Headers vs. Exhaust Manifolds ~ 'Let The Debate Begin'

Generally,

On a 'stock' Small Block High-Performance Engine,

The gain can honestly be stated to be at +15 Horsepower without
exaggerating.

That is running with the Headers closed as well.

Remember too, that there will be an increase in RPM's of approximately
{+250}, as there will be less exhaust restriction.
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Back-in-the-Day {Our '1972' Performance Test for 'Cyclone Headers'}

Car .................... 1970 Plymouth 'Duster 340' ~ 4-Speed ~ 3.55 Gears ~ G70 x 14" Tires.
Odometer ............ 23,000
Improvements ......'None' {All Stock}

Best Performance.... 14.37 @ 97.10 MPH

Improvements ..... 'Cyclone' Headers ~ 1 5/8" Primary Pipes ~ 3" Collector
Note ................... Closed Headers, running thru the 'factory' stock exhaust system.

Best Performance ... 14.18 @ 98.94 MPH

Gain ................. {-.19 off Elapsed Time} and {+1.84 MPH}