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That all depends on how good the cast Item is. If it is real close to your combo in runner size, plenum, etc, you won't see a lot of difference. The old tunnelrams for mopar BBs were small runner, and responded to porting. If you are building an indy head motor, or B1 deal, the cast Indys work pretty good,same for the B1.
There is always room for improvement, just a matter of budget and where you want to spend money. A custom manifold is way down my list of what to do next, as the 3x single four I run is pretty good as is. Put a 2100 cfm terminater on it, and there shouldn't be much improvement. I have ported 440-1 heads, 528 cubes.

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