High output small block Chevrolet engines produce the most power possible with high compression ratios, or CRs -- ratios as great as 15:1. Competition and racing engines are designed this way, but require the use of high octane racing fuels. High performance "pump gas" engines can produce relatively high power levels as well, but the lower octane ratings of available pump fuel restrict CRs to levels of under 11:1. However, a carefully planned engine combination can frequently approach -- or exceed -- 500 horsepower.
Plan an engine build that targets 1.5 horsepower per cubic inch. Design the engine with as much displacement as possible. Two popular SBC combinations based on the 3.75-inch 400 engine stroke are the 383- and 406ci engines. The 383 uses the 400 crankshaft in a 350 block that is overbored .030-inch (4.030-inches). The 406ci engine uses the 400 block which is overbored .030-inch (4.155-inches).
Locate and purchase the shortblock components. These include a serviceable engine block and high-strength crankshaft, connecting rods and pistons. Late-model 350 blocks -- for the 383ci combination -- use roller camshafts. This is an advantage since roller cams produce power more easily than flat-tappet cams. Forged steel crankshafts and connecting rods are preferable in high-output engines, but aftermarket cast steel crankshafts are suitable for limited high RPM usage.
Select connecting rods with upgraded bolts. Many lower-end component failures are associated with connecting rod bolts breaking. Select pistons only after you have decided which cylinder head will be used. The shape of the piston head impacts compression ratio. The block deck-height, head gasket thickness and head chamber volume need to be known beforehand.
An alternative is to buy a professionally-built shortblock that is assembled with high quality components.
Select aluminum cylinder heads with combustion chambers that -- when combined with the shape of the piston head -- yield a compression ratio of about 10.5:1. SBC heads are commonly available with chamber volumes from 64 to 76 cubic centimeters. Choose heads with intake ports of approximately 220cc of intake port volume and rated airflow of greater than 250 CFM at .500-inch lift.
Select an intake manifold and carburetor with flow capacity that is matched to the heads. A single plane manifold and 750 to 850 CFM carburetor will make more high RPM horsepower.
Select the camshaft. Contact a cam manufacturer or custom cam grinder for a cam design which offers optimum performance. It's essential that all details about the engine be provided -- heads, head flow, intake system, compression ratio, fuel, etc. -- as well as the type of car it will be used in and your intended usage. A flat tappet cam with 260 degrees or more of duration -- measured at .050-inch lift -- and .600-inch net valve lift may be used. A roller cam requires less duration.
Have the components machined and prepared -- blueprinted to exacting tolerances -- by a qualified machine shop. Once prepared, assemble the engine in accordance to the instructions in the performance engine assembly manual.
