The bigger ports with bigger valves cost velocity off idle, like having too big a cam
they do make more power once you get past the flat spot
I'd use the smaller valves too
all small block heads the same?
308's exhaust is much better than X or J or SBM 915 heads and the intakes just as good or better, combustion chamber is better
I had 302-308 but switched to Direct Connection Magnum heads when they had them on sale fore $200 each with good valves and springs
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Swirl Port Heads http://www.hotrod.com/articles/mopp-0607-mopar-cylinder-head/
Chrysler’s small-block cylinder head design changed very little until 1987 when the 302 casting (last three digits of the casting number), commonly known as the swirl port, was introduced. While the 302 casting, as well as the later 308 casting, were most commonly found on low-performance 318s, they shouldn’t be overlooked for a performance build. Even with small 1.75-inch intake and 1.50-inch exhaust valves, the design of this head’s heart-shaped quench-style combustion chamber makes it a good choice for mild to moderate performance. An engine with these heads in stock form will outperform an engine with stock X or J castings simply due to the improved combustion chamber design. Additionally, these heads have hardened valve seats, which make them compatible with unleaded fuel. The swirl port heads were utilized through 1991 (302 casting) and 1992 (308 casting) and respond well to port and bowl work, offering better power potential than all of the earlier small-block heads. Another nice feature of these heads is they utilize valvetrain and intake manifolds common to early small-blocks, so parts are readily available. Simply bolt these heads to your small-block and enjoy the extra power they provide."
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