How to Fix Chevy 305 Problemsby Contributor
The eight cylinder Chevrolet 305 is poorly regarded because of poor factory performance, especially when compared to Chevy's larger 350 engine. While General Motors had to make some compromises in the initial design, the result is an engine that older car owners can tune to a "sweet spot" between horsepower and fuel efficiency.
The 305 cubic inch (5.0 liter) engine was introduced in 1975 to meet federal emissions standards while improving GM's fleet mileage average. Performance was not a initial concern. Stodgy models like the Chevrolet Caprice, Oldsmobile Cruiser wagon, and the Buick Skylark used the engine, giving it a reputation as under powered. However, the 305 can also be found in the more demanding Camaro line as well as light trucks and vans. The engine was replaced by the Vortec 5000 in 1996.
Because the 305 was designed to provide moderate power while retaining relative efficiency, drivers looking for pure horsepower or great mileage are going to be disappointed. This engine could be modified to deliver 350 hp, but a balance of snappy (though not outstanding) performance with decent gas mileage better fits most drivers' needs. Sensible upgrades to the 305 correct the initial engineering problems with modern components that let the engine operate more efficiently, boosting both power and mileage.
The primary problem with the Chevrolet 305 engine is the narrow 3.736 bore, which results in insufficient airflow compared to a larger block. The stock headers further restricted air intake because of pollution concerns, though replacement Edlebrock or Vortec headers increase airflow while still maintaining proper emissions. Upgrade the air filter assembly with a modern unit to let the engine "breathe" more easily on the intake. The stock exhaust system is generally sufficient - though pre-1981 catalytic converters should be replaced with a higher-flow "honeycomb" converter.
If rebuilding the engine, address some of the internal shortcomings of the 305. Change the pre-1990 factory flat-tappet cams to hydraulic roller cams to reduce engine friction. Likewise, improved flat-top pistons are superior to the original dished pistons, offering power while still maintaining emissions standards. The stock large valve heads can lead to valve shrouding which reduces airflow. A solution is to install a larger exhaust valve so shrouding will be less of a concern. Also, take a look at replacing the a two barrel carburetor with a Throttle Body fuel injector (TBI) or a four barrel "Quadrajet" carburetor. Both systems offer economy at highway speeds, yet provide power when you need it.
As with all older vehicles, peripheral systems can wear out long before the engine does. The cam drive gear is of special concern on 305 engines. 305s manufactured before 1988 used a nylon-toothed cam gear to reduce noise. This gear is prone to failure and should be replaced with a solid steel gear. If replacing the stock water pump consider an electric version to reduce the workload of the engine. Stock radiators of this vintage frequently need replacement as well. Opt for a lightweight three row aluminum model that reduces weight.