The 383 CI small-block Chevy has the potential to produce far more torque than the base 350 SBC engine. Using the same crankshaft stroke as the 400 SBC provides an immediate boost in torque, yet there are other modifications that can improve on that baseline. Most of the additional torque improvements are a result of careful selection of components which have to do with how -- and where, in the operating range -- the engine "breathes." While large volumes of airflow are required to produce high-rpm horsepower, efficient airflow velocity is the key to producing torque.
Locate and purchase a set of Chevrolet 5.7L L31 (350 CI) Vortec cylinder heads from a late 1990s model GM truck or van. These heads have design features that make them better than even the best and most-modified of earlier GM heads. Alternately, you can select a set of heads from an aftermarket vendor such as EQ, Dart, RHS, etc., as long as they have similarly small ports and small combustion chambers.
Select an intake manifold and carburetor designed for lower-rpm operating ranges. A dual-plane manifold such as the Edelbrock Performer will work well in rpm ranges to around 5500 while the Performer RPM will extend the power range beyond 6,000 rpm. A single-plane, open-plenum manifold design can produce more upper-rpm horsepower, but does so at the expense of low-rpm torque. A carburetor rated at around 750 CFM of airflow should be used.
Choose pistons that provide a compression ratio (CR) which is as high as can safely be run on available pump gasoline. Generally, 9.5 to 1 CRs should be about the maximum, but if careful attention is paid to other details, slightly more compression can be used.
Select a cam with the proper duration and intake valve closing point. Camshaft selection is critical. Too much duration places the camshaft's intended rpm range too high and also has a later intake valve closing point. As a result, the cylinder can't build enough pressure to operate well at low rpm, so the engine will feel "soggy" at low speed. Conversely, a cam with too little duration and too early an intake valve closing point will build too much pressure in the cylinder and be prone to pinging and detonation. A cam in the 210 to 220 degrees of duration range -- at .050-inch lift -- should be close.
Take the engine block and components to a competent machine shop for cleaning, inspection and preparation. Provide the machinist with the rotating assembly, as well -- crank, rods, bearings, and pistons, pins and rings. Instruct him to machine/recondition the block and balance the rotating assembly.
Clean all the parts and test-assemble for proper fit. Pay attention to critical tolerances. If there are any problems with clearances, disassemble and correct the issue.
Install the engine and test-run, being certain to adjust the fuel system for proper flow and air/fuel mixtures as well as ignition timing. Peak torque is made only when the tune is optimal. An alternative is to have the engine "run in" and tuned on an engine dynamometer. This can save days, weeks or months of tuning in the vehicle.
