Big block V8: 348, 409, 366, 396, 402, 427, 454

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Note: Data are taken from several different sources and may not always agree or be complete.

The "big block" Chevrolet engines included here are the 1958 through early 1965 348 cid and 409 cid engines as well as the Mark IV 366/396/402/427/454 "rat motors." Unless otherwise noted, the term "big-block" on these pages refer to the 366-454 engines. The late 1969 on 396 cid big-block engines were actually 402 cid. Chevrolet introduced a 400 cid small-block engine in 1970 used in full-size passenger cars and Monte Carlos. Chevrolet also often called the 402 cid big-block a 400 cid engine. To help avoid issues as to which engine was being referred to, the small-block version was called a 400-2 (since it was only available with a 2-barrel carburetor) or Turbo-Fire 400; the 400 (402) big-block engine was called a 400-4 (since it was equipped with a 4-barrel carburetor) or Turbo-Jet 400.

The big block engine ranged in cubic-inch size from 348 to 454 in factory form. Factory horsepower ratings ranged from 250 horsepower with the 348 V8 engine to a whopping 450 horsepower with the LS6 454 "King of the Hill" engine before horsepower ratings were drastically cut in 1971 due to government emissions requirements.

The 396/454 Mark IV has standard performance and high performance versions. Both are cast iron with additional high performance heads cast in aluminum. Both versions have push rod guides and screw-in valve spring studs; items not found on factory-built small-block heads. Standard performance heads have the letters "PASS," high performance head have the letters "HI-PERF" while truck applications will have "TRUCK." Standard performance heads have what are referred to as "oval-port" intake runners while high performance heads have "square-port" or "rectangle-port" intake runners.

Two combustion chamber types are found, "open chamber" and "closed chamber." Beginning in 1970, big-block Chevrolet heads use a tapered-seat spark plug which eliminated the washer to make the seal although some early 1970 castings continued to use the longer reach "N" reach spark plug. These new tapered-seat spark plugs are often referred to as "peanut" plugs and required a smaller spark plug wrench size of 5/8-inch, as opposed to the older 13/16-inch size. Since the spark plug holes were cut into the cylinder head's casting, the basic casting could be machined for either spark plug.

The Mark-IV cylinder heads used a new "splayed" valve arrangement where the intake and exhaust valves were moved closer to their respective runners and canted at various angles. Due to the valve arrangement, these picked up the nickname of "porcupine" and "semi-hemi" heads.

Screw-in rocker arm studs and valve guides	"Splayed" valve arrangement
Big-block oval-port runners	Big-block square-port runners
Big-block open combustion chamber	Big-block closed combustion chamber
348-409 cylinder heads	Some early big-block heads have this boss drilled and tapped for applications requiring spark plug shields.
Typical "N" reach spark plug	Typical tapered-seat (peanut) spark plug

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