[SD455DJ]

This conversation has got me curious…

For the 350 (354) and 400 engines with #11 heads to have the same advertised compression ratio of 8.80 to 1, the 350 heads would need to measure 77.2 cc’s and the 400 heads would need to measure 88.3 cc’s (both with ‘zero’ deck, 8 cc piston valve reliefs, and 0.040” quench using the Butler on-line compression calculator).

With Rocky’s information about the #11 heads having a design cc of 87.17, the 350 (354) would have 8.04 to 1 compression and the 400 would have 8.88 to 1 compression cc’s (both with ‘zero’ deck, 8 cc piston valve reliefs, and 0.040” quench using the Butler on-line compression calculator). Using a thicker head gasket (0.052” as per Rocky’s info), the 400’s compression drops to 8.69 to 1.

According to the Pontiac Parts Catalog – Models Thru 1975 (circa May 1979) - the “1970 400 2-B.C. w/8.6 ratio” uses #9790911 head gasket that measures 0.048”. This would increase the compression slightly to 8.75 to 1.

However, since factory engines were built with pistons at least 0.015”, or greater, below deck and head chambers 5-7 cc’s larger than advertised, the compression ratios are a good half point below advertised as well.

I have both the factory ’70 WU code 350 and XX code 400 2-bbl engines, so I’ll get them Whistlered to see what their actual compression ratios are.

Dennis