13.63 12.85 12.16 11.55 11.00 10.50 10.06 9.65 9.28 8.93 8.62 8.33 8.06
#400 bored .060" %428 bored .060" ~455 bored .060" *Actual displacement of 350.
The above figures assume 10 cc for .042" (crushed) head gasket, 6 cc valve reliefs for 400 stock pistons, 5 cc for -. 023" deck height on 4.120" bore and flat top pistons (no dome). To correct for pop-up pistons, subtract dome volume from chamber volume. To correct for deep valve reliefs, add relief volume to chamber volume. Note that stock claimed compression ratio is usually 0.50 more than actual compression ratio.
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