The 400 CID Chevy small block engine was produced from 1970 until 1980 and was the largest displacement engine made in that platform. It was intended as a low performance, high-torque engine primarily used in Chevy's heavier passenger cars and light-duty trucks. The general design was the same as earlier small block Chevrolet engines and incorporated many similar external features and dimensions while being internally different. The 400 has gained notoriety -- with modifications -- as a powerful performance engine.
The 400 CID small block engine kept the same external dimensions as the other small block engines, but increased its displacement through internal modifications. The engine shares the same length, width and height as other SBCs at approximately 28 by 26 by 27 inches respectively. The identical external measurements made the 400 SBC readily interchangeable with other Chevy engines, such as the 350 CID small block engine, across many of the Chevy and GMC truck and Chevrolet car lines. Engine weight was also similar at about 261 Kilogram. This engine shared the same ignition system -- distributor, accessories and intake and exhaust systems as the smaller engines.
General Motors achieved the 400 SBC engine's displacement by increasing the bore to 4.125 inches and the crankshaft stroke to 3.75 inches -- actual displacement is just over 400.9 cubic inches. In contrast, the Chevy 350 CID engine uses a bore of 4.00 inches and a stroke of 3.48 inches. GM had to cast the block for the 400 CID engine differently to gain the extra displacement. To gain the extra 1/8-inch bore diameter, the cooling system water jackets between adjacent cylinders were eliminated on the 400 block. Additionally, GM designed the 400 crankshaft with larger main journals than the 350 -- 2.65 inches versus 2.45 inches. The connecting rods were also shorter than the others were -- 5.565 inches as opposed to 5.7 inches in the smaller engines. As a result of the short rod and 1/4 inch longer stroke, GM was required to use external crankshaft balancing on the flywheel/flexplate and harmonic balancer -- this is a shared characteristic of the 454 CID big block Chevy.
The 400 CID small block was intended as a low RPM, high torque engine, because of the long stroke. It was available with both a two-barrel and four-barrel carburettor. While power output measurements changed in the early 1970s from gross engine power to "net" ratings --at the rear wheels, output remained relatively comparable. Gross horsepower ratings were at as much as 265 horsepower while net ratings were little as 150.