Here's a breakdown of the issues and what you can do:
Problems using a 305 head on a 350:
* Smaller Valves: 305 heads have smaller valves than 350 heads. This restricts airflow, limiting the 350's potential power output. You'll see a significant horsepower and torque loss.
* Smaller Combustion Chamber: Similar to the valves, the smaller combustion chamber reduces the efficiency of the larger displacement engine.
* Weaker Construction: 305 heads are generally built lighter for fuel economy, not high performance. They may not withstand the higher cylinder pressures of a 350, potentially leading to cracking or warping under stress.
* Flow Restrictions: The ports and intake runners are designed for a smaller engine, again restricting the airflow a 350 needs to perform well.
"Beefing it up" limitations:
While you can improve a 305 head to some degree, it will never perform as well as a proper 350 head. The fundamental limitations of size and design are difficult to overcome. Possible improvements, but with diminishing returns, include:
* Valve Job: Larger valves can be installed, but this often requires machining the head and may still leave you with suboptimal flow characteristics due to the port size.
* Porting and Polishing: Improving the flow through the intake and exhaust ports can help, but again, it's working with a smaller base design.
* Hardening: Some strengthening can be achieved through specialized heat treatments, but this is costly and doesn't address the fundamental size limitations.
The Bottom Line:
Using a 305 head on a 350 is possible, but not recommended for performance applications. The performance gains you'd get from "beefing it up" are unlikely to outweigh the cost and effort involved. It's far more effective and cost-efficient to source and use a set of 350 heads (or even better performing aftermarket heads) designed for the larger engine. Sticking with 305 heads would only be suitable for a mild build, maybe a lightly modified 350 for cruising, but not for any serious performance goals.
