Which cylinder head was offered on the 220 hp 400 in 1978 and 79?
Two versions of the 1975 – 1977 “6X” cylinder head were produced. The head designed for the 400 cid has a chamber volume of 98.09 CC’s. The head designed for the 350 cid has only a 91.11 CC chamber. The difference is in the size of the chamber scallop on the spark plug side. The big chamber head matches the 4.12′ bore size while the smaller chamber matches the 3.88″ bore size. The heads can be identified without removing from the car by a number stamped into a flat machined pad just below the valve cover surface, above the temperature gauge fitting. The small chamber heads are stamped with a “4” and the large chamber heads are stamped with an “8”. However, when installed in a car the number faces the ground so a mirror and a flashlight comes in handy. To increase compression ratio, and in turn horse power, the factory substituted the small chamber 6X-4 heads on the performance 400 engines in 1978 and 1979. Among Pontiac heads designed for un-leaded gas, the 6X is considered the best fl
