Chevy 400 Engine Performance Tipsby Moss Strohem
The small block Chevy engines, introduced in 1955 at 265 cubic inches, continued expanding in size even while maintaining the same external dimensions. The 400 cubic inch version was produced from 1970 to 1980 and reached practical limits of displacement by increasing the cylinder bore to 4.125 inches and lengthening the stroke to 3.75 inches. It was available in Chevrolet cars until 1976 and in Chevy and GMC trucks through 1980.
The 400 cubic inch engine was never intended to be performance oriented, but it still became recognized as a very capable high-performance platform. Few completely original 400 engines exist today. In attempting to use this engine in a performance application, the first area to address is the tuneup. Ensure that the ignition, fuel and cooling systems are all operating optimally. The cooling system is critical; because of its larger-bore diameter, the 400 was designed with no coolant flowing between the cylinders. As a result, they are prone to damage from excess heat. A cool-running, well-tuned 400 will make good power in stock form, yet many enthusiasts prefer more. The two most cost-effective improvements are exterior engine modifications. The first area of focus should be the exhaust system. The factory equipment is designed for quiet, reliable operation but is restrictive. Replacing the cast-iron exhaust manifolds with aftermarket headers and dual exhaust will improve power and efficiency. The next area to address is the fuel system, particularly the intake manifold and carburetor. Not all 400 engines were originally equipped with a four-barrel (4V) carburetor--again, because performance was never the intention. For those that were so equipped, replacing the factory cast-iron intake manifold with an aftermarket dual-plane manifold will improve intake airflow. The GM Rochester four-barrel spread-bore carb on the 400/4V engine is adequate for mild performance applications, but the bore pattern requires a manifold with the same pattern. If replacing a 2V carb and intake manifold, an aftermarket 4V manifold and carburetor will be necessary. These two modifications will improve airflow and can increase power output by 10 to 20 percent or more in normal operating ranges, from idle through approximately 5,000 rpm, without making the car difficult to drive. Note that installing an induction system that is designed for high rpm use will decrease low-end power and overall performance unless you make other substantial modifications.
Other performance improvements are more complex and require disassembling part (or all) of the engine. Since this engine had an intended rpm range below 5,000, further upgrades require more thought. First, the original camshaft is designed for a smooth idle and low maintenance. Carefully select an aftermarket camshaft to make a meaningful difference in output. Modifications to the cylinder heads (porting or outright replacement with more modern heads) will complement the camshaft change. These upgrades can make as much or more of an improvement to performance as do the intake/exhaust upgrades. Beyond these "bolt-on/bolt-in" upgrades, internal modifications to the connecting rods, pistons and crankshaft are essential. The stock internal components were not designed for high rpm use or power output, so be careful when planning and selecting parts. Properly configured, however, this engine is capable of outputs much higher than 500 horsepower.