Holley/Demon Carb stumble

Start with butterflies that don't have holes. Then drill if you need them.

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Exactly.

And I wouldn’t be too eager to drill them either.

Mitty Demon main body 1.560 (850)

Proform main body...... 1.380 (850)

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The Demon flows more than 850, the Proform would be pretty close to 850(wet flow).

I’m not sure where I’d even start to look for my notes on my made-from-parts Holley HP950, but with some mildly blended venturi’s iirc it flowed about 865cfm(calculated wet flow...... 92% of dry flow).
I seem to recall the Demon 1.560 venturi carbs being closer to 920cfm(calculated wet).
The Mighty Demon 850, Race Demon 1000, and Race Demon RS 1025 are all the same “size”, and all flow basically the same.