The big block V8 Chevrolet engine was called the "W" series and was released in 1958 for passenger car and truck use. The engine had offset valves and unique scalloped rocker covers, giving it a distinctive appearance. The offset valves were not designed with any mechanical advantage in mind- it was done to reduce the block's overall length. It did, however allow the fitment of larger valves without any major redesign or machining.

The "W" series was produced from 1958 to 1965, with three displacements offered: 348 cu. in., offered from 1958-1961 in cars, and through 1964 in trucks; 409 cu. in., offered from 1961-1965; and 427 cu. in., available only in 1963 in a special Z11 package. The "W" big block had a dry weight of approximately 665 pounds, depending on intake manifold and carburetion, and was a physically massive engine when compared to the Chevrolet small block.

The block had 4.84" bore centers, two-bolt main bearing caps, a lubrication system with the main oil gallery located low on the driver's side of the crankcase similar to Ford's "side oiler" system and unique for its day. The heads used on the high performance 409 and 427 engines had larger ports and valves than those used on the 348 and the base 409 passenger car and truck engines, but were externally identical to the standard units and must be identified by casting number.

A minor difference between the 348 and 409-427 engines was the location of the engine oil dipstick: it was on the driver side on the 348 and passenger side on 409-427. As with the 265 and 283 cubic inch small block engines, the "W" engine valve gear consists of tubular steel push rods operating stud-mounted, stamped steel rocker arms. The pushrods also act as a conduit for oil flow to the valve gear. Due to the relatively low mass of the valve train, the "W" engine were capable of operating at RPMs beyond 6000 RPM, however, the head design limited HP above 5,800 rpm.

Unlike many of its contemporaries, the "W" combustion chamber was in the upper part of the cylinder, not the head, the latter having only tiny recesses for the valves. This arrangement was achieved by machining the block at milled at a 74 degree angle rather than 90, creating a raised portion of the block toward the outward side which became the combustion chamber. The concept for this unique head was to maximize brake mean effective pressure (BMEP) at relatively low engine speeds, resulting in a very fat torque curve. It was essentially the same basic idea as used by Ford's MEL engine, which also appeared in 1958. Both of these designs were known more for their torque than their top end

A crowned piston was used to designate the combustion chamber, a concept in American production engines shared only with the afore mentioned Ford MEL block. As the piston approached top dead center, the angle of the crown combined with that of the head deck to form a wedge shaped combustion chamber with a pronounced quench area. The spark plug protruded vertically into this chamber, which tended to cause a rapidly moving flame front during combustion.

Eventually, this design would we considered difficult for production and especially service, in that truing up the head by milling, required re-sinking the valves and trimming the tops of the valve stems. But he major issue was that the horsepower fell off in racing applications.

With various tweaks and some serious addition of cubes, the “W” held its own on the street and drag strip. Where it did not fare as well was in NASCAR, where the configuration did not lend itself to sustained high rpm.  Thus, development began on a second generation big-block in final iteration, when released in 1965 would be called the “Mark IV”.

Blocks:

348

The first "W" engine was the 1958 "Turbo-Thrust" 348-cubic-inch V8, thought to be originally designed and intended for use in Chevrolet trucks, but  this was not completely true. Engineers realized that the 265-283 "small block" could not produce enough torque to add effective performance at moderate rpms in the larger, heavier 1958 passenger car line that was projected. It was also a response to the climbing engine sizes of the competition. The bore was 4.125" and stroke was 3.25", a substantially oversquare design.

With a four-barrel carburetor, the base Turbo-Thrust produced 250 HP. A special "Tri-Power" triple-two-barrel version, called the "Super Turbo-Thrust" produced 280 HP, and a "Special Turbo-Thrust" upped the output to 305, utilizing with a single large four-barrel. The higher horsepower engines were equipped with dual point distributors, improved head's and 2 1/2"exhaust. 

Mechanical lifters and the three two-barrel carburetors brought the "Special Super Turbo-Thrust" up to 315 HP. (See a Road Test HERE) In 1959 and 1960, high-output versions of the top two engines were produced with 320 HP and 335 HP (See a Road Test HERE) respectively. In 1961, power was again increased to 340 HP for the single four-barrel model, and 350 HP when equipped with three two-barrels.

409

A 409 cubic inch version of the "W" engine was released in 1961 on December 17, 1960 for Chevrolets along with the Impala SS (Super Sport) model. (See an expose of the new Engine HERE). Bore and stroke were upped from the 348 to 4.312" x 3.50". (See a Road Test HERE). The 409 became Chevrolet's top-of-the-line regular production engine from 1962 through 1965, with a choice of single or dual-four-barrel carburetion. The initial version of the 409 engine produced 360 HP, with a single-four-barrel Carter AFB carburetor. The same engine was upped to 380 HP in 1962. (See an expose of the refined 409 HERE). A 409 HP version of this engine was also available, with a dual-four-barrel aluminum intake manifold and two Carter AFB carburetors. (See a Road Test HERE).

In the 1963 model year, the 409's output reached 425 HP at 6200 rpm with the same 2 4-barrel manifold, 11.25:1 compression and a new solid lifter camshaft. In addition, a 340 HP version of the 409 engine was available from 1963-1965, with a single-four-barrel cast-iron intake mounting a Rochester 4GC carburetor, and a hydraulic-lifter camshaft. The 425 HP engine was available through mid-1965 when it was replaced by the 396-cubic-inch 425 HP Mark IV big-block engine.

427 Z11

A special 427 cubic inch version of the 409 engine was available for the 1963 Chevrolet Impala Sport Coupe when ordered under Chevrolet Regular Production Option (RPO) Z11. This was a special package created for drag racers, including some aluminum engine and body parts and a cowl-induction air intake system, along with this new 427 "W" engine. Unlike the later second generation Mark IV 427, it was still based on the W-Series 409 engine, but with a longer 3.65" stroke. (See more Z-11 info HERE).

A high-rise two piece aluminum intake manifold and dual Carter AFB carbs fed a 13.5:1 compression ratio which produces a very under-rated 430 HP and 435 ft. lbs of torque. Fifty RPO Z11 cars were eventually produced. GM Documents show 50 Z11 engines were built at the GM Tonawanda engine plant, and that 20 partial engines were made for replacement/over the counter sale.