edelbrock magnum head flow?

yellow rose said

I hope you can find a top if it didn’t come with one. I don’t care for that big bread box top but you may not have a choice.

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i have one of these that i can adapt to fit, or fab my own plenum. not too big of a deal. I have seen those really big plenum tops for it but not sure exactly how they perform...what is the reasoning behind them?