Here's why:
* Smaller Valves and Ports: 305 heads typically have smaller valves and combustion chambers, and smaller intake and exhaust ports compared to heads designed for a 350. This restricts airflow, which is crucial for horsepower. A larger displacement engine like the 350 needs to breathe more efficiently to realize its potential.
* Chamber Design: The combustion chamber design in 305 heads might not be optimal for the larger displacement of the 350, leading to less efficient combustion.
* Flow Characteristics: Even if the valve size were similar, the port design and flow characteristics of 305 heads are likely not as good as 350 heads for a 350 cubic inch engine. They will restrict the flow of air and fuel.
* Carb Conversion: The carb conversion itself is a separate factor. Properly tuning a carb for the engine's needs is critical. Poor carb tuning can mask or even worsen the limitations of using smaller heads.
When 305 heads *might* seem better (but probably not):
The only scenario where 305 heads *might* produce comparable power would be if the 350's original heads were in extremely poor condition (heavily damaged or severely worn). In this case, even less-than-ideal 305 heads might offer a slight improvement. However, this is not ideal and finding properly refurbished or new 350 heads would always be the preferred solution.
In summary: For optimal power from your 350, invest in heads specifically designed for that engine size. Using 305 heads would be a step backward. Focus on finding good quality 350 heads, or possibly even aftermarket performance heads for significant horsepower gains.
