The 305 stock heads (typically 624 and 416 casting numbers) have 1.84/1.50 valve combination and small combustion chambers. The stock springs on 305 heads are good for about .375” lift. They are a limiting factor.

The Trick Flow are 1.94/1.50 units with an enhanced combustion chamber and larger intake runners that provide a 25% increase in flow (245 cfm versus the stock 190). They also handle .480 lift.