351 Cleveland carb size — 600 or 750?

Engine dyno, as configured — flywheel horsepower. Standard test config: stock manifolds through 18" of pipe, air cleaner lid flipped, no accessories driven. Factory SAE net ratings are measured as‑installed and read 15–25% lower. Rev ceiling for this combination: 6000 rpm.

Where each one wins

Torque, lb-ft, at the flywheel. Everything except the part named is identical between the two runs.

Every forum thread on this ends in opinion. It does not have to. Airflow demand is displacement times rpm times efficiency, and once the carburettor is not restricting that, a bigger one cannot add anything.

What the published swap tests show: one size step below the restriction point is worth two to five percent, one step above is worth nearly nothing, and two steps above starts costing you midrange. On a four-carb Buick test the biggest carburettor of the four was the worst in every column.

So the honest answer is that carburettor size buys headroom, not horsepower — and the point where it starts costing you is further up than most people think.

The build

The other one

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