The Chevrolet 350ci engine is an excellent platform for performance modifications such as upgraded cylinder heads. The application and purpose of the car it will be used in influences head selection. Heads designed for a competition racing engine will perform poorly in a daily-driven street car, and heads designed for a street application will restrict the output of a car designed for racing. Fortunately, there are many options in aluminum and iron castings for nearly all intended uses from both General Motors and aftermarket manufacturers.
If greater performance levels are desired while maintaining good street behavior, important guidelines need to be considered. First, greater airflow capacity improves output potential, but this has the drawbacks noted above: Too much airflow will hinder torque and power at low engine speeds. With a relatively short stroke, the 350 is sensitive to port size, and torque is easily affected. Yet, a lighter car can allow room for a little “cheating” and can hide the drawbacks of a port that is too large.
A general indicator of airflow potential is the volume of the intake and exhaust ports. Early 350s had heads with standard 23-degree valve angles and intake port volumes of around 160cc or less, but extensive port modifications could enhance flow and increase output. GM Performance Parts began offering castings with larger ports for competition usage in the 1970s, and aftermarket manufacturers introduced higher-flowing castings soon after. Now several aftermarket companies offer heads for many applications with intake port volumes exceeding 235cc and airflows greater than 300cfm. (Heads with volumes this large are intended for competition only, and some require specific shaft-mounted aftermarket rocker arms.)
Some manufacturers produce higher-quality castings than others, and their prices and reputations reflect this. Aside from other subtle distinctions, the most important consideration will be port design and size, and air velocity. Airflow advertisements can be misleading, and the relatively smaller port is usually better—with the exception of all-out competition engines. While port volume determines airflow potential, the quality of airflow is critical. It must have high, consistent velocity, especially in a heavier car. Simply put, a larger-diameter “pipe” reduces air speed, and this impacts torque output.
Keep in mind that more output at high engine speed sacrifices power at low speeds. For a more specific selection, here are some guidelines.
A daily street-driven car with an unmodified short block (lower engine) assembly is generally safe for operation up to 5,000 to 5,500 revolutions per minute (RPMs). A head with an intake port volume in the range of 160 to 180cc is suitable. A camshaft and fuel-induction system (carb or fuel injection) should be selected to work within this operating range. A reasonable power output of 350 bhp can be achieved.
For engine speeds up to 6,500 RPM, heads with port volume volumes from 180 to 210cc offer improved power output, but even more careful attention must be paid to the camshaft, fuel and exhaust systems. Low-speed performance will be hindered even more, however. An engine operating at this speed should have upgraded components in the lower end, as well. Power can exceed 450 to 500 bhp.
A 350ci engine in a competition environment where RPMs consistently operate beyond 4,000 and up to 8,000 RPMs can use large port heads to achieve outputs ranging from 600 to 700 bhp or more.
