How much horsepower will headers add to a 351 Cleveland?

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.

Headers are the most oversold part in the catalogue and still one of the most worthwhile. The honest number on a small block is high single digits to low double digits in percent, not the thirty percent the ad copy implies.

Cleveland exhaust ports are enormous to begin with, which is part of why the factory manifolds are less of a restriction here than on some engines. You still gain, just not as much as the guy at the swap meet says.

Tube size matters less than people think too: on a same-engine test, long tubes from two different manufacturers landed within a horsepower of each other, and adding mufflers cost about one percent.

The build

The other one

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